There are plenty of low-cost logic analysers but they all share a common characteristic; a USB link is used to transfer the data into a PC for analysis.
If the equipment is in a safe & comfortable office environment, then this isn’t a problem, but in many cases it is operating in an distant, inaccessible or hostile location, so remote monitoring is desirable. If the analyser unit is small and low-cost, it can remain attached on a semi-permanent basis, enabling long-term monitoring & diagnosis of remote equipment
The initial specification of the logic analyser unit is
Digital inputs: 16 for each unit
Input threshold: programmable
Sample rate: up to 20 megasamples per second
Sample store: up to 250 kilosamples
Network interface: WiFi
Network protocols: TCP and HTTP
Control method: full remote control
Display method: Web pages with Javascript
Remote API: Python class
The project is fully open-source, and is documented in the following posts:
There are various ways that the Pi Pico (RP2040) can be given a wireless interface; in a previous post I used a Microchip ATWINC1500, now I’m using the Espressif ESP32 WROOM-32. The left-hand photo above shows an Adafruit Airlift ESP32 co-processor board which must be hand-wired to the Pico, whilst the right-hand is a Pimorini Wireless Pack that plugs directly into the Pico, either back-to-back, or (as in the photo) on an Omnibus baseboard that allows multiple I/O boards to be attached to a single Pico.
So you can add WiFi connectivity to your Pico without any additional wiring.
The resulting hardware would normally be programmed in C, but I really like the simplicity of MicroPython, so have chosen that language, but this raises an additional question; do I use the Pimorini MicroPython that is derived from the Pi Foundation version, or CircuitPython, which is a derivative created by Adafruit, with various changes?
CircuitPython includes a lot of I/O libraries as standard, but does lack some important features (such as direct access to memory) that are useful to the more advanced developer. So I’ll try to support both, but I do prefer the working with MicroPython.
SPI interface
The ESP32 does all the hard work of connection to the WiFi network and handling TCP/IP sockets, it is just necessary to send the appropriate commands over the SPI link. In addition to the usual clock, data and chip-select lines, there is a ‘reset’ signal from the Pico to the ESP, and a ‘ready’ signal back from the ESP to the Pico. This is necessary because the Pico spends much of its time waiting for the ESP to complete a command; instead of continually polling for a result, the Pico can wait until ‘ready’ is signalled then fetch the data.
My server code uses the I/O pins defined by the Adafruit Pico Wireless Pack:
Function GPIO Pin num
Clock 18 24
Pico Tx data (MOSI) 19 25
Pico Rx data (MISO) 16 21
Chip select (CS) 7 10
ESP32 ready 10 14
ESP32 reset 11 15
Software components
Pico software modules and ESP32 interface
ESP32 code
The ESP32 code takes low-level commands over the SPI interface, such as connecting and disconnecting from the wireless network, opening TCP sockets, sending and receiving data. The same ESP32 firmware works with both the MicroPython and CircuitPython code and I suggest you buy an ESP32 module with the firmware pre-loaded, as the re-building & re-flashing process is a bit complicated, see here for the code, and here for a guide to the upgrade process. I’m using 1.7.3, you can check the version in CircuitPython using:
Note that some ESP32 modules are preloaded with firmware that provides a serial interface instead of SPI, using modem-style ‘AT’ commands; this is incompatible with my code, so the firmware will need to be re-flashed.
MicroPython or CircuitPython
This has to be loaded onto the Pico before anything else. There are detailed descriptions of the loading process on the Web, but basically you hold down the Pico pushbutton while making the USB connection. The Pico will then appear as a disk drive in your filesystem, and you just copy (drag-and-drop) the appropriate UF2 file onto that drive. The Pico will then automatically reboot, and run the file you have loaded.
The standard Pi Foundation MicroPython lacks the necessary libraries to interface with the ESP32, so we have to use the Pimorini version. At the time of writing, the latest ‘MicroPython with Pimoroni Libs’ version is 0.26, available on Github here. This includes all the necessary driver code for the ESP32.
If you are using CircuitPython, the installation is a bit more complicated; the base UF2 file (currently 7.0.0) is available here, but you will also need to create a directory in the MicroPython filesystem called adafruit_esp32spi, and load adafruit_esp32spi.py and adafruit_esp32spi_socket.py into it. The files are obtained from here, and the loading process is as described below.
Loading files through REPL
A common source of confusion is the way that files are loaded onto the Pico. I have already described the loading of MicroPython or CircuitPython UF2 images, but it is important to note that this method only applies to the base Python code; if you want to add files that are accessible to your software (e.g. CircuitPython add-on modules, or Web pages for the server) they must be loaded by a completely different method.
When Python runs, it gives you an interactive console, known as REPL (Read Evaluate Print Loop). This is normally available as a serial interface over USB, but can also be configured to use a hardware serial port. You can directly execute commands using this interface, but more usefully you can use a REPL-aware editor to prepare your files and upload them to the Pico. I use Thonny; Click Run and Select Interpreter, and choose either MicroPython (Raspberry Pi Pico) or CircuitPython (Generic) and Thonny will search your serial port to try and connect to Python running on the Pico. You can then select View | Files, and you get a window that shows your local (PC) filesystem, and also the remote Python files. You can then transfer files to & from the PC, and create subdirectories.
At the time of writing, Thonny can’t handle drag-and-drop between the local & remote directories; you have to right-click on a file, then select ‘upload’ to copy it to the currently-displayed remote directory. Do not attempt a transfer while the remote MicroPython program is running; hit the ‘stop’ button first.
In the case of CircuitPython, you need to create a subdirectory called adafruit_esp32spi, containing adafruit_esp32spi.py and adafruit_esp32socket.py.
Server code
To accommodate the differences between the two MicroPython versions, I have created an ESP32 class, with functions for connecting to the wireless network, and handling TCP sockets; it is just a thin wrapper around the MicroPython functions which send SPI commands to the ESP32, and read the responses.
Connecting to the WiFi network just requires a network name (SSID) and password; all the complication is handled by the ESP32. Then a socket is opened to receive the HTTP requests; this is normally on port 80.
There are significant differences between conventional TCP sockets, and those provided by the ESP32; there is no ‘bind’ command, and the client socket is obtained by a strangely-named ‘avail_server’ call, which also returns the data length for a client socket – a bit confusing. This is a simplified version of the code:
Most of this information is irrelevant to a tiny Web server, since there is little choice over the information it returns. The first line has the important information, namely the resource that is being requested, so instead of decoding the whole message, we can do simple tests to match the line to known resources:
DIRECTORY = "/"
INDEX_FNAME = "rpscope.html"
DATA_FNAME = "data.csv"
ICON_FNAME = "favicon.ico"
DISABLE_CACHE = "Cache-Control: no-cache, no-store, must-revalidate\r\n"
DISABLE_CACHE += "Pragma: no-cache\r\nExpires: 0\r\n"
req = esp.get_http_request()
if req:
r = req.split("\r")[0]
if ICON_FNAME in r:
esp.put_http_404()
elif DATA_FNAME in r:
esp.put_http_file(DIRECTORY+DATA_FNAME, "text/csv", DISABLE_CACHE)
else:
esp.put_http_file(DIRECTORY+INDEX_FNAME)
Since we are dealing with live data, that may change every time it is fetched, the browser’s caching mechanism must be disabled, hence the DISABLE_CACHE response, which aims to do so regardless of which browser version is in use.
Sending the response back to the browser should be easy, it just needs to be split into chunks of maximum 4095 bytes so as to not overflow the SPI buffers. However I had problems with unreliability of both the MicroPython and CircuitPython implementations; sometimes the network transfers would just stall. The solution seems to be to drastically reduce the SPI block size; some CircuitPython code uses 64-byte blocks, but I’ve found 128 bytes works OK. Further work is needed to establish the source of the problem, but this workaround is sufficient for now.
MAX_SPI_DLEN = const(128)
HTTP_OK = "HTTP/1.1 200 OK\r\n"
CONTENT_LEN = "Content-Length: %u\r\n"
CONTENT_TYPE = "Content-type %s\r\n"
HEAD_END = "\r\n"
def put_http_file(self, fname, content="text/html; charset=utf-8", hdr=""):
try:
f = open(fname)
except:
f = None
if not f:
esp.put_http_404()
else:
flen = os.stat(fname)[6]
resp = HTTP_OK + CONTENT_LEN%flen + CONTENT_TYPE%content + hdr + HEAD_END
self.send_data(self.client_sock, resp)
n = 0
while n < flen:
data = f.read(MAX_SPI_DLEN)
self.send_data(self.client_sock, data)
n += len(data)
self.send_end(self.client_sock)
Dynamic web server
A simple Web server could just receive a page request from a browser, match it with a file in the Pico filesystem, and return the page text to the browser. However, I’d like to report back some live data that has been captured by the Pico, so we need a method to return dynamically-changing values.
There are three main ways of doing this; server-side includes (SSI), AJAX, and on-demand page creation.
Server-side includes
A Web page that is stored in the server filesystem may include tags that trigger the server to perform specific actions, for example when the tag ‘$time’ is reached, the server replaces that text with the current time value. A slightly more sophisticated version embeds the tag in an HTML comment, so the page can be displayed without a Pico server, albeit with no dynamic data.
The great merit of this approach is its simplicity, and I used it extensively in my early projects. However, there is one major snag; the data is embedded in an HTML page, so is difficult to extract. For example, you may have a Web page that contains the temperature data for a 24-hour period, and you want to aggregate that into weekly and monthly reports; you could write a script that strips out the HTML and returns pure data, but it’d be easier if the Web server could just provide a data file for each day.
AJAX
Web pages routinely include Javascript code to perform various display functions, and one of these functions can fetch a data file from the server, and display its contents. This is commonly known as AJAX (Asynchronous Javascript and XML) though in reality there is no necessity for the data to be in XML format; any format will do.
For example, to display a graph of daily temperatures, the Browser loads a Web page with the main formatting, and Javascript code that requests a comma-delimited (CSV) data file. The server prepares that file dynamically using the current data, and returns it to the browser. The Javascript on the browser decodes the data, and displays it as a graph; it can also perform calculations on the data, such as reporting minimum and maximum values.
The key advantage is that the data file can be made available to any other applications, so a logging application can ignore all the Javascript processing, and just fetch the data file directly from the server.
With regard to the data file format, I prefer not to use XML if I can possibly avoid it, so use Javascript Object Notation (JSON) for structured data, and comma-delimited (CSV) for unstructured values, such as data tables.
The first ‘A’ in AJAX stands for Asynchronous, and this deserves some explanation. When the Javascript fetches the data file from the server, there will be a delay, and if the server is heavily loaded, this might be a substantial delay. This could result in the code becoming completely unresponsive, as it waits for data that may never arrive. To avoid this, the data fetching function XMLHttpRequest() returns immediately, but with a callback function that is triggered when the data actually arrives from the server – this is the asynchronous behaviour.
There is now a more modern approach using a ‘fetch’ function that substitutes a ‘promise’ for the callback, but the net effect is the same; keeping the Javascript code running while waiting for data to arrive from the server.
On-demand page creation
The above two techniques rely on the Web page being stored in a filesystem on the server, but it is possible for the server code to create a page from scratch every time it is requested.
Due to the complexity of hand-coding HTML, this approach is normally used with page templates, that are converted on-the-fly into HTML for the browser. However, a template system would add significant complexity to this simple demonstration, so I have used the hand-coding approach to create a basic display of analog input voltages, as shown below.
This data table is created from scratch by the server code, every time the page is loaded:
Even in this trivial example, there is significant work in ensuring that the HTML tags are nested correctly, so for pages with any degree of complexity, I’d normally use the AJAX approach as described earlier.
Running the code
The steps are:
Connect the wireless module
Load the appropriate UF2 file
In the case of CircuitPython, load the add-on libraries
Get the Web server Python code from Github here. The file rp_esp32.py is for MicroPython with Pimoroni Libraries, and rp_esp32_cp.py is for CircuitPython.
Edit the top of the server code to set the network name (SSID) and password for your WiFi network.
Run the code using Thonny or any other MicroPython REPL interface; the console should show something like:
Connecting to testnet...
WiFi status: connecting
WiFi status: connected
Server socket 0, 10.1.1.11:80
Run a Web browser, and access test.html at the given IP address, e.g. 10.1.1.11/test.html. The console should indicate the socket number, data length, the first line of the HTTP request, and the type of request, e.g.
Client socket 1 len 466: GET /test.html HTTP/1.1 [test page]
The browser should show the voltages of the first 3 ADC channels, e.g.
The Web pages produced by the MicroPython and CircuitPython versions are very similar; the only difference is in the table headers, which either reflect the I/O pin numbers, or the analog channel numbers.
If a file called ‘index.html’ is loaded into the root directory of the MicroPython filesystem, it will be displayed in the browser by default, when no filename is entered in the browser address bar. A minimal index page might look like:
So far, I have only presented very simple Web pages; in the next post I’ll show how to fetch high-speed analog samples using DMA, then combine these with a more sophisticated AJAX functionality to create a Web-based oscilloscope.
Copyright (c) Jeremy P Bentham 2021. Please credit this blog if you use the information or software in it.
In part 1, we got as far as connecting an ATWINC1500 or 1510 module to a WiFi network; now it is time to do something vaguely useful with it.
Sockets
A network interface is frequently referred to as a ‘socket’ interface, so first I’d better define what that is.
A socket is a logical endpoint for network communication, and consists of an IP address, and a port number. The IP address is often assigned automatically at boot-time, using Dynamic Host Configuration Protocol (DHCP) as in part 1, but can also be pre-programmed into the unit (a ‘static’ address).
The 16-bit port number further subdivides the functionality within that IP address, so one address can support multiple simultaneous conversations (‘connections’). Furthermore, specific port numbers below 1024 are generally associated with specific functions; for example, an un-encrypted Web server normally uses port 80, whilst an encrypted server is on port 443. Port numbers 1024 and above are generally for user programs.
Clients and servers, UDP and TCP
A sever runs all the time, waiting for a client to contact it; the client is responsible for initiating the contact and providing some data, probably in the form of a request. The server returns an appropriate response, then either side may terminate the connection; the client may end it because it has received enough data, or the server because there are limits on the maximum number of simultaneous clients it can service.
There are 2 fundamental communication methods in TCP/IP: User Datagram Protocol (UDP) and Transmission Control Protocol (TCP).
UDP is the simpler of the two, and involves sending a block of data to a given socket (port and IP address) with no guarantee that it will arrive. TCP involves sending a stream of data to a socket; it includes sophisticated retry mechanisms to ensure that the data arrives.
There are those in the networking community who shun UDP, because they think the unreliability makes is useless; I disagree, and think there are various use-cases where the simple block-based transfer is perfectly adequate, possibly overlaid with a simple retry mechanism, so we’ll start with a simple UDP server.
UDP server
The simplest UDP server is stateless, i.e. it doesn’t store any information about the client; it just responds to any request it receives. This means that a single socket can handle multiple clients, unlike TCP which requires a unique socket for each client it is communicating with.
For a classic C socket interface, the steps would be:
Create a datagram socket using socket()
Bind to the socket to a specific port using bind()
When a message is received on that port, get the data, return address and port number using recvfrom()
Send response data to the remote address and port number using sendto()
Go to step 3
The code driving the ATWINC1500 module does the same job, but the function calls are a bit different, as they reflect the messages sent to & received from the WiFi module:
Initialise a socket structure for UDP
Send a BIND command to the module, with the port number
Receive a BIND response
Send a RECVFROM command to the module
Wait until a RECVFROM response is received, get the data, return address & port number
Send a SENDTO command to the module with the response data, return address & port number
Go to step 4
Note that there may be a very long wait between steps 4 and 5, if there are no clients contacting the server. Fortunately the module will signal the arrival of a message by asserting the interrupt request (IRQ) line, so the RP2040 CPU can proceed with other tasks while waiting.
UDP Reception
There are 4 steps when the module receives a packet (‘datagram’) from a remote client:
Get the group ID and operation. This identifies the message type; for UDP it will generally be a response to a RECVFROM request, but it could be something completely different. My software combines the group ID and operation into a single 16-bit number.
Get the operation-specific header. This is generally 16 bytes or less, and in the case of RECVFROM, gives the IP address and port number of the sender, also a pointer & offset to the user data in the buffer.
Get the user data. The application doesn’t need to fetch all the incoming data; for example, in the case of a Web server, it might just get the first line of the page request, and discard all the other information.
Handle socket errors. If there is an error, the data length-value will be negative, and the code must take appropriate action, such as closing and re-opening the server socket. Since a UDP socket is connectionless, it generally won’t see many errors, but a TCP socket will flag an error every time a client closes an active connection.
Having fetched these two blocks, control is passed to a state machine that takes appropriate action. If we’ve just received an indication that DHCP has succeeded, then we bind the server sockets.
if (gop==GOP_DHCP_CONF)
{
for (sock=MIN_SOCKET; sock<MAX_SOCKETS; sock++)
{
sp = &sockets[sock];
if (sp->state==STATE_BINDING)
put_sock_bind(fd, sock, sp->localport);
}
}
When we get a message indicating the binding has succeeded, then if it is a TCP socket, we need to send a LISTEN command. If UDP, we can just send a RECVFROM, and wait for data to arrive. We can tell whether the socket is TCP or UDP by looking at the socket number; the lower numbers are TCP, and higher are UDP.
else if (gop==GOP_BIND && (sock=rmp->bind.sock)<MAX_SOCKETS &&
sockets[sock].state==STATE_BINDING)
{
sock_state(sock, STATE_BOUND);
if (sock < MIN_UDP_SOCK)
put_sock_listen(fd, sock);
else
put_sock_recvfrom(fd, sock);
}
If a UDP server, we may now get a RECVFROM response, indicating that a packet (‘datagram’) has arrived. If so, we save the return socket address (IP and port number), call a handler function, then send another RECVFROM request.
else if (gop==GOP_RECVFROM && (sock=rmp->recv.sock)<MAX_SOCKETS &&
(sp=&sockets[sock])->state==STATE_BOUND)
{
memcpy(&sp->addr, &rmp->recv.addr, sizeof(SOCK_ADDR));
if (sp->handler)
sp->handler(fd, sock, rmp->recv.dlen);
put_sock_recvfrom(fd, sock);
}
A very simple handler just echoes back the incoming data:
uint8_t databuff[MAX_DATALEN];
// Handler for UDP echo
void udp_echo_handler(int fd, uint8_t sock, int rxlen)
{
if (rxlen>0 && get_sock_data(fd, sock, databuff, rxlen))
put_sock_sendto(fd, sock, databuff, rxlen);
}
UDP client for testing
For testing, I use a simple UDP client written in Python, that can run on a Raspberry Pi, or any PC running Linux or Windows. It sends a message every second, and checks for a response. You’ll need to change the IP address to match the DCHP value given by the module.
# Simple Python client for testing UDP server
import socket, time
ADDR = "10.1.1.11"
PORT = 1025
MESSAGE = b"Test %u"
DELAY = 1
def hex_str(bytes):
return " ".join([("%02x" % int(b)) for b in bytes])
print("Send to UDP %s:%s" % (ADDR, PORT))
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.settimeout(0.2)
count = 1
while True:
msg = MESSAGE % count
sock.sendto(msg, (ADDR, PORT))
print("Tx %u: %s" % (len(msg), hex_str(msg)))
count += 1
try:
data = sock.recvfrom(1000)
except:
data = None
if data:
bytes, addr = data
s = hex_str(bytes)
print("Rx %u: %s\n" % (len(bytes), s))
time.sleep(DELAY)
TCP server
A TCP connection is more complex than UDP, since the module firmware must keep track of the data that is sent & received, in order to correct any errors. The steps for a classic C socket interface would be:
Create a ‘stream’ socket using socket()
Bind to the socket to a specific port using bind()
Set the socket to wait for incoming connections using listen()
When a connection request is received on the main socket, open a new socket for the data using accept()
When data arrives on the new socket, get it using recv()
Send response data using send()
If socket error or transfer complete, close socket. Otherwise go to step 5
The corresponding operations for the WiFi module are:
Initialise a socket structure for TCP
Send a BIND command to the module, with the port number
Receive a BIND response
Send a LISTEN command
Receive a LISTEN response
Receive an ACCEPT notification when a connection request arrives on the main socket. Save the new socket number.
Send a RECV command on the new socket.
Receive a RECV response when data arrives on the new socket
Send response data using SEND
Go to step 7, or close the new socket
TCP reception
The first step (binding a socket to a port number) is the same as for UDP, but then we send a LISTEN command, which activates the socket to receive incoming connections. When a client connects, we get an ACCEPT response containing 2 socket numbers; the first is the one that we used for the original BIND command, and the second is a new socket that will be used for the data transfer; we need to issue a RECV on this socket to get the user data.
When data is available, the RECV command will return, and we can call a data handler function, then send another RECV for more data. Alternatively, if the data length is negative, then there is an error, and the socket needs to be closed. This isn’t necessarily as bad as it sounds; the most common reason is that the client has closed the connection, and we just need to erase the 2nd socket for future use.
else if (gop==GOP_RECV && (sock=rmp->recv.sock)<MAX_SOCKETS &&
(sp=&sockets[sock])->state==STATE_CONNECTED)
{
if (sp->handler)
sp->handler(fd, sock, rmp->recv.dlen);
if (rmp->recv.dlen > 0)
put_sock_recv(fd, sock);
}
The TCP data handler is again a simple echo-back of the incoming data, but with an added complication: if the data length is negative, there has been an error. This isn’t as bad as it sounds; the most common error is that the client has closed the TCP connection, so the server must also close its data socket, to allow it to be re-used for a new connection.
// Handler for TCP echo
void tcp_echo_handler(int fd, uint8_t sock, int rxlen)
{
if (rxlen < 0)
put_sock_close(fd, sock);
else if (rxlen>0 && get_sock_data(fd, sock, databuff, rxlen))
put_sock_send(fd, sock, databuff, rxlen);
}
TCP client for testing
This is similar to the UDP client; it can run on a Raspberry Pi or PC, running Linux or Windows:
# Simple Python client for testing TCP server
import socket, time
ADDR = "10.1.1.11"
PORT = 1025
MESSAGE = b"Test %u"
DELAY = 1
def hex_str(bytes):
return " ".join([("%02x" % int(b)) for b in bytes])
print("Send to TCP %s:%s" % (ADDR, PORT))
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.settimeout(0.5)
sock.connect((ADDR, PORT))
count = 1
while True:
msg = MESSAGE % count
sock.sendall(msg)
print("Tx %u: %s" % (len(msg), hex_str(msg)))
count += 1
try:
data = sock.recv(1000)
except:
data = None
if data:
s = hex_str(data)
print("Rx %u: %s\n" % (len(data), s))
time.sleep(DELAY)
# EOF
Source files
The C source files are in the ‘part2’ directory on Github here
The default network name and passphrase are “testnet” and “testpass”; these must be changed to match your network, then the code will need to be rebuilt & run using the standard Pico devlopment environment.
The default TCP & UDP port numbers are 1025, and the Python programs I’ve provided can be used to perform simple simple echo tests, providing the IP address is modified to match that given when the Pico joins the network.
The Raspberry Pi Pico is an incredibly useful low-cost micro-controller module based on the RP2040 CPU, but at the time of writing, there is a major omission: there is no networking capability.
This project adds low-cost wireless networking to the Pi Pico, and any other RP2040 boards. The There are various modules on the market that could be used for this purpose; I have chosen the Microchip ATWINC1500 or 1510 modules as they low-cost, have an easy hardware interface (4-wire SPI), and feature a built-in TCP/IP software stack, which significantly reduces the amount of software needed on the RP2040.
The photo above shows the module mounted on an Adafruit breakout board, and the module itself; this is the variant with a built-in antenna, but there is also a version with an antenna connector, that allows an external antenna to be used.
The only difference between the ATWINC1500 and 1510 modules is that the latter have larger flash memory size (1 MB, as opposed to 0.5 MB). There is also an earlier series of low-level interface modules named ATWILC; I’m not using them, as the built-in TCP/IP software of the ATWINC saves a lot of code complication on the RP2040.
Hardware connections
Pi Pico and WiFi module
For simplicity, I have used the Adafruit breakout board, but it is possible to directly connect the module to the Pico, powered from its 3.3V supply.
Wiring Pico to Adafruit WINC1500 breakout
Pi Pico pins
SCK 18 SPI clock
MOSI 19 SPI data out
MISO 16 SPI data in
CS 17 SPI chip select
WAKE 20 Module wake
EN 20 Module enable
RESET 21 Module reset
IRQ 22 Module interrupt request
No extra components are needed, if the wiring to the module is kept short, i.e. 3 inches (76 mm).
SPI on the RP2040
Initialising the SPI interface on the RP2040 just involves a list of API function calls:
When using the standard SPI transfer API function, I found that occasionally the last data bit wasn’t being received correctly. The reason was that the API function returns before the transfer is complete; the clock signal is still high, and needs to go low to finish the transaction. To fix this, I inserted a loop that waits for the clock to go low, before negating the chip-select line.
// Do SPI transfer
int spi_xfer(int fd, uint8_t *txd, uint8_t *rxd, int len)
{
gpio_put(CS_PIN, 0);
spi_write_read_blocking(SPI_PORT, txd, rxd, len);
while (gpio_get(SCK_PIN)) ;
gpio_put(CS_PIN, 1);
}
Interface method
The WiFi module has its own processor, running proprietary code; it is supplied with a suitable binary image already installed, so will start running as soon as the module is enabled.
Pico WINC1500 block diagram
The module has a Host Interface (HIF) that the Pico uses for all communications; it is a Serial Peripheral Interface (SPI) that consists of a clock signal, incoming & outgoing data lines (MOSI and MISO), and a Chip Select, also known as a Chip Enable. The Pico initiates and controls all the HIF transfers, but the module can request a transfer by asserting an Interrupt Request (IRQ) line.
The module is powered up by asserting the ‘enable’ line, then briefly pulsing the reset line. This ensures that there is a clean startup, without any complications caused by previous settings.
There are 2 basic methods to transfer data between the PICO and the module; simple 32-bit configuration values can be transferred as register read/write cycles; there is a specific format for these, which includes an acknowledgement that a write cycle has succeeded. The following logic analyser trace shows a 32-bit value of 0x51 being read from register 0x1070; the output from the CPU is MOSI, and the input from the module is MISO.
ATWINC1500 register read cycle
Now the corresponding write cycle, where the CPU is writing back a value of 0x51 to the same 32-bit register.
ATWINC1500 register write cycle
There are a few unusual features about these transfers.
The chip-select (CS) line doesn’t have to be continuously asserted during the transfer, it need only be asserted whilst a byte is actually being read or written.
The command value is CA hex for a read cycle, and C9 for a write.
The module echoes back the command value plus 2 bytes for a read (CA 00 F3), or plus 1 byte for a write (C9 00), to indicate it has been accepted.
The register address is 24-bit, big-endian (most significant byte first)
The data value is 32-bit, little-endian in the read cycle (51 00 00 00), and big-endian in the write cycle (00 00 00 50).
The last point is quite remarkable, and when starting on the code development, I had great difficulty believing it could be true. The likely reason is that the SPI transfer is is big-endian as defined in the Secure Digital (SD) card specification, but the CPU in the module is little-endian. So the firmware has to either do a byte-swap on every response message, or return everything using the native byte-order, with this result.
In addition to reading & writing single-word registers, the software must read & write blocks of data. This involves some negotiation with the module firmware, since that manages the allocation & freeing of the necessary storage space in the module. For example, the procedure for a block write is:
Request a buffer of the required size
Receive the address of the buffer from the module
Write one or more data blocks to the buffer
Signal that the transfer is complete
Reading is similar, except that the first step isn’t needed, as the buffer is already available with the required data.
Operations
The above transfer mechanism is used to send commands to the module, and receive responses back from it; there is generally a one-to-one correspondence between the command and response, but there may be a significant delay between the two. For example, the ‘receive’ command requests a data block that has been received over the network, but if there is none, there will be no response, and the command will remain active until something does arrive.
The commands are generally referred to as ‘operations’, and they are split into groups:
Main
Wireless (WiFi)
Internet Protocol (IP)
Host Interface (HIF)
Over The Air update (OTA)
Secure Socket Layer (SSL)
Cryptography (Crypto)
Each operation is assigned a number, and there is some re-use of numbers within different groups, for example a value of 70 in the WiFi group is used to enable Acess Point (AP) mode, but the same value in the IP group is a socket receive command. To avoid this possible source of confusion, my code combines the group and operation into a single 16-bit value, e.g.
To invoke an operation on the module, you must first send a 4-byte header that gives an 8-bit operation number, 8-bit group, and 16-bit message length.
The next 4 bytes of the message are unused, so can either be sent as zeros, or just skipped. Then there is the command header, which varies depending on the operation being performed, but are often 16 bytes or less, for example the IP ‘bind’ command:
I’ll be discussing the IP operations in detail in the next part.
The interrupt request (IRQ) line is pulled low by the module to indicate that a response is available; for simplicity, my code polls this line, and calls an interrupt handler.
if (read_irq() == 0)
interrupt_handler();
Joining a network
I’ll start with the most common use-case; joining a network that uses WiFi Protected Access (WPA or WPA2), and obtaining an IP address using Dynamic Host Configuration Protocol (DHCP). This is remarkably painless, since the module firmware does all of the hard work, but first we have to tackle the issue of firmware versions.
As previously explained, the module comes pre-loaded with firmware; at the time of writing, this is generally version 19.5.2 or 19.6.1. There is a provision for re-flashing the firmware to the latest version, but for the time being I’d like to avoid that complication, so the code I’ve written is compatible with both versions.
The reason that this matters is that 19.6.1 introduced a new method for joining a network, with a new operation number (59, as opposed to 40). Fortunately the newer software can still handle the older method, so that is what I’ll be using by default, though there is a compile-time option to use the new one, if you’re sure the module has the newer firmware.
The code to join the network is remarkably brief, just involving some data preparation, then calling a host interface transfer function to send the data. It searches across all channels to find a signal that matches the given Service Set Identifier (SSID, or network name). A password string (WPA passphrase) is also given; if this is a null value, the module will attempt to join an ‘open’ (insecure) network, but there are very obvious security risks with this, so it is not recommended.
There are 3 source files in the ‘part1’ directory on Github here:
winc_pico_part1.c: main program, with RP2040-specific code
winc_wifi.c: module interface
winc_wifi.h: module interface definitions
The default network name and passphrase are “testnet” and “testpass”; these will have to be changed to match your network.
Normally I’d provide a simple Pi command-line to compile & run the files, but this is considerably more complex on the Pico; you’ll have to refer to the official documentation for setting up the development tools. I’ve provided a simple cmakelists file, that may need to be altered to suit your environment.
There is a compile-time ‘verbose’ setting, which regulates the amount of diagnostic information that is displayed on the console (serial link). Level 1 shows the following:
Firmware 19.5.2, OTP MAC address F8:F0:05:xx.xx.xx
Connecting...........
Interrupt gid 1 op 44 len 12 State change connected
Interrupt gid 1 op 50 len 28 DHCP conf 10.1.1.11 gate 10.1.1.101
[or if the network can't be found]
Interrupt gid 1 op 44 len 12 State change fail
Verbose level 2 lists all the register settings as well, e.g.
It has been a long & complicated journey to get to this point, and you might be thinking that it’ll take a lot more effort to join a network, particularly if it involves WPA security.
However, this isn’t true; because we’re dealing with an intelligent network interface, all the complexities of WPA are handled by the on-chip firmware, so it only takes a few simple IOCTL commands to join a secure network.
For the purposes of this blog, I’m assuming there is an existing infrastructure network with an Access Point (AP) using a Service Set Identifier (SSID) of ‘testnet’, and a WPA or WPA2 password of ‘testpass’ – this is purely for test purposes, and must be changed.
IOCTL calls
IOCTLs were described in detail in the previous part, but to recap: there are over 300 possible calls, covering all aspects of the configuration and monitoring of the network interface. Some calls can take a significant amount of time to process, so the host CPU has to poll the SDIO interface, checking for a response.
In the previous part, I put a ‘while’ loop round some of the calls to account for this delay, but this was getting rather messy, so I’ve expanded the IOCTL calls to include a millisecond retry time; if this is non-zero, the code will keep polling for a response until the time is exceeded.
An example of this is the way the country is set:
if (!ioctl_set_data("country", 100, &country_struct, sizeof(country_struct)))
printf("Can't set country\n");
After sending the command, the code waits for up to 100 milliseconds for a response, returning its length; zero if there was an error or no response.
There isn’t a standard data format for all the IOCTLS, so it is very easy to have a mismatch between the command & data; ideally we’d attach a diagnostic function to each call, to report any errors. To do this, I’ve created a simple macro to invoke the IOCTL function, with token-pasting to report the function name and first argument in the event of an error. For example, the code to set the pre-shared key is:
If the data is incorrect, for example the key string is too short, an error will be reported on the console at run-time:
Error: ioctl_wr_data(WLC_SET_WSEC_PMK ...)
I’ve found this really helpful in tracking down IOCTL programming errors and timing issues.
Joining an insecure network
We’ll start with a simple test network that has security disabled; at the risk of stating the obvious, this mode presents a major security risk, so should be used with great caution.
All we need to set is a two-letter country, and an SSID (network name), which are used to populate the IOCTL structures, e.g.
#define SSID "testnet"
#define COUNTRY "GB"
#define COUNTRY_REV -1
#define SECURITY 0 // Zero to disable security
wlc_ssid_t ssid={sizeof(SSID)-1, SSID};
wl_country_t country_struct = {.ccode=COUNTRY, .country_abbrev=COUNTRY, .rev=COUNTRY_REV};
The country defines which radio channels may be used. A few countries have multiple revisions of their channel settings; to use the default revision, a value of -1 is entered.
Joining the network just involves setting ‘infrastructure’ mode, disabling the various forms of security, then sending a ‘SET_SSID’ IOCTL command to join the network:
The joining process takes a few seconds, then success or failure is signalled by asynchronous events, hence the ‘enable_evts’ line – this specifies which events we’d like to receive when the device is connecting, and after it has connected. The event messages are described below.
Joining a secure network
There are IOCTL commands covering a wide range of security options; I’ve tested WPA-TKIP and WPA2 in pre-shared key (PSK) mode as they are in widespread use.
The settings aren’t complex; the hard work is done by the chip firmware. I’ve used a compile-time definition to select the security mode:
Some of the IOCTL calls require two 32-bit integer arguments, the first being an interface number that is zero in this implementation. The function ‘set_intx2’ was created to simplify the handling of the 2 arguments:
// Set 2 integers in IOCTL variable
int ioctl_set_intx2(char *name, int wait_msec, int val1, int val2)
{
int data[2] = {val1, val2};
return(ioctl_cmd(WLC_SET_VAR, name, wait_msec, 1, data, 8));
}
Response when joining network
The status reports are in the form of asynchronous event messages; prior to joining, we did an ioctl_enable_events() call to specify which events we’re interested in receiving. This call also stores a string value for each enabled event, to simplify decoding.
The messages are obtained by polling the SDIO interface with a command 53. If one is available, the first 2 bytes are a (little-endian) length, the next 2 are the bitwise inverse of that length; otherwise the returned values are zero if there is no message.
My code first loads and decodes the 12-byte event header, then the remainder of the message. For example, this is the response if the Access Point (AP) doesn’t support the requested security scheme:
The total length (including the 12-byte header) is 67 hex, 103 decimal; the bitwise inverse of this is 98FF hex. Some care is needed when doing the decoding; the first part of the message is in the usual little-endian byte order, while the later parts are in network (big-endian) format. The most important part of the decode is the last line, giving the name of the event (0, SET_SSID) and the status code (1, FAIL).
It would be nice if all the responses were as easy to understand as this, but they can be a bit misleading. This is the response when the password is incorrect:
The event fields haven’t been decoded because this isn’t really an event; it is a network frame. The all-1s destination address shows that it is a broadcast transmission; it is quite usual for a network to carry lots of broadcasts, that have to be decoded by all the attached systems, so the wireless interface is dutifully passing them on to our software for processing.
These network frames are a handy way of showing that our wireless interface is connected to a real network, but to understand them we’ll need to tackle the TCP/IP protocols that underpin all our network communications; that’ll be in the next part.
If you want to run the code so far, see the end of the previous part for instructions; there is a batch file ‘make_join.bat’ for Windows, and ‘make_join’ for Linux.
Interrupts
Thinking ahead to the TCP/IP stack, we’ll need a quick way of detecting when network data (a network ‘frame’) has arrived; continually sending out 12-byte SDIO data requests, in the hope of receiving something back, is really inefficient.
If you search the CYW43438 data sheet for the word ‘interrupt’, you’ll see that SDIO data line 1 also acts as an interrupt line – there is no separate pin for that function. This works because generally the 4 data lines are just floating (pulled high by resistors); they’re only driven high or low during block transfers. So while they are idle, the wireless chip can pull one of them (DATA1) low to request a data transfer; as soon as the transfer starts, the line reverts to carrying data bits.
It is quite easy to set this up, just write to the Interrupt Enable register:
The value of 7 includes a master interrupt enable, plus enabling functions 1 & 2. [I’m not clear on what these 2 functions are: the network traffic seems to use function 1 only].
Handling the interrupt should just be a case of checking the I/O pin, then doing the necessary read cycle:
if (gpio_in(SD_D1_PIN) == 0)
{
while ((n=ioctl_get_event(&ieh, eventbuff, sizeof(eventbuff))) > 0)
..process the network data..
}
Unfortunately this approach doesn’t work very well; the interrupt line has a tendency to remain low, causing lots of unnecessary attempts to get non-existent data. The reason is that the interrupt needs to be explicitly acknowledged, so that the I/O pin goes high again. The Broadcom driver handles this situation by disabling asynchronous messaging; when the interrupt pin is asserted, it does read & write cycles to the interrupt status register, to check for the interrupt, and acknowledge it. Here is my interpretation of this method, which seems to work well:
// After security is set up, enable async events
ioctl_enable_evts(join_evts);
// Start joining a network
CHECK(ioctl_wr_data, WLC_SET_SSID, 100, &ssid, sizeof(ssid));
// Enable interrupts
sdio_cmd52_writes(SD_FUNC_BUS, BUS_INTEN_REG, 0x07, 1);
// ..wait until the events show we have joined the network, then..
// Disable events
ioctl_enable_evts(no_evts);
// Fetch a network frame
if (gpio_in(SD_D1_PIN) == 0)
{
sdio_bak_read32(SB_INT_STATUS_REG, &val);
if (val & 0xff)
{
sdio_bak_write32(SB_INT_STATUS_REG, val);
if ((n=ioctl_get_event(&ieh, eventbuff, sizeof(eventbuff))) > 0)
...process the network data...
}
}
In the early stages of code development, I tend to use polling in place of interrupts, as it makes debugging much easier. When the code is debugged, I’ll switch to using real CPU interrupts: any I/O pin can be configured to trigger an interrupt, so the change shouldn’t be difficult.
It has been a long haul, but we are now getting close to doing something useful with the WiFi chip; we just need to tackle the issue of IOCTLs.
You may already be familiar with these from configuring a serial link, or network hardware; they provide a programming interface into a vendor-specific driver. Since the BCM/CTW43xxx chips are intelligent (they have their own CPU) the IOCTL calls are handled directly by the firmware we’ve programmed into the chip. So even though we’re in ‘bare metal’ mode, without an operating system, we still need to handle IOCTLs.
The IOCTL calls are listed in the wwd_wlioctl.h in WICED or WiFi Host Driver, or wlioctl_defs.h, and there are over 300 of them; this post will concentrate on the code that sends IOCTL requests and handles the responses, and we’ll check they are working by doing a quick network scan – more interesting things, like transmission & reception, will have to wait for the next part.
Message structure
When using IOCTL calls, you are essentially writing a data packet to the WiFi RAM, waiting for an acknowledgement, then reading back the response. As you’d expect, there is a specific data format for the requests and responses, though it does have some strange features:
The best feature of the IOCTL data is that it always starts with a 16-bit length word, followed by the bitwise inverse of that length (least-significant byte first). For example, here is the decode of a request to set a variable ‘bus:rxglom’ to a value of 1:
You can check this is an IOCTL message by adding the first two bytes to the second two: 002B + FFD4 = FFFF. It uses a command 53 to send a 44-byte request (actually 43 bytes, rounded up to nearest 4-byte value) to the RAD function, containing a header of mostly zeros with an IOCTL number of 107 hex (263 decimal) to set a variable, a null-terminated variable name, then the binary value.
It is then necessary to poll the WiFi chip to check when the response is available, and if so, acknowledge it:
19.291055 * Cmd 53 15404004 Rd BAK 180000:A020 len 4
Data 4 bytes: 40 00 80 00 *
19.291081 * Rsp 53 00001000 Flags 10
19.291179 * Cmd 53 95404004 Wr BAK 180000:A020 len 4
19.291205 * Rsp 53 00001000 Flags 10
Data 4 bytes: 40 00 00 00 *
19.291259 Ack 28 3F
The value of 40 hex in backplane register 2020 (A020 for a 32-bit value) shows there is a response, which is acknowledged by writing 40 hex to that register, then the response is read:
The response is the same length as the request, and when writing to a variable it is largely a copy of the command. When receiving the response, it is important to check that it matches the request, as the two can easily get out of step. Unfortunately the sequence number can’t be used for this purpose (in this example the response is 2, and request is 0) instead the most-significant 16 bits of the ‘flags’ are a ‘request ID’ that should be the same for request & response, while the lower 16 bits are set to 2 for a write cycle, 0 for a read.
Equally strange is that the command length always seems to be the same as the response, so for a short command with a long response (such as ‘ver’) the command is 296 bytes long, just to carry a 3-character name. This is a bit crazy; sometime I’ll experiment with the header fields to see if there is a way round it.
Glom
I’ll admit this word wasn’t in my vocabulary until I encountered it in the WiFi drivers, and I’m still not entirely clear what it means. The transaction above sets ‘rxglom’ to 1, which enables ‘glom’ mode for incoming commands (‘rx’ refers to the WiFi chip command reception, not the host).
After this is set, another header is introduced into commands sent to the WiFi chip; I have accommodated this using another structure, and a union to cover both.
The good news is that the first 4 bytes of any message remain the same (16-bit length, and its bitwise inverse); the bad news is that the new header is shoehorned in after that, pushing the other headers out by 8 bytes.
Here is an example: getting ‘cur_ethaddr’ which is the 6-byte MAC address:
I’m sure there must be some point to the extended header, but right now I’m not at all sure what it is. There doesn’t seem to be any official marker in the glom header to show that it has been included, which makes life difficult for any software attempting to decode the IOCTLs. For the time being, I’m hedging my bets by using a global variable to enable or disable this option, and leaving it disabled; hopefully its true purpose will be clear soon.
Partial data read
If the IOCTL command has a long response, and the software doesn’t read it all, the remainder will still be available for the next read. This can be demonstrated by the version (‘ver’) command; even though it is sent as a single 296-byte block, the Linux driver receives it as one block of 64 bytes, then another of 224:
This serves to emphasise the important of reading all the data from every response, and checking that the Request ID matches that of the response; it’d be all to easy for the network driver to lose track.
Events
So far, we’ve dealt had a strict one-to-one matching between request and response, but how does the WiFi chip indicate when it has extra data available? For example, a single network scan may generate 10 or 20 data blocks (one for every access point), how does the host know when this data is available? There is mention of an interrupt pin (which we’ll save for a future blog) but how can the driver software check for data pending?
I puzzled over this for some time, on the assumption there must be a special register to indicate this, but in the end it seems that the driver just issues a normal data read; if data is available it can be recognised by the length header, if not zeros are returned.
The WiFi chip has a finite amount of buffer space to queue up such events; this is the ‘credit’ value in the IOCTL header; presumably the network driver should check this to see if events have been lost due to running out of buffers.
Network scan
Finally, we get to do something vaguely useful; scan for WiFi networks. There are 2 types: ‘iscan’ and ‘escan’. The first is an incremental scan, that seems easier to use, but is marked as ‘deprecated’ in some source code. The second is supposed to be more versatile (i.e. more complicated) but is the preferred option, so that is what we’ll be using.
We need to fill in a structure with the scan parameters; due to the large number of networks in the vicinity, I usually scan a single channel:
The scan is triggered by sending this data in an ‘escan’ IOCTL call, but first we must tell the chip that we’re interested in the response events. This is done by sending a very large bitfield, with a bit set for each event you want to receive; there are over 140 possible events, so you need to pick the right one. I got the list from whd_events_int.h which is part of the Cypress WiFi Host Driver project; if you don’t know what that is, please refer to part 1 of this blog, which describes all the resources I’m using.
Surprisingly easy, until we get back the results of the scan, which has one varying-length record for every WiFi access point found. There is a lot of data, around 300 to 600 bytes per record, so we need to do some heavyweight decoding.
Decoding the scan data
So far, I’ve avoided including any of the standard Cypress / Broadcom header files in my project. This is because any one header file often depends on another 2, which then depends on another 5, and so on… Quite rapidly, you’re including a large chunk of the Operating System which isn’t at all necessary; it just makes the decoding process much harder to follow.
Fortunately for this project, there is a way to avoid these major OS dependencies; use header files that were created for use in embedded systems, namely the Cypress WiFi Host Driver described in part 1 of this blog. Here are the structures that are needed for decoding the scan response data, and the files they’re in:
So only 6 extra files need to be included at this stage, and we’ve avoided the unnecessary complexity of an Operating System interface – after all, this driver is supposed to be bare-metal code.
The code to print the MAC address, channel number and SSID (network name) is:
The scan result data fields are in network-standard byte-order (big endian) so the channel number needs to be byte-swapped.
Running the code
If you want to try out the code so far, you’ll need a Pi ZeroW with a USB-serial cable attached, the arm-none-eabi-gcc compiler and gdb debugger. You can find full details and a simple test program here; it is worth running this before attempting the Zerowi project.
The source code is at https://github.com/jbentham/zerowi, ‘make_scan.bat’ will create zerowi.elf on windows, which is downloaded into the target using the ‘run’ batch file. This executes alpha_speedup.py to accelerate the serial link from 115200 to 921600 baud, then runs Arm gdb using the setup commands in run.gdb.
I have provided Linux scripts ‘make_scan’ and ‘run’, these need to be made executable using ‘chmod +x’. The Alpha debugger does require arm-none-eabi-gdb, which isn’t included in many Linux distributions (including Raspbian Buster) so may need to be built from source.
My Windows system uses serial port COM7, and Linux uses /dev/ttyUSB0; yours may well be different, so you’ll need to change scripts accordingly. If the Cypress firmware is included in the build image (i.e. ‘INCLUDE_FIRMWARE’ is non-zero) then it will take around 10 seconds to load the executable image onto the ZeroW. When the code runs you should see a list of access points; to keep the number of entries low, I only scan a single channel, by default channel 1:
In the previous post we sent some commands to the WiFi chip, and got a response. To make the chip do anything useful, we need to program its internal CPU, as it doesn’t have code in ROM.
It does have configuration tables in ROM, that indicate what resources it possesses, and their locations, so the chip variants can all be programmed by a single driver. However, parsing these tables isn’t easy; the simplest code I’ve found is in the the Plan9 driver (see part 1 of this blog for details), and that is moderately impenetrable; here is the parser output for the ZeroW (values in hex):
I think these are the Intellectual Property (IP) cores within the chip, and the locations they occupy in the memory map, but in the absence of documentation, a lot of guesswork is required. So I decided to ignore the configuration tables, and just use the same addresses as the Linux driver, after it has done the decode. This makes my driver a lot less flexible, as the addresses will have to be changed for each new chip, but there aren’t many of them, so only a handful of definitions will need changing.
The most important number is the chip ID; it should be A9A6 hex, so it’d be a good idea to check our chip matches that. In part 3 my code did some preliminary SDIO initialisation, now to follow on from that:
// SD function numbers
#define SD_FUNC_BUS 0
#define SD_FUNC_BAK 1
#define SD_FUNC_RAD 2
// Maximum block sizes
#define SD_BAK_BLK_BYTES 64
#define SD_RAD_BLK_BYTES 512
// [0.243831] Set bus interface
sdio_cmd52_writes(SD_FUNC_BUS, BUS_SPEED_CTRL_REG, 0x03, 1);
sdio_cmd52_writes(SD_FUNC_BUS, BUS_BI_CTRL_REG, 0x42, 1);
// [17.999101] Set block sizes
sdio_cmd52_writes(SD_FUNC_BUS, BUS_BAK_BLKSIZE_REG, SD_BAK_BLK_BYTES, 2);
sdio_cmd52_writes(SD_FUNC_BUS, BUS_RAD_BLKSIZE_REG, SD_RAD_BLK_BYTES, 2);
The SD function numbers allow Command 52 & 53 to access 3 different interfaces within the chip: think ‘hardware functions’ rather than ‘software functions’. The SDIO bus interface is configured using the ‘bus’ function, and is set into high-speed mode (as discussed in part 2). Then the block sizes for the backplane (‘BAK’) and radio (‘RAD’) functions are set; these are limited to 64 & 512 bytes by the hardware. These will be used by command 53 when operating in multi-block mode.
We now use the ‘bus’ function to enable the ‘backplane’ interface; by default, the IP cores in the chip are switched off to conserve power, and they need to be enabled; the second line of code checks that the core has actually powered up (I/O enabled -> I/O ready). Once the backplane function is enabled, we set a window pointing to the common base address (‘chipcommon’ in the Plan9 driver) then do a read, and we get hex values A6 A9 41 15, which is correct. However, some explanation is needed with regard to the backplane window.
Backplane window
You may recall that the commands we’re using here, CMD52 and CMD53, only have a 17-bit address range, yet the chip uses 32-bit addresses internally. The way this is handled is by writing a 24-bit value to 3 of the backplane registers, to act as an offset within the internal space.
// Backplane window
#define SB_32BIT_WIN 0x8000
#define SB_ADDR_MASK 0x7fff
#define SB_WIN_MASK (~SB_ADDR_MASK)
// Set backplane window, don't set if already OK
void sdio_bak_window(uint32_t addr)
{
static uint32_t lastaddr=0;
addr &= SB_WIN_MASK;
if (addr != lastaddr)
sdio_cmd52_writes(SD_FUNC_BAK, BAK_WIN_ADDR_REG, addr>>8, 3);
lastaddr = addr;
}
// Do 1 - 4 CMD52 writes to successive addresses
int sdio_cmd52_writes(int func, int addr, uint32_t data, int nbytes)
{
int n=0;
while (nbytes--)
{
n += sdio_cmd52(func, addr++, (uint8_t)data, SD_WR, 0, 0);
data >>= 8;
}
return(n);
}
It is important to realise that this is a simple windowing scheme where the bottom 15 bits are provided by the offset, and the top 17 bits by the window: the two values aren’t added together. An additional complication (yes, really) is that there are 2 copies of the lower 15-byte address space; 0 – 7fff hex is for byte accesses, and 8000 – ffff hex is for 32-bit word accesses (offset SB_32BIT_WIN).
To give a concrete example, here is the analysis of the RPi driver fetching the CPU ID:
18.001455 * Cmd 52 92001400 Wr BAK 1000A 00
18.001481 * Rsp 52 00001000 Flags 10 data 00
18.001618 * Cmd 52 92001600 Wr BAK 1000B 00
18.001644 * Rsp 52 00001000 Flags 10 data 00
18.001750 * Cmd 52 92001818 Wr BAK 1000C 18 Bak Win 180000
18.001777 * Rsp 52 00001018 Flags 10 data 18
18.001905 * Cmd 53 15000004 Rd BAK 180000:8000 len 4
Data 4 bytes: a6 a9 41 15 *
You can see the 3 CMD52 write cycles to set the window address, then the 4-byte read cycle, with the offset into the 32-bit area. The ‘win 180000’ and ‘180000:8000’ labels are my analysis code trying to be helpful, by saving the window value, and repeating it at the subsequent read cycle.
Firmware file
There are various firmware versions that could be used (see Cypress WICED) but I’m using the same version as the RPi driver, available here. It is around 300K bytes; eventually, it’ll be stored in the SD card filesystem, but for the time being I wanted a simpler storage mechanism, so attached an external SPI memory device, that can be programmed by a standard RPi utility, and is really easy to read back.
This extra hardware isn’t compulsory; there is an INCLUDE_FIRMWARE option in the source code to link the firmware file into the binary image; the functionality is the same, it just takes longer to load over the target serial link.
The device I used is an EN25Q80B, which has a megabyte of serial flash memory. MikroElektronika sell a small flash click board that is simple to connect to the ZeroW, as follows:
The version of flashrom I used does issue a warning that the Eon chip isn’t fully supported, but still programs it OK. Reading the chip is really easy:
If you don’t want to bother with this, just set the INCLUDE_FIRMWARE option in the source code, which links the firmware file into the main executable.
File upload
Before we can upload the code, there is a lot more initialisation to be done; another 34 commands that I won’t be describing here, mainly because I’m having difficulty understanding them in the absence of documentation; for now, the source code is the only explanation you’ll get.
The process of transferring the file is made a bit more complicated by the windowing scheme I described earlier; we have to move that along after every 32K. Command 53 is used in multi-block mode, so one command is issued for multiple data blocks.
// Upload blocks of firmware from flash to chip RAM
int write_firmware(void)
{
int len, n=0, nbytes=0, nblocks;
uint32_t addr;
flash_open_read(0);
while (nbytes < FIRMWARE_LEN)
{
addr = sdio_bak_addr(nbytes);
len = MIN(sizeof(txbuffer), FIRMWARE_LEN-nbytes);
nblocks = len / SD_BAK_BLK_BYTES;
if (nblocks > 0)
{
flash_read(txbuffer, nblocks*SD_BAK_BLK_BYTES);
n = sdio_write_blocks(SD_FUNC_BAK, SB_32BIT_WIN+addr, txbuffer, nblocks);
if (!n)
break;
nbytes += nblocks * SD_BAK_BLK_BYTES;
}
else
{
flash_read(txbuffer, len);
txbuffer[len++] = 1;
sdio_cmd53_write(SD_FUNC_BAK, SB_32BIT_WIN+addr, txbuffer, len);
nbytes += len;
}
}
flash_close();
return(nbytes);
}
// Write multiple 64-byte command 53 blocks (max 32K in total)
int sdio_write_blocks(int func, int addr, uint8_t *dp, int nblocks)
{
int n=0;
SDIO_MSG rspx, cmd={.cmd53 = {.start=0, .cmd=1, .num=53,
.wr=1, .func=func, .blk=1, .inc=1, .addrh=(uint8_t)(addr>>15)&3,
.addrm=(uint8_t)(addr>>7), .addrl=(uint8_t)(addr&0x7f),
.lenh=(uint8_t)(nblocks>>8)&1, .lenl=(uint8_t)nblocks, .crc=0, .stop=1}};
clk_0(1);
add_crc7(cmd.data);
log_msg(&cmd);
sdio_cmd_write(cmd.data, MSG_BITS);
if (sdio_rsp_read(rspx.data, MSG_BITS, SD_CMD_PIN))
{
gpio_write(SD_D0_PIN, 4, 0xf);
gpio_mode(SD_D0_PIN, GPIO_OUT);
gpio_mode(SD_D1_PIN, GPIO_OUT);
gpio_mode(SD_D2_PIN, GPIO_OUT);
gpio_mode(SD_D3_PIN, GPIO_OUT);
while (n++ < nblocks)
{
sdio_block_out(dp, SD_BAK_BLK_BYTES);
sdio_rsp_read(rspx.data, BLOCK_ACK_BITS, SD_D0_PIN);
dp += SD_BAK_BLK_BYTES;
clk_0(2);
}
gpio_mode(SD_D0_PIN, GPIO_IN);
gpio_mode(SD_D1_PIN, GPIO_IN);
gpio_mode(SD_D2_PIN, GPIO_IN);
gpio_mode(SD_D3_PIN, GPIO_IN);
}
clk_0(1);
return(n);
}
Once that is complete, we must load in the configuration data, which is available here. A small amount of pre-processing is required, namely removing the comment lines, and replacing all the newline characters with nulls. Since the file is small, command 53 is used in single-block mode.
// Upload blocks of config data to chip NVRAM
int write_nvram(void)
{
int nbytes=0, len;
sdio_bak_window(0x078000);
while (nbytes < config_len)
{
len = MIN(config_len-nbytes, SD_BAK_BLK_BYTES);
sdio_cmd53_write(SD_FUNC_BAK, 0xfd54+nbytes, &config_data[nbytes], len);
nbytes += len;
}
return(nbytes);
}
After another 12 initialisation commands, we can check if the code was loaded OK:
If the first value is D0 hex, and the second is 6, then all is well, and after another 21 initialisation commands, we can think about doing something useful with the chip…
This diagram shows the internals of the CYW43xxx chip in simplified form; the important point is that the chip has its own CPU, RAM and ROM; it is a computer-within-a-computer.
In part 2 I mentioned the lack of documentation, and now this becomes a major issue; how to program this complex chip, with no data on its internals. Cypress have partially solved this problem by issuing a standard binary ‘blob’ (roughly 300K bytes) that contains all the code for the embedded CPU; we’ll just be feeding data and control messages into that program, not knowing (or caring) what it is doing to the chip hardware.
I say the problem is ‘partially’ solved because we have to set up the chip to receive this program, upload the code into its RAM, then configure the chip to run it.. a sizeable task, as I’ve discovered.
First steps
The first task is to get the WiFi chip to respond to our commands. The SD and SDIO specifications offer plenty of flowcharts that describe how such a device might be initialised, but I had minimal success with these; they may be applicable to older chips, but maybe the later incarnations of the CYW43xxx just treat the SDIO bus as a convenient parallel interface, rather than slavishly following a specification that was designed for plug-in SD cards.
Then there are the timing issues; a quick glance at the existing code shows many instances where SDIO commands are artificially delayed; after you change something within the chip, it needs time to react before receiving the next command – and if that is sent too quickly, the chip just ignores it, with no error response.
The best way I could find to tackle these problems is to capture all the SDIO commands, responses & data when the Linux driver is running. The capture method is described in the previous part of this blog, and it results in a sizeable file: over 5 gigabytes as a CSV. It contains over 2,200 commands and 13,000 data blocks, so I wrote an application (‘sd_decoder’) to decode the file and display the commands. It turns out that there is a lot of redundancy (for example, the driver loads the 300K binary into the chip, then reads it all back again) so by focusing purely on one WiFi chip, we can make major simplifications.
Fragments of the simplified command sequence can then be replayed into the chip, and eventually, it starts to respond – the first time you get a response from a new chip is a happy day! Here are the first few commands the Linux driver uses to start up the chip:
To explain the format; firstly the time in seconds, then 6 command/response bytes, ‘*’ to indicate CRC is correct, or ‘?’ if incorrect, then a partial decode.
A few points to note:
The first 4 commands produce no responses.
There are jumps in the timestamp, presumably caused by intentional delays.
CMD5 is described in the SDIO specification as enabling I/O mode, but the response we get has an incorrect CRC.
The CMD3 – CMD7 sequence is described in the SD specification; it is the way that a host selects one card from multiple card slots; CMD3 gets a Relative Card Address (RCA), then command 7 selects the card using that address.
It’d be nice to understand why two of the commands have failed CRCs, but the Linux driver ignores that error, so I will as well. The above sequence is implemented in my code as follows:
Note the time delays; it is tempting to reduce them, but I have found from bitter experience that this can result in major problems much later on (as a critical setting has been ignored), so I wouldn’t recommend doing that.
Raspberry Pi I/O
Now it is necessary to translate the SDIO commands into hardware I/O cycles. The good news about bare-metal programming is that is isn’t necessary to use a fancy driver, or seek permission from the operating system; we can just control the I/O directly.
The primary source of information is the ‘BCM2835 ARM Peripherals’ document; armed with that and knowledge of the I/O base address (0x20000000 for the Pi ZeroW) we can create suitable low-level functions.
Configuring a pin as an input or output is done by setting a 3-bit values. Writing 1 or 0 to a pin is (sadly) done using separate set & clear registers; this does make any speed-optimisations (e.g. direct DMA to I/O ports) significantly harder, so I haven’t tried this yet.
Running under Linux
Although the whole purpose of this project is to run without Linux, after I’d written the above code, I did wonder whether it’d speed up development if I ran it under Linux with the WiFi interface shut down, using mmap() to gain access to the devices at low level.
This experiment failed; I never got reliable communications with the WiFi chip, and the operating system had a tendency to crash after my code was run. This isn’t too surprising, since the whole point of the OS is to control the hardware, and having a user-mode program controlling it as well, is really asking for trouble.
Timer
In addition to I/O cycles, we need a microsecond timing reference, that can be used to provide accurate delays. Fortunately there is a 32-bit register clocked at 1 MHz that is ideal for the purpose.
#define USEC_BASE (REG_BASE + 0x3000)
#define USEC_REG() ((uint32_t *)(USEC_BASE+4))
// Delay given number of microseconds
void usdelay(int usec)
{
int ticks;
ustimeout(&ticks, 0);
while (!ustimeout(&ticks, usec)) ;
}
// Return non-zero if timeout
int ustimeout(int *tickp, int usec)
{
int t = *USEC_REG();
if (usec == 0 || t - *tickp >= usec)
{
*tickp = t;
return (1);
}
return (0);
}
SDIO output
The Raspberry Pi CPU is sufficiently fast that we can easily toggle the clock line at 500 kHz, while shifting the command bits out.
#define SD_CLK_DELAY 1 // Clock on/off time in usec
// Write command to SD interface
void sdio_cmd_write(uint8_t *data, int nbits)
{
uint8_t b, n;
gpio_mode(SD_CMD_PIN, GPIO_OUT);
for (n=0; n<nbits; n++)
{
if (n%8 == 0)
b = *data++;
gpio_out(SD_CMD_PIN, b & 0x80);
b <<= 1;
usdelay(SD_CLK_DELAY);
gpio_out(SD_CLK_PIN, 1);
usdelay(SD_CLK_DELAY);
gpio_out(SD_CLK_PIN, 0);
}
gpio_mode(SD_CMD_PIN, GPIO_IN);
}
This code could be made much faster, by eliminating the delays. When writing the data output function I took a more aggressive approach to the timing; the transfers are error-free with a 2 MHz clock (8 Mbit/s of data) and could go faster with some optimisation.
Reception is a lot more tricky, handling command responses, data on CMD53 read-cycles, and acknowledgements on write-cycles. This requires multiple state-machines, triggered by the clock edges and ‘start’ bit detection; see the source code for details.
The 7- and 16-bit CRC calculations for commands & data have already been explained in part 2 of this blog.
I’m starting this project with the Raspberry Pi ZeroW, which uses the Cypress WiFi chip CYW43438. It interfaces to the ARM processor using Secure Digital I/O (SDIO), which consists of the following signals:
Clock (1 line, O/P from CPU)
Command (1 line, I/O)
Data (4 lines, I/O)
Later in this blog, I’ll be describing what these pins do, in case you are a newcomer to the strange world of SDIO.
The I/O bit numbers are defined in the DeviceTree file for the board:
The ‘pull’ settings show that pullup resistors are enabled for pin 23 to 27 hex (GPIO35 to 39), and an initial guess would be that these pins are the command and data, while 22 hex (GPIO34) is the clock.
The datasheet mentions a power-on signal, and a quick trawl on the Web suggests that this could be GPIO41, which must be high to power up the WiFi interface. There is also mention of a low-speed (32 kHz) clock that may be needed when waking up the chip from low-power mode; it turns out this is on GPIO43. This can be verified by dumping the I/O configuration registers when the WiFi interface is running:
Each pin has a 3-bit mode value, that shows whether it being used for simple input, output, or is connected to an internal peripheral (ALT0 – 5). The values above can be decoded by referring to the ‘BCM2835 ARM Peripherals’ data sheet, but an easier way is to use the ‘pigs’ front-end for the PIGPIO library, thus:
sudo pigpiod [load PIGPIO daemon]
pigs mg 34 [get mode of GPIO pin 34]
7 [returned value 7: pin is ALT3]
pigs mg 43 [get mode of GPIO pin 43]
4 [returned value 4: pin is ALT0]
Pins 34 to 39 are all set to ALT3, which is unhelpfully labelled in the BCM2835 datasheet as ‘reserved’; in reality this means they are connected to the (undocumented) Arasan SD controller. GPIO43 is configured as ALT0, which is the clock source GPCLK2, configured for 32.768 kHz.
Attaching a logic analyser
To understand what the Linux driver is doing, I need to attach a logic analyser to the SDIO bus. This isn’t easy on most boards; the interface runs very fast (up to 50 MHz) so the only means of attachment is by soldering onto extremely small surface-mount components, that can easily be damaged.
However, the Pi Zerow has some interesting pads on the underside.
SDIO pads
Those 7 gold circles are clearly attached to some internal signals, since they have conductive holes (known as ‘vias’) to tracks on other layers. Also, they’re in the right area for the SDIO interface, and it is possible they’re needed for testing the WiFi/ Bluetooth interface after the PCB is assembled. Monitoring these signals with WiFi running proved that they do have almost all of the SDIO signals, aside from the most important one: the clock. Further probing suggested that the only way to pick up that signal is on the other side of the board at a resistor, but connecting to this point is tricky; you need good surface-mount soldering skills to avoid damaging the board.
SDIO clock connection
The main problem with the logic analyser interface is the sheer volume of data that’ll be accumulated. The boot process takes around a minute, with sporadic activity on the SDIO interface; catching all that, with a data rate of 50 MHz, would require a very complicated and/or expensive setup. Fortunately, the Raspberry Pi has an ‘overclocking’ setting in the boot file config.txt, which sets the clock rate to be used when the OS requests 50 MHz. This doesn’t just speed up the interface; a value of 1 or 2 MHz can be used to slow it right down, e.g.
# Add to /boot/config.txt:
dtparam=sdio_overclock=2
This allows a lower-cost analyser to be used (see part 1 of this blog for details) – and surprisingly, the change doesn’t make a lot of difference to the boot-time, since there are long pauses in SDIO activity, where the OS is doing other things. This can be seen by zooming the analyser display out to the maximum, showing 50 seconds of data:
SDIO activity during Linux boot
The bottom trace is the clock, the next is the command (CMD) line, then there are the 4 data lines. Despite the long periods with no activity, there is a lot going on: over 2,200 commands and 13,000 data blocks are being exchanged between the CPU and the WiFi chip.
SDIO protocol
If, like me, you have some experience of the Serial Peripheral Interface (SPI), you may expect SDIO to be similar, in that it uses a clock line to synchronise the sender & receiver; the rising edge of the clock indicates that the data is stable, and can be read by the receiver.
However, there are a few key differences:
Bi-directional. All the lines, apart from the clock, are bi-directional; either side can drive them.
Command and data lines. There are separate lines for commands and data, and the 4 data lines act as a 4-bit parallel bus.
Start & end bits. Instead of the SPI chip-select, the data and command lines idle high, then go low to signal the start of a transfer; this is referred to as a ‘start bit’, and is a single bit-time with a value of zero. At the end of the transfer there is a single bit with a value of 1, an ‘end bit’.
Format. The format of SDIO commands and responses is standardised, with specific meaning to the transferred bytes.
It is well worth reading the SDIO specification; at the time of writing, the latest version that is available from the SD Association is “SD Specifications Part E1, SDIO Simplified Specification Version 3.00”. For a few of the commands you need to refer back to the “SD Specifications Part 1 Physical Layer Simplified Specification”, for example:
SD command and response
This is SD command 3 (generally abbreviated to CMD3) and response 6 (R6) from the target (WiFi chip). Both are specified at being 48 bits long, and you can see they begin with a 0 start-bit, and finish with a 1 end-bit. Between the two, the command line is briefly idle. It is a bit confusing that the reply to a command 3 is not a response 3; this is because there are a lot of commands (over 50) but many of them share the same response format, so only 7 possible responses have been defined.
The most common commands used in the SDIO interface are CMD52 and 53. Command 52 is used to read or write a single 8-bit value, while CMD53 transfers blocks of data, either singly or in batches. The following trace shows command 53 reading a single block of 4 data bytes; the command and response look similar to command 52, but there is also activity on the data lines, starting with a 4-bit value of zero, and ending with F hex.
SDIO command 53
SDIO interface code
In the absence of the necessary documentation, writing code for the ‘Arasan’ SD controller on the Raspberry Pi would be quite fraught, so I decided to use direct control (‘bit-bashing’ or ‘bit-banging’) of the I/O lines. The more experienced among you might be thinking this is a really bad idea, as it can be very CPU-intensive and slow, but I believe that the end-result (for example, booting the WiFi chip from scratch in 1 second) vindicates my decision – and if you want to use the controller, you can modify my code to do so.
The bit-patterns within the SDIO commands and responses are quite complex, and the code I’ve seen makes heavy use of bit-masking and shifting to combine the individual values into a single message. I’m not a fan of this approach, and prefer to use C language bitfields. For example, CMD52 has the following fields in the 6-byte message:
Start: 1 bit (always 0)
Direction: 1 bit (1 for command, 0 for response)
Command index: 6 bits (52 decimal for command 52)
R/W flag: 1 bit (0 for read, 1 for write)
Function number: 3 bits (select the bus, backplane or radio interface)
RAW flag: 1 bit (1 to read back result of write)
Unused: 1 bit
Register address: 17 bits (128K address space)
Unused: 1 bit
Data value: 8 bits (byte to be written, unused if read cycle)
CRC: 7 bits (cyclic redundancy check)
End: 1 bit (always 1)
You’ll see that the values don’t all line up on convenient 8-bit boundaries; furthermore the data sheet defines the values with most-significant value first, whereas standard C structures have least-significant first.
My solution is to use macros to reverse the order of bits in the byte, so the command structure looks similar to the specification. These are used to create structures for the commands and responses, which are combined in a union.
#define BITF1(typ, a) typ a
#define BITF2(typ, a, b) typ b, a
#define BITF3(typ, a, b, c) typ c, b, a
..and so on..
typedef struct
{
BITF3(uint8_t, start:1, cmd:1, num:6);
BITF5(uint8_t, wr:1, func:3, raw:1, x1:1, addrh:2);
BITF1(uint8_t, addrm);
BITF2(uint8_t, addrl:7, x2:1);
BITF1(uint8_t, data);
BITF2(uint8_t, crc:7, stop:1);
} SDIO_CMD52_STRUCT;
typedef union
{
SDIO_CMD52_STRUCT cmd52;
SDIO_RSP52_STRUCT rsp52;
SDIO_CMD53_STRUCT cmd53;
uint8_t data[MSG_BYTES+2];
} SDIO_MSG;
The code to split the 17-bit address into 3 bytes is still a bit messy, but the structure definition does simplify the process of creating a command:
// Send SDIO command 52, get response, return 0 if none
int sdio_cmd52(int func, int addr, uint8_t data, int wr, int raw, SDIO_MSG *rsp)
{
SDIO_MSG cmd={.cmd52 = {.start=0, .cmd=1, .num=52,
.wr=wr, .func=func, .raw=raw, .x1=0, .addrh=(uint8_t)(addr>>15 & 3),
.addrm=(uint8_t)(addr>>7 & 0xff), .addrl=(uint8_t)(addr&0x7f), .x2=0,
.data=data, .crc=0, .stop=1}};
return(sdio_cmd_rsp(&cmd, rsp));
}
For speed, the CRC is created using a byte-wide lookup table in RAM, which is computed on startup:
#define CRC7_POLY (uint8_t)(0b10001001 << 1)
uint8_t crc7_table[256];
// Initialise CRC7 calculator
void crc7_init(void)
{
for (int i=0; i<256; i++)
crc7_table[i] = crc7_byte(i);
}
// Calculate 7-bit CRC of byte, return as bits 1-7
uint8_t crc7_byte(uint8_t b)
{
uint16_t n, w=b;
for (n=0; n<8; n++)
{
w <<= 1;
if (w & 0x100)
w ^= CRC7_POLY;
}
return((uint8_t)w);
}
// Calculate 7-bit CRC of data bytes, with l.s.bit as stop bit
uint8_t crc7_data(uint8_t *data, int n)
{
uint8_t crc=0;
while (n--)
crc = crc7_table[crc ^ *data++];
return(crc | 1);
}
Data CRC
The data transfer includes a CRC for every line, for example this is the transfer of the 4 bytes A6, A9, 41, and 15 hex.
Command 53 data read
A total of 12 bytes are transferred, because each data line has an added 16-bit CRC. This was a bit of a headache, since splitting the 4-bit data into individual 1-bit values for CRC calculation would considerably slow down the command generation & checking. Fortunately there is a easy way to calculate & check the CRC for each line, while still keeping the 4 values together. This comes from the realisation that the exclusive-or operation in the CRC doesn’t care what order the bits are in; we can rearrange the bits to match our data. So we can compute all 4 CRCs using a single 64-bit value:
Bit 0: bit 0 of 1st CRC
Bit 1: bit 0 of 2nd CRC
Bit 2: bit 0 of 3rd CRC
Bit 3: bit 0 of 4th CRC
Bit 4: bit 1 of 1st CRC
..and so on, up to..
Bit 63: bit 15 of 4th CRC
Once they have been computed, the 4 CRCs are transmitted by just shifting out the next 4 bits of the 64-bit value. To speed up the calculation, a 4-bit lookup table is initialised on startup:
#define CRC16R_POLY (1<<(15-0) | 1<<(15-5) | 1<<(15-12))
uint64_t qcrc16r_poly, qcrc16r_table[16];
// Initialise bit-reversed CRC16 lookup table for 4-bit values
void qcrc16r_init(void)
{
qcrc16r_poly = quadval(CRC16R_POLY);
for (int i=0; i<(1<<SD_DATA_PINS); i++)
qcrc16r_table[i] = (i & 8 ? qcrc16r_poly<<3 : 0) |
(i & 4 ? qcrc16r_poly<<2 : 0) |
(i & 2 ? qcrc16r_poly<<1 : 0) |
(i & 1 ? qcrc16r_poly<<0 : 0);
}
// Spread a 16-bit value to occupy 64 bits
uint64_t quadval(uint16_t val)
{
uint64_t ret=0;
for (int i=0; i<16; i++)
ret |= val & (1<<i) ? 1LL<<(i*4) : 0;
return(ret);
}
Now when transmitting, the 64-bit (i.e. 4 x 16-bit) CRC is updated with every 4-bit value:
uint64_t qcrc=0;
// For each 4-bit value 'd':
qcrc = (qcrc >> 4) ^ qcrc16r_table[(d ^ (uint8_t)qcrc) & 0xf];
After the data has been sent, the CRC values are transmitted:
Command 53 can be used to transfer a single block (where the block size is specified in the command) or multiple blocks (where the block size has previously been set, and the number of blocks is specified in the command).
If the CPU is sending a single command, then writing multiple blocks to the WiFi chip, how does it check that the blocks are being received and processed OK? The answer is that when writing blocks, the recipient generates a brief acknowledgement back. Here is an example of a CMD53 write.
Command 53 write
It is a bit difficult to see what is going on; the command and response are similar to all the others, but then (after a surprisingly long pause) the data is transferred from the CPU to the WiFi chip. Zooming in on that data:
Command 53 write data
4 bytes with the values 3, 0, 0, 0, are being transferred, then 8 bytes of CRC. However, after that, the recipient acknowledges the received data by driving the least-significant data line with a bit value of 00101000 00111111 (28 3F hex). To be honest, I haven’t been able to find a proper description of these bits; I assume there is a single byte value, then the recipient holds the line low until it has finished processing, but the meaning of the byte bits isn’t at all clear. So for the time being, my code reads in the byte value, then waits for the line to go high, effectively treating it as a ‘busy bit’.
Clock polarity
A small but significant detail is the relationship between the data changes and the clock edges – when is the data stable so it can be read? I previously suggested that the data is read on the positive clock-edge, but look at this trace showing the transition between the command and the response:
Command and response clocking
For the command, the data changes on the negative-going clock edge, so can be read on the positive-going edge. The response appears to be the opposite way around, with the data changing on the positive-going edge – what is going on?
The answer is that the WiFi chip has been set to ‘SDIO High-Speed’ mode; the data changes very shortly after the positive-going edge of the clock, so as to enable fast transfers. The timing is described in the chip data sheet if you want to know the details, but the end result is that the logic analyser isn’t fast enough to capture the gap between clock & data changes, so the software that analyses the logic traces has to use the last state before the clock goes high.
Bit bashing
The bit-bash (or bit-bang) code was quite difficult to write; it has to toggle the clock line, feed out the single-bit command, then get the response whilst simultaneously sending or receiving the 4-bit data. Also, although the examples above show the data being shorter than the response, in reality it can be considerably longer, up to 512 bytes, so will finish long after the activity on the command line. Then there is the issue of the acknowledgements of block data writes, and the need for a timeout in case the chip goes unresponsive…
There is no point describing the code here; if you are interested, take a look at the source. In theory, it’d be a good idea to replace it with a driver for the Arasan SD controller, but I’m not sure there would be a large speed gain – the Linux driver seems to spend a lot of time idle, waiting for the SD controller to complete a task. Also the bit-bashing code is more universal: it shouldn’t be too difficult to port it to other processors such as the STM32, which is frequently paired with the Cypress chip in a standalone module.
SPI interface
For completeness, I need to mention that according to the data sheet, the WiFi chip has a Serial Peripheral Interface (SPI), that can be used instead of SDIO. This is enabled by sending a reset command to the chip, while certain I/O lines are held in specific states.
I originally thought this interface would be easier to use than SDIO, but all my attempts to get it working failed. Also, the SPI connections don’t seem to line up with the SPI master in the BCM2835, so the interface would have to be bit-bashed, which would be really slow as the data bus is only 1-bit-wide. So I abandoned SPI, and focused exclusively on SDIO.
The WiFi chips on the Raspberry Pi boards are from the Broadcom BCM43xxx range, which has been taken over by Cypress, and renamed CYW43xxx. The Pi ZeroW uses the CYW43438, the PDF data sheet is here.
The obvious starting point for creating a new driver is the existing Linux driver, known as ‘Broadcom Full MAC’, or BRCMFMAC. The source is available here.
A more recent driver is included in the Cypress WICED development system. This is based on Eclipse, and is very comprehensive, covering a wide range of wireless chips and functionality.
A more compact version of the code is available as the Cypress Wifi Host Driver, which is intended for integration in real-time systems.
Another (highly unusual) WiFi driver is available for the Plan9 operating system, contained within a single file ether4330.c. This has a large number of unconventional operating-system dependencies, and would require significant modifications to run on a bare-metal platform, but is interesting as the author has succeeded in creating some remarkably compact code.
To see an example of an SPI interface with a CYW43439, take a look at my Zerowi standalone driver for the Raspberry Pi Pico W here.
It is great to have such a range of source-code available, but in practice it has been of minimal use in this project; even the simplest versions are exceedingly complex, with hidden dependencies & timing issues, so rather than simplifying existing code, I have written all the code from scratch.
Documentation on the Broadcom/Cypress chips is very limited; the data sheet gives minimal information about the chip internals. There is a programming manual, but that only available from Cypress under a confidentiality agreement. I haven’t had access to that document, as it would place severe restrictions on what I can write in this blog. So I have just used freely-available information, logic analysis, and a lot of trial-and-error.
With regard to a logic analyser, the requirements for this project are a bit demanding. The Raspberry Pi ZeroW takes nearly a minute to boot, so a recording of the hardware activity will be quite long, overflowing the memory of most logic analysers. So it is necessary to using a ‘streaming’ analyser, that can send data continuously to a PC’s hard disk; I’ve been using the DSLogic Basic from DreamSourceLab, as it can stream 8-bit values at 20 megasamples per second, and has an easy-to-use GUI based on Sigrok, but a simpler device streaming 10 megasamples per second might be adequate for most analysis tasks.
Sigrok pulseview (the open-source logic analyser GUI) has a built-in Secure Digital (SD) decoder, but this is of limited use on the Secure Digital I/O (SDIO) interface of the Broadcom/Cypress chips, so for analysis I export the data as a very large CSV file (over 5 gigabytes!), then use my own software tools written in the C language to analyse it – Python would be much too slow.
A variant of this analysis code has been used to draw the logic analyser diagrams for this blog – they are all derived from real-world data.
The data analysis tools run on a Windows PC, and have been compiled using gcc 7.4.0 or 8.2.0 from the cygwin project. I suspect the code would also run under Linux with minimal modifications, but haven’t had the time to try this out. The programs can be compiled directly from the command-line, e.g. to produce sd_decoder.exe:
gcc -Wall -o sd_decoder sd_decoder.c
If you want to understand the SDIO interface, the specifications are essential reading; they are available at the SD Association.
Lean2 