Wio RP2040 review

12.08.2021 7:44

I've been following the development of the ecosystem around the new RP2040 microcontroller from the Raspberry Pi Foundation. I've found the microcontroller interesting in the combination with MicroPython since it appeared suitable for development in high-level language while still offering reasonably good real-time performance. For the common low-level bit banging microcontroller stuff I'm not sure if Python beats C/C++. However as soon as any kind of networking is involved, I think using a high-level language is significantly easier. With all sorts of necessary error handling and multi-tasking, networking code quickly becomes unreadable in C.

Hence I've been curiously waiting for RP2040 development boards to appear that would integrate some kind of a network interface. The two products that were most prominent on my radar were Arduino Nano RP2040 Connect and Seeed Wio RP2040. Both were announced earlier this year, but were more or less unobtainable. In June however Seeed reached out to me and offered to send me a free sample of their Wio RP2040 development board in return for a review. Two months later I've finally got one on my desk.

Seeed Wio RP2040 mini dev board on top of its box.

Wio RP2040 itself is a surface-mount module with castellated holes suitable for machine mounting on custom PCBs. It contains just the RP2040 microcontroller, the radio and an integrated inverted-F antenna for connecting to a Wi-Fi network. Voltage supply for the module is 5 V, the GPIO pins use 3.3 V levels. There is also a 3.3 V regulator output pin available on the module, however I could not find any information on how much current you can safely draw for your own use.

To make development easier, Seeed also sells the module already mounted on the Mini Dev Board, which is what I got in the box you see above. The Mini Dev Board adds two LEDs, two buttons, a USB C connector and breaks out all the module pins to two 14 pin 100 mil headers. Schematic and PCB layout files are available for the Mini Dev Board, but not for the module itself.

Also worth noting is the declaration of an EU representative on the box. This is most likely related to the requirements of the European Radio Equipment Directive.

Wio RP2040 compared to Raspberry Pi Pico.

Compared to Raspberry Pi Pico, Wio RP2040 Mini Dev Board is slightly shorter and wider. It has a USB C connector instead of USB Micro for programming and getting power from a PC. In addition to the Pico's BOOT button, the Mini Dev Board also has a RUN button for manually resetting the microcontroller. There's also a power LED that is hard-wired to the power supply line.

Same as on Pico, the Mini Dev Board has space for 100 mil headers on its edge. The headers themselves are not included in the box, so if you want to mount this on a breadboard you need to supply and solder them yourself. GPIO 20, 21 and 22 are not available on the Wio RP2040 headers. They are probably used for communication with the wireless chip inside the module.

Curiously, I found zero information on which 2.4 GHz 802.11 b/g/n radio is used in the module. I'm yet to peek under the RF shield can, but I strongly suspect it hides an ESP8285 from Espressif. ESP8285 is a variant of the popular ESP8266 with built-in flash memory. This guess comes from the fact that the host name the module uses when obtaining an IP address from a DHCP server is espressif. The official firmware image also has a number of strings that mention ESP:

$ strings firmware.uf2|grep -i esp
esp8285
o rp2040] %s | esp8285_ipconfig could'n get ip
[wio rp2040] %s | esp8285_ipconfig could'n get gateway
[wio rp2040] %s | esp8285_ipconfig could'n get netmask
[wio rp2040] %s | esp8285_config could'n get ip
couldn't init nic esp8285 ,try again please
esp8285 power off

Speaking of firmware, Seeed provides a firmware.uf2 file that contains a customized MicroPython interpreter with some added modules related to networking. Unfortunately, it's not clear at the moment what is the source used for building this file. Another problem is that the file linked from the Wiki seems to silently change without notice. Since July I've seen at least two files being distributed with the same name and URL but different contents.

The procedure for loading the firmware is the same as with Pico. Power up the module with BOOT button depressed and then copy the firmware image into the emulated USB storage device. Using rshell, this is how the module presents itself, running the firmware.uf2 downloaded on August 5:

$ rshell
Connecting to /dev/ttyACM0 (buffer-size 512)...
Trying to connect to REPL  connected
Testing if sys.stdin.buffer exists ... Y
Retrieving root directories ... 
Setting time ... Aug 05, 2021 07:43:39
Evaluating board_name ... pyboard
Retrieving time epoch ... Jan 01, 1970
Welcome to rshell. Use Control-D (or the exit command) to exit rshell.
> ls /pyboard
> repl
Entering REPL. Use Control-X to exit.
>
MicroPython v1.15 on 2021-07-06; Seeed Wio with RP2040
Type "help()" for more information.
>>> help('modules')
__main__          machine           uasyncio/funcs    urandom
_boot             math              uasyncio/lock     ure
_onewire          micropython       uasyncio/stream   uselect
_rp2              mqtt              ubinascii         usocket
_thread           network           ucollections      ustruct
_uasyncio         onewire           uctypes           usys
builtins          rp2               uerrno            utime
cmath             uarray            uhashlib          uzlib
ds18x20           uasyncio/__init__ uio
framebuf          uasyncio/core     ujson
gc                uasyncio/event    uos

The embedded flash filesystem is empty by default, however there are some extra importable modules available in the interpreter: network, mqtt and a few others. Again, unfortunately there is very little information on these, apart from a few examples in the Wiki. No source available as far as I can tell either. MQTT module seems similar to umqtt.simple described here with some differences - there is no check_msg() method, for example.

I didn't have much luck with using these networking Python modules. Some examples in the wiki are apparently outdated and I didn't manage to get any of them to a usable state.

Specifically, the firmware I was using seemed to have problems receiving data from the network. I could connect to the Wi-Fi network and successfully open a usocket to another host. Sending data using usocket.send() worked. However as soon as the socket received anything from the other end, the MicroPython interpreter would apparently crash and I could never get anything back using usocket.recv(). The program stopped running and the REPL would not respond. I couldn't connect to the board over USB anymore until I reset the processor using the RUN button.

I had similar problems with Seeed's MQTT example code. After fixing it to account for the fact that WLAN_UART class is not defined, Wio RP2040 connects to my MQTT broker. I can successfully publish messages and subscribe to topics from MicroPython. However as soon as some other client sends a message to the topic that the Wio RP2040 is subscribed to, the interpreter crashes. There's definitely something still alive running on the MCU because the broker keeps getting periodic MQTT pings from Wio RP2040. The Python code doesn't seem to be executing though and neither Thonny nor rshell will connect to it.

I tried to find the problem, but without the source and any kind of debug info I was pretty much stuck. I also asked my Seeed contact about it and after a week I have yet to receive a reply.

Update: On 17 August I received a MicroPython firmware image from Seeed that fixes the interpreter crashes related to the networking I describe above. They say that they will fix the image linked from the wiki at a later date.

Screenshot of Thonny with Wio RP2040 MQTT example.

It's obviously very early in the product cycle. I actually don't know if these modules have shipped in any quantity so far. Each time I check, they are out of stock and the banner on Seeed website currently says they will start shipping in September. Still, I was disappointed to see that networking, the main feature of this module, doesn't seem to be functional at the moment. It seems Seeed's customized MicroPython port still needs some work. There's also support for programming the module in C/C++ using Arduino IDE. I have not tried that, but it seems other people are not having much success with that either.

Apart from fixing the software, I hope Seeed also adds some more documentation in the future. Having examples is great, but the custom Python modules should come with a reference. If the firmware image is open source, instructions on building one would be welcome as well. I'm also missing a proper hardware datasheet with some electrical specifications for the module.

The problems I encountered are even more puzzling since Wio RP2040 seems to be focused on being a base for a product than a development board for one-off projects. Its bare-bones design doesn't include any extra sensors that Arduino is shipping on their RP2040 boards. This makes it less inviting for playing around compared to the kitchen-sink-included approach of the Arduino. On the other hand, that's obviously a feature when you're designing a custom board with only the peripherals you need. Seeed is also running a promotional campaign and gives you some free modules when using their assembly services.

Another thing worth noting is that with recently introduced EU import regulations, getting these modules shipped in small quantities from China is quite troublesome and expensive. Even when receiving this free sample I had to deal with import customs paperwork and pay approximately 20 EUR in VAT and processing fees. Add shipping costs and the 13 USD base price shown in the Seeed store effectively becomes around 50 EUR. On the other hand, I have noticed that the modules are listed on Mouser, so this might improve in the future.

In summary, this module promises to be a cheap and simple basis for small network-connected sensors and actuators. I like the simplicity of connecting to MQTT using a few lines of Python. Unfortunately, current software does no deliver on that promise and I can only recommend waiting until the quality improves.

Posted by Tomaž | Categories: Digital

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