Pico fails silently when initialising SD Card

[craigdrayton]

I have a Wokwi project using micropython and the Pi Pico, with the wokwi-microsd-card module.

Upon attempting to initialize the SD card using the standard micropython sdcard library, the application hangs on Line 75 of the library self.spi.write(b"\xff")

Is this simply because SPI hasn't been implemented yet on Pico, or is something else going on? https://docs.wokwi.com/parts/wokwi-pi-pico

Cheers, Craig

[urish]

Hi Craig, SPI has been implemented (3c7c5c73).

Mind sharing the link to the your project so I can have a look?

[craigdrayton]

Thanks for such a fast reply. Sure thing: https://wokwi.com/arduino/projects/322461882944324178

[urish]

Thanks!

Apparently, the SIO wasn't connected on Wokwi's end (that's why the docs said it was unimplemented).

I added the missing bits, and now your code seems to communicate with the SD Card. It seems to fail on this command:

        if self.cmd(16, 512, 0) != 0:
           raise OSError("can't set 512 block size")

Command 16 hasn't been implemented yet, but the block size is 512 by default (at least in the simulator), so I believe you can remove this check. I haven't had a chance to look where it gets stuck next.

However, adding a Logic Analyzer can be very valuable in diagnosing the communication. Here is a screen shot showing how pulse view decodes the beginning of the communication:

[craigdrayton]

Thanks! The SD card seems to initialize correctly now, I can read sd.sectors and get 8MB.

Now I may be running up against limitations of the current wokwi sdcard module.

• I'm unable to mount the SD card (fails silently) uos.mount(sd, "/sd")
• If I skip mounting the SD card, uos.listdir("/") shows my project files
• I can then write to, say, "/test01.txt" and then later read the data, but the file does not persist

[urish]

I'm unable to mount the SD card (fails silently)

That's interesting. Any idea why?

If I skip mounting the SD card, uos.listdir("/") shows my project files

That makes sense - Wokwi copies the project files to the Pi Pico's flash memory, so that's why you see them

I can then write to, say, "/test01.txt" and then later read the data, but the file does not persist

This is true for both the internal flash memory and the SD Card - we don't persist the data at the moment. Still need to figure out what's the best way to go about it.

p.s. - I see your code also uses ulab. In general, it shouldn't be too hard to include it in Wokwi - we'd just need to have the uf2 here: https://github.com/wokwi/firmware-assets/tree/gh-pages/micropython, and then it'll be able to load it into Wokwi (there's already on with ulab for the ESP32)

[urish]

I'm unable to mount the SD card (fails silently)

After some digging, I think I found the reason: the code is using spi.write_readinto(), passing a buffer of 512 bytes. Looking at the MicroPython implementation, it seems to use DMA for buffer sizes of 32 bytes or more:

https://github.com/micropython/micropython/blob/26faf74d52e3e953c9ddbcd800a2f9e117477ce7/ports/rp2/machine_spi.c#L214-L215

The simulated SPI peripheral does not (yet) implemented DMA. A quick workaround would be to split the read into smaller chunks (less than 32 bytes each), so MicroPython won't try to use DMA.

You can also open an issue for SPI + DMA support.

[urish]

Thanks to @v923z, we now have a ulab-enabled firmware for the Raspberry Pi Pico as well!

https://wokwi.com/arduino/projects/322595929479709267

If you want to switch to it in your project, change the value of the "env" attr in your diagram.json to "micropython-ulab-26faf74d5-ccd9481"

[craigdrayton]

Thanks @urish I really appreciate the quick responses (apologies for my delay on this one).