RasPi Pico: Often (but not always) Errno 19 on i2c writes

[ajs256]

I'm sometimes getting "OSError: [Errno 19] Unsupported operation" when attempting to write to an i2c device (a SparkFun Serial LCD) using a RasPi Pico.

code.py:

import board
import busio

i2c = busio.I2C(board.GP17, board.GP16)

while not i2c.try_lock():
    print("i2c not locked, retrying...")

print(i2c.scan())
i2c.writeto(114, "|-") # Clear
i2c.writeto(114, "Hello world!") # Show some text. Error happens here.

The error appears to be happening on line 11, according to the output. Am I trying to send an illegal character? Or is it a bug in CircuitPython?

EDIT: Oh. I suppose I should say that I am running Adafruit CircuitPython 6.2.0-beta.2 on 2021-02-11; Raspberry Pi Pico with rp2040. I don't know why I didn't say that in my initial comment!

[dhalbert]

There are issues with some I2C devices. The I2C peripherals on the RP2040 do not allow zero-length writes, which are the typical way to do scanning. We are working on a workaround for that.

If you remove the i2c.scan(), does the problem go away?

[ajs256]

Hmm. I commented out the scan call, and I now get the same error every time! I don't think the scan() is what's causing it.

[dhalbert]

Could you try some other I2C device? What is the part number of the serial LCD you are using?

[ajs256]

The LCD is SparkFun ID LCD-14073, running firmware version 1.4. That one is now discontinued, but LCD-16397 and LCD-16396 are the same apart from the addition of a QT/Qwiic connector if you want to test yourself. Give me a bit to scrounge up another device.

[ajs256]

I don't get the error at all using a 8x8 bicolor LED matrix backpack (Adafruit PID 902). I am using the adafruit_ht16k33 library and not writing raw bytes though.

[ajs256]

Just tested it using the absolute newest build as of https://github.com/adafruit/circuitpython/commit/b19e7c914844634306745ff6858f52df308f85ff, and the error still happens.

[ajs256]

Tested again as of https://github.com/adafruit/circuitpython/commit/15615effc2f8a9f991ace105f8a9f7bb51982059, and it still happens.

Sorry, but because I do not have an SWD debugger or a logic analyzer, I cannot help with anything requiring that.

[ajs256]

Tested using bus_device, and it appears to hang when attempting to init the device, unless I pass in probe=False. Then it gives me the OSError I was getting before. Here's the code I'm using:

import board
import busio
from adafruit_bus_device import i2c_device

i2c = busio.I2C(board.GP17, board.GP16)

while not i2c.try_lock():
    pass # Interestingly, it doesn't work without locking the bus. bus_device examples don't seem to require this. 
print("i2c locked.")

lcd = i2c_device.I2CDevice(i2c, 114, False)
print("Device initted.")

lcd.write("|-") # Clear display
lcd.write("Hello, world!") # Print text
print("Text shown.")

[dhalbert]

Try using bitbangio.I2C() on absolute latest. I am still working on getting busio.I2C() to work.

[ajs256]

bitbangio.I2C works perfectly, thank you.

[ajs256]

Well... not exactly perfectly. I have to pass in probe=False, or else it just infinitely hangs.

[dhalbert]

Moving to a 6.x.x milestone since there's now a workaround.

[dhalbert]

This is now fixed by #4499 and #4508.

[ajs256]

This doesn't appear to be fixed in

Adafruit CircuitPython 6.2.0 on 2021-04-05; Raspberry Pi Pico with rp2040

I still need to use bitbangio, otherwise I occasionally get OSError: [Errno 19] Unsupported operation when writing a string to an I2CDevice.

[dhalbert]

Which device(s) in particular? I tested a bunch.

[ajs256]

The SparkFun SerLCD, which is ATmega328-based, and the firmware is open source here.

[dhalbert]

Could you open a specific issue for that device? It must depend on how I2C peripheral is implemented there. There are further minor fixes to I2C that are in the works upstream, but I'm not sure they would fix this. Does it work some of the time with the native I2C?

[ajs256]

Could you open a specific issue for that device?

Give me a bit to get a minimal example working.

Does it work some of the time with the native I2C?

Sometimes with busio.I2C and all the time with bitbangio.I2C.

[ajs256]

Just opened #4621.