SPI: deinit changing pin function?

OpenMV Boards
1

I was having some trouble with using the camera as an SPI peripheral (slave), specifically with the rpc library, but I think the issue is more with the SPI bus, in general. The problem manifested in odd behaviour with the SS pin (P3): after the first time it went LOW, the pin would no longer have any effect on the SPI transactions. For example, after the first transaction, I could connect the SS pin to 3V3 and SPI would more or less work(!) (but I suspect the ‘less’ part is due to the SS pin not working right, which leads to frame errors).

I traced the issue to SPI.deinit(). When that function is called, the pin mode for P3 is changed to an alternate function and it always reads LOW – even if connected to 3V3. So when testing the SS pin for the start of a transaction, it always returns LOW, so framing errors can occur.

I ran a short code to demonstrate:

import pyb

pin = pyb.Pin("P3", mode = pyb.Pin.IN, pull = pyb.Pin.PULL_UP)
spi = pyb.SPI(2)

print(pin.mode())
print(pin.af())
print(pin.af_list())
print(pin.value())

spi.init(pyb.SPI.SLAVE, polarity=0, phase=0)
print('init')

print(pin.mode())
print(pin.af())
print(pin.af_list())
print(pin.value())

spi.deinit()
print('deinit')

print(pin.mode())
print(pin.af())
print(pin.af_list())
print(pin.value())

When you run this code, the pin mode changes to 3 after deinit() is called, which corresponds to an alternate function, in this case AF = 0 --> Pin.AF1_TIM1.

I get a lost in the library files, and I’m struggling to figure out where exactly deinit() lives – I’d be curious to see what that function does and try to fix it.

The full output of the above code is:

0
0
[Pin.AF1_TIM1, Pin.AF5_SPI2, Pin.AF7_USART3, Pin.AF9_CAN2]
1
init
1
0
[Pin.AF1_TIM1, Pin.AF5_SPI2, Pin.AF7_USART3, Pin.AF9_CAN2]
1
deinit
3     ##  <------ pin mode has changed!
0
[Pin.AF1_TIM1, Pin.AF5_SPI2, Pin.AF7_USART3, Pin.AF9_CAN2]
0     ## <------ always reads LOW!
2

Will look into this. This could be the reason for the SPI mode in the RPC library not working. We re-factored SPI code recently.

I can’t test what’s going on until the week of December 6th.

3

Right now we kinda unconditionallity deinit the pin on the spi bus deinit: https://github.com/openmv/openmv/blob/master/src/omv/ports/stm32/stm32fxxx_hal_msp.c#L523C8-L523C8

This is probably not playing well with the pyb module API.

Try reiniting the pin afterwards. This should fix the issue.

…

As for fixing this… not sure if our code is broken. Deiniting the pin is the behavior we wanted. It might make sense to always re-init the pin after deiniting the SPI bus.

Have to think about that… I suppose we could just switch the pin to a gpio output on deinit versus AF if it was in that mode. Not sure what to do here.