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adafruit / circuitpython

CircuitPython - a Python implementation for teaching coding with microcontrollers
https://circuitpython.org
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Allow 4-16 bits SPI transfer size on RP2040, and other non-8-bit size on other chips. #8179

Open mbutsch-sts opened 1 year ago

mbutsch-sts commented 1 year ago

I am new at CircuitPython and may be misunderstanding some things.

I am trying to use CP's SPI functions on a Raspberry Pi Pico to access a SPI device (AT93C46D) with a transfer size of 16 bits. When I use the configure method, it reports that only 8 or 9 bits are allowed. I believe this is happening because of the line referenced below. https://github.com/adafruit/circuitpython/blob/4b4fb47088dca33e85a5eaeb14096aa16692340f/shared-bindings/busio/SPI.c#L202

The datasheet for the RP2040 seems to indicate that the SPI controller supports a transfer size of 4 to 16 bits.

Is there a reason for CP not to support it?

How would I get this issue fixed?

Thanks, Mark

dhalbert commented 1 year ago

SPI doesn't really distinguish byte boundaries: a 16-bit value is two concatenated 8-bit values. What happens if you send or receive two 8-bit-bytes? We have plenty of libraries where 16-bit values (or larger) are sent that way.

dhalbert commented 1 year ago

I looked at the datasheet for this device. The protocol is approximately SPI, but the chip-select line appears to be inverted (active high).

https://ww1.microchip.com/downloads/aemDocuments/documents/MPD/ProductDocuments/DataSheets/20006224B.pdf

mbutsch-sts commented 1 year ago

I tried sending/receiving two 8 bit values and it works. Is there a reason to not allow configuration of the actual transfer size, especially since there is already a parameter for 'bits'?

dhalbert commented 1 year ago

Different chip families have different SPI frame size capabilities. Eight bits is always availble. RP2040 is unusual in allowing 4-16 bits. Other chips are just 8 bits (e.g., nRF), 8 or 9 bits (MicroChip SAMD), 8 or 16 bits (STM), etc.

If we were writing a library for the AT93C46D or helping someone, we would recommend using an 8-bit frame size, so that the library would usable without changes and without conditional code inside it, across all chips that implement SPI.

So yes, we could support the RP2040's availability of 4-16 bits, but it's supporting the unique capabilities of a particular chip, so it's a lesser priority than just supporting a universal 8-bit SPI protocol in general (which we already do).

dhalbert commented 1 year ago

The 9-bit size on the SAMD's I think may be a remnant of a use case of 8-bits + parity, which you don't see much anymore.

mbutsch-sts commented 1 year ago

OK, thanks. This got me over my issue. I appreciate the responses / assistance.