busio.UART reports not supporting 9 bit data

[ericboxer]

Based on the Documentation busio.UART should support 9 bit data. However, when i try to create a UART port

serial485 = busio.UART(board.TX, board.RX, baudrate=38400, bits=9)

I get the following NotImplimentedError:

NotImplementedError: bytes > 8 bits not supported

[hierophect]

From what I can tell, the only Circuitpython devices that will support variable UART word length are the STM32s, and even they can only do 8 or 9, not 7. Atmel, NRF and CXD all either throw errors or ignore it. That said the policy of the documentation might be inclusive of all possible protocol options with not implemented errors handled on the chip level, I'm not sure.

[dhalbert]

We dropped support for 9 bits for atmel-samd due to complications when upgrading from the atmel-samd ASF3 to ASF4 libraries. The documentation should be clearer.

@hierophect I do see that stm32f4 supports 9 bits, but the read/write buffering seems to assume max 8-bit bytes. Maybe this is a bug? We need to decide whether it's worth supporting 9-bit bytes.

@ericboxer What is your application that needs 9-bit bytes?

[ericboxer]

@dhalbert Gotcha, that makes sense.

I needed a quick and dirty to connect up to an existing RS485 bus that uses 9 bit. The main device on the bus is an STM32F4, and one of the other slave devices is an older SiLabs 8051.

[hierophect]

@dhalbert the 9th bit would be handled by the peripheral as part of the parity system, so it isn't actually part of the data. I assume when parity is used in 8 bit mode it simply disregards the 8th bit of data if you provide it.

[dhalbert]

@hierophect Looks like RS485 can use the 9th (formerly parity) bit to distinguish between data and command (eek). On SAMD, you can send 9 bits of data plus a parity bit plus a stop bit. 9-bit SPI is also available. So it can be a real data bit.

[hierophect]

@dhalbert You're right, I was confused by the HAL wording. I looked into it more and while it appears at first glance that you'd need to use the LLs to properly handle 9 bit data (they have specific 9 bit send and receive functions) it actually looks like the ST peripheral will simply spread the data out across two bytes for every send and receive when using 9 bit data without parity through the HAL. From the user's perspective they'll appear to receive and send twice as much data, they just have to manage that conversion themselves.

[hierophect]

I'm not sure what the python equivalent of this would be but if it was in C you'd simply cast the 16 bit array holding your 9 bit words as 8 bit and it should work ok.
https://community.st.com/s/question/0D50X00009XkWx7SAF/9bit-uart-transmission-stm32-cubemx-hal-driver