Consider a slower I/O speed for CAN/CANFD on STM32

[liamkinne]

Doing some design validation on a board that uses Embassy I noticed some overshoot on the TX signal going to the CAN transceiver.

The GPIO speed is set to VeryHigh which has much faster edges than even an 8Mbit/s bus would require.

My suggestion is to change the default TX speed to Medium (which should be good for up to 10MHz even when running at 1.8V) and possibly even make it user configurable. Happy to do the work on this one and submit a PR if someone gives me the go ahead.

For reference, here's the I/O AC characteristics table for the STM32G474.

[Dirbaio]

sounds good. PRs welcome :)

Other drivers like SPI etc already make this configurable.

[liamkinne]

@Dirbaio unlike how it's done with SPI, the CanConfigurator constructor doesn't have configuration passed to it, only peripherals.

Should there just be another argument to CanConfigurator::new for the GPIO speed or is there somewhere better to put that config? It would make sense to have it happen in start() but by then the pin peripheral isn't accesible except for .info().tx_pin_port which I don't think can be used with set_as_af!

[Dirbaio]

i'd add a non_exhaustive config struct. This way we can add more stuff there in the future.