feather-stm32f405: add SPI support

[ardnew]

This adds support for the three SPI peripherals on the target feather-stm32f405.

This included extending the STM32 family's SPI implementation to now support the following options:

- Direction (full/half duplex)
- Hardware or software chip/slave-select
- 8-bit or 16-bit data frame format

[deadprogram]

Also, @ardnew please note merge conflict that will need resolution via rebase against dev branch. Thanks.

[deadprogram]

Seems like the requested changes have all been made to the PR, is that correct?

[aykevl]

The code looks clean, however I'm wondering about this:

• The SPI peripheral is enabled at the end of the Configure call
• The SPI peripheral is enabled at the start of Transfer, if it isn't enabled already
• The SPI peripheral is disabled at the end of Transfer.

Is it intentional to disable the SPI peripheral between transferring each byte? Why do you enable it then in Configure?

[ardnew]

Is it intentional to disable the SPI peripheral between transferring each byte?

It was intentional on my part, yes, as this is what the STM32 HAL/LL drivers do before and after each transfer - I'm not sure why exactly they do it though. Clears pending flags? Paranoid attempt to minimize bus traffic? Helps prevent the user from re-configuring the interface while its enabled?

Why do you enable it then in Configure?

Good catch, indeed one of these is unnecessary. I think enabling/disabling for each transfer is the safer option, if not the most efficient. But I also think its a little pedantic and would be fine leaving it enabled between transfers unless someone objects.

[deadprogram]

I do not think that you want to enable/disable on event call to Tx(), only on Configure(). It might "work" but will slow down operation considerably.

[deadprogram]

Hi @ardnew any update on removing the register enable from the Tx() method? Thank you.

[ardnew]

@deadprogram Sorry -- got a little distracted with other efforts!

Requested change implemented.

[deadprogram]

Thanks @ardnew now squash/merging.