Add SPI driver for samg family

[mcbridejc]

Currently based off feature/samg-usb, on the assumption those will land in develop before this.

[mcbridejc]

It pains me to admit that I've failed to understand the GPIO connect system implemented here, and not for lack of trying.

As I recall, the method of leaving some pins unconnected in the STM32 drivers is to pass, e.g. GpioUnused::Sck as a connect template parameter. I've tried to support a similar thing here for the sake of consistency, but have failed. Instead, I've just made all of the pins optional, and the connect() method will connect whatever subset of pins are provided.

[salkinium]

As I recall, the method of leaving some pins unconnected in the STM32 drivers is to pass, e.g. GpioUnused::Sck as a connect template parameter.

No, you just don't pass anything. The template arguments have a variable length for this purpose. The STM32 connector just returns GpioUnused instead of void when searching for the signal in the parameter pack, which has all the same APIs as the other Gpio types, just with empty implemtnations.

Instead, I've just made all of the pins optional, and the connect() method will connect whatever subset of pins are provided.

That's fine, you haven't failed at all. This is also a valid way to implement it.

I remember something about the Flexcom getting in the way of the GpioUnused API, so I guess that's why the SAM port uses a different way?

[mcbridejc]

No, you just don't pass anything. The template arguments have a variable length for this purpose. The STM32 connector just returns GpioUnused instead of void when searching for the signal in the parameter pack, which has all the same APIs as the other Gpio types, just with empty implemtnations.

Interesting. I have been doing SPI::connect<SCK::Sck, MOSI::Mosi, GpioUnused::Miso>();, and I vaguely remember figuring that out after getting errors when providing just two pins...but now I've removed the GpioUnused arg and indeed it works! So I'm not sure how I ended up coming to believe that...

I remember something about the Flexcom getting in the way of the GpioUnused API, so I guess that's why the SAM port uses a different way?

You're probably right; though I certainly don't know. It all feels overly complex to me, and I definitely feel like I'm poking at something I only vaguely understand. However I don't want to spend days/weeks figuring it out, and this seems to work, so I'm prepared to push on :).

[salkinium]

Awesome, thanks!

[mcbridejc]

Awesome, thanks!

Thank you for maintaining this! You have done all the hard parts :)