Represent pins A6 and A7 of Arduino Nano similar to other pins

[Fundevoge]

As can be seen in the image, the Arduino Nano gives access to ADC_MUX 111 and ADC_MUX 110 via the pins A6 and A7.
It would be nice if these pins could be used somehow but it probably would require some refactoring as the availability of these pins depends on the package/board variant (TQFP and VQFN Package Only; Arduino Uno has the wrong package, Arduino Pro Mini does not forward it) and not on the processor variant.

[Rahix]

They are supported, access just works a bit different than for all the other pins. Check the example code here:

avr-hal/examples/arduino-nano/src/bin/nano-adc.rs

Lines 47 to 53 in 06ad11f

	// Arduino Nano has two more ADC pins A6 and A7. Accessing them works a bit different from
	// the other pins as they are not normal IO pins. The code below shows how it works.
	let (a6, a7) = (
	adc.read_blocking(&adc::channel::ADC6),
	adc.read_blocking(&adc::channel::ADC7),
	);
	ufmt::uwrite!(&mut serial, "A6: {} A7: {}", a6, a7).void_unwrap();

[Fundevoge]

I see, thanks for pointing it out.
It would be convenient to be able to use pins.a6 and then get a Pin<Analog, ADC6> or similar that does not implement the PinOps so it cannot be converted, but implements AdcChannel so a6.analog_read() is possible.
But I also understand that this is not the most important feature...

[Rahix]

If you manage to make this work nicely, I'd be happy to take a PR implementing this :)

So far my reasoning was that writing code generic over ADC channels is already possible using AdcChannel so I didn't bother going any further here.