port: Add methods to temporarily switch a pin to a different mode

[Rahix]

This allows temporarily making an output pin an input and vice versa:

let mut pin = pins.d0.into_output();

pin.with_pin_as_floating_input(|p| {
    // p is input here
    p.is_high();
});

Similarly:

• pin.with_pin_as_pull_up_input()
• pin.with_pin_as_output()
• pin.with_pin_as_output_high()

TODO

• [ ] Deduplicate implementation.
• [ ] Check that all invariants are upheld and pin-state is always returned to what it was before.
• [ ] Investigate whether we want the pin to be in the same state, or just the same mode - an alternative API could be one where a correct pin must be returned from the closure again:

  let pin = pins.d0.into_output();
  assert!(pin.is_set_low())
  pin.with_temporary(|p| {
    let p = p.into_floating_input();
    // ...
    p.into_output_high()
  })
  assert!(pin.is_set_high())
  

[Rahix]

FWIW, other HALs have come up with a very similar API. For example: https://docs.rs/stm32f4xx-hal/0.11.1/stm32f4xx_hal/gpio/struct.Pin.html#method.with_floating_input

[mutantbob]

I wonder how this would behave if the pin were currently being used as the CS for an SPI transaction.

[Rahix]

Well, the CS pin must be managed manually anyway so you can do weird stuff in any case... I don't think this changeset here would make it worse than it already is, or am I missing something?

[haennes]

Hello, I really need this for my project. Is there any way I can help? Should I fork it and then open a new PR?

[Rahix]

@haennes, giving me some feedback and testing this PR here is all I need. I just held off merging it because I wanted to hear from users first. Maybe you can comment on some of the open questions from the OP? What would be best for your usecase?

[haennes]

It works on an Arduino Nano (from Elegoo) (Atmega m328p). I will test it on an Arduino Uno R3 (from Elegoo) (Atmega 328p) too. Although the 2nd implementations seems better at a first glance, the way it currently is is quite perfect, as it removes a lot of boilerplate code in the users project.

[hiszd]

Am I correct in saying that implementing the IoPin trait from embedded-hal would take care of this as well? https://docs.rs/embedded-hal/latest/embedded_hal/digital/v2/trait.IoPin.html

[Rahix]

@hiszd, not really. IoPin moves the pin to change its state more permanently while the methods in this PR only change the pin state for the duration of a closure.

[RenjiSann]

Hi,

I just finished implementing a driver for the DHT11 sensor on an Elegoo Arduino Uno R3. I had to use a pull up pin with temporary output mode to send the data request signal, and I can attest that it worked without any problem.

This might need a little refactoring before it is anywhere close to presentable, but it could eventually be integrated into the examples someday.