Add OpenDrainIO pin state (with InputPin capability)

[Finomnis]

As already reported in #339, there is currently no way to create an I/O pin in nrf-hal.

There are several reasons why having this would be useful. My personal usecase is the DHT22/AM2302 sensor. Controlling it requires a single-wire pulled-up open drain IO.

Sadly, this means that all libraries that can interface with it require InputPin + OutputPin.

Solution

There is no inherent reason why nrf chips shouldn't be able to implement this. The input buffer is always available and allows reading back the pin values during every pin configuration. Although for energy saving reasons, the input buffer is currently disabled during the OpenDrain state.

My proposal is to add an OpenDrainIO state that does not disable the input buffer and implements InputPin.

[Finomnis]

Will write tests for this as well, as soon as I have an nrf52840-DK.

[jonas-schievink]

I wonder if this should just be the behavior of OpenDrain pins in general? I agree with #340 (comment) that it's a footgun for PushPull pins, but for OpenDrain, not so much, since those are legitimate input pins.

[Finomnis]

I wonder if this should just be the behavior of OpenDrain pins in general?

To my knowledge this was an intentional decision, because having an input buffer connected to a floating pin can cause substantially increased power drain. So people wanted to have an explicit opt-in. But I'm open to dispute that decision.
Note that this would be potentially a (behavioral) breaking change.

[jonas-schievink]

Okay, that sounds good!

bors r+

[bors]

Build succeeded:

• ci

[Finomnis]

@jonas-schievink Added tests in #402