Add example code for enabling pull-up resistor on a pin and sampling the pin.

[mutantbob]

When I connect a switch to a teensy pin, the voltage floats around 0.67V instead of being pulled up to 3.3V. If I install an external 10K pull-up resistor it works fine.

https://www.pjrc.com/teensy/td_digital.html includes an example of how to enable a pull-up resistor in C:

pinMode(PIN_D7, INPUT_PULLUP);

I have tried using

let cfg = teensy4_bsp::hal::iomuxc::Config::zero().set_pullupdown(PullUpDown::Pullup27k);
iomuxc::configure(&mut switch_pin, cfg);

but this does not appear to work like I want it to.

(some more detail and failed attempts at https://stackoverflow.com/questions/70102418/how-do-you-use-a-teensy-4-pin-via-the-teensy4-bsp-rust-crate-as-an-input-with-th )

I would like a code example that enables the Teensy4.0 pull-up resistor and reads the value

[mciantyre]

Sorry for the trouble! See imxrt-rs/imxrt-hal#112 for a confirmed, working configuration of a pull-down resistor. Change the set_pullupdown value to your preferred pull-up resistance.

Notice the additional set_pull_keep and set_pull_keep_select calls on the configuration. They're important for a correct configuration, and missing in our troublesome attempt.

---

Alternatively, use this new API to configure a PU/PD:

// Old:
// let cfg = teensy4_bsp::hal::iomuxc::Config::zero().set_pullupdown(PullUpDown::Pullup27k);

// New:
use teensy4_bsp::hal::iomuxc::{Config, PullKeeper};
let cfg = Config::zero().set_pull_keeper(Some(PullKeeper::Pullup27k));

This should be available with a recent update to a teensy4-pins dependency. If it doesn't build, run cargo update, and try another build. (imxrt-rs/imxrt-iomuxc#8 introduced this simplification.)

[mciantyre]

Thanks again for working through this. Reach out if you hit any other issues.

I enabled discussions, which provides a Q&A, StackOverflow-like service. I'm happy to answer your questions there, too.