Seeed XIAO PWM Issue/Questions

[fujexo]

Hi!

For controlling a LED strip, I need to use PWM. I've got a Seed XIAO MCU which I'm trying to use for this because it's small and fits into the original controller case. Also it has rust support 🎉

Unfortunately, I'm not able to generate the PWM output using rust (A small example with Arduino proofed that it works). I'm not understanding the Alternate Pin functions and probably using them wrong. Also, I don't get where the pins are assigned to the PWM Channel 0 (magic?).

This is my code so far, I've tried to get it together from other BSP examples.

#![no_main]
#![no_std]

extern crate panic_halt;

use hal::{clock::GenericClockController, delay::Delay, prelude::*};
use pac::{CorePeripherals, Peripherals};

use bsp::{
    hal::{
        pwm::{Channel, Pwm0}, 
        self, 
        gpio::{
            v2::{AlternateE, PA07, PA05, Pin}
        }
    }, 
    entry, 
    pac
};
use xiao_m0 as bsp;


#[entry]
fn main() -> ! {
    let mut peripherals = Peripherals::take().unwrap();
    let core = CorePeripherals::take().unwrap();

    let mut clocks = GenericClockController::with_internal_32kosc(
        peripherals.GCLK,
        &mut peripherals.PM,
        &mut peripherals.SYSCTRL,
        &mut peripherals.NVMCTRL,
    );
    let mut delay = Delay::new(core.SYST, &mut clocks);
    let pins = bsp::Pins::new(peripherals.PORT);

    let _a8: Pin<PA07, AlternateE> = pins.a8.into_alternate();
    let _a9: Pin<PA05, AlternateE> = pins.a9.into_alternate();

    let gclk0 = clocks.gclk0();
    let mut pwm0 = Pwm0::new(
        &clocks.tcc0_tcc1(&gclk0).unwrap(),
        1.khz(),
        peripherals.TCC0,
        &mut peripherals.PM,
    );
    let max_duty = pwm0.get_max_duty();

    loop {
        pwm0.set_duty(Channel::_0, max_duty / 2);
        delay.delay_ms(1000u16);
        pwm0.set_duty(Channel::_0, max_duty / 8);
        delay.delay_ms(1000u16);
    }
}

[bradleyharden]

I haven't used the PWM code at all, so I can't be much help. But have you tried using a TC instead of a TCC? Maybe there's a bug in the TCC code that hasn't been caught, because most people use TC.

[fujexo]

Thanks for your feedback!
It is unclear for me how to use a TC on these pins. I think I now get how the pin assignment magic happens, I found this table in the datasheet on page 21 where my used pins (A8, A9) are assigned to TCC0/TCC1.
To use for example TC3, I would have to select other pins, is that right?

I also just found something interesting. Because I started to distrust myself, I've written a simple blinky program to verify my pins:

#[entry]
fn main() -> ! {
    let mut peripherals = Peripherals::take().unwrap();
    let core = CorePeripherals::take().unwrap();

    let mut clocks = GenericClockController::with_internal_32kosc(
        peripherals.GCLK,
        &mut peripherals.PM,
        &mut peripherals.SYSCTRL,
        &mut peripherals.NVMCTRL,
    );
    let mut delay = Delay::new(core.SYST, &mut clocks);
    let pins = bsp::Pins::new(peripherals.PORT);

    let mut a8 = pins.a8.into_push_pull_output();
    let mut a9 = pins.a9.into_push_pull_output();

    a8.set_low().unwrap();
    a9.set_low().unwrap();

    loop {

        a8.set_high().unwrap();
        delay.delay_ms(1000u16);
        a8.set_low().unwrap();
        delay.delay_ms(1000u16);

        a9.set_high().unwrap();
        delay.delay_ms(1000u16);
        a9.set_low().unwrap();
        delay.delay_ms(1000u16);

    }
}

For some reason, my LEDs are driven when nothing is pulling the pins down (might be my level shifter - topic for after the coding). So I've noticed that when set both these pins to alternate mode E (which I found in the datasheet is TC/TCC, finally found the explanation to the alternate modes :)), the Warm LED turns off (probably because the pin is set to low) and the cold LED is staying on. Can't find an explanation to this after the blinky program worked fine and I think it should be the same for both of the pins, right?

    let _cold_led = pins.a8.into_alternate::<E>();
    let _warm_led = pins.a9.into_alternate::<E>();

[bradleyharden]

It is unclear for me how to use a TC on these pins. ... To use for example TC3, I would have to select other pins, is that right?

Correct. I assumed your could find some other pins to try out, but I don't know your board at all.

In your latest example, it sounds like you're trying to use AlternateE mode without using the TCC, is that right? I don't know what would happen in that case. It probably depends on the default state of the TCC peripheral. Are there any dedicated LEDs on the board? Could you use those for a blinky example?

The best way to diagnose the PWM issue is probably to reproduce one of the TC examples on a different set of pins (if you can find one). At least that's probably what I would do.

[bradleyharden]

@fujexo, any progress with this?