Timers

One of the big advantages of a "bare-metal" embedded system is that you control everything that happens on your machine. This allows you to have really precise control of time: nothing will slow you down unless you let it.

However, we've seen that if we really want to get time right, we probably need help. Embedded MCUs like the nRF52833 all provide this kind of help in the form of "timers". A timer is a peripheral that, as its name implies, acts like a little clock that keeps very precise track of time.

The nRF52833 contains four timers. If you look at the documentation for the chip, you'll find that they are pretty complicated to set up and use. Luckily, the HAL provides a wrapper around timers that makes common uses easy. The most common use of a timer is to delay for a precise amount of time: just what our wait() function of the previous sections was trying to do.

Take a look at examples/timer-blinky.rs. This code sets up a timer and uses it to delay for 500ms (0.5s) between each toggle.

#![no_main]
#![no_std]

use cortex_m_rt::entry;
use embedded_hal::{delay::DelayNs, digital::OutputPin};
use nrf52833_hal::{gpio, pac, timer};
use panic_halt as _;

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

    let p0 = gpio::p0::Parts::new(peripherals.P0);
    let mut row1 = p0.p0_21.into_push_pull_output(gpio::Level::High);
    let _col1 = p0.p0_28.into_push_pull_output(gpio::Level::Low);

    let mut timer0 = timer::Timer::new(peripherals.TIMER0);

    loop {
        timer0.delay_ms(500);
        row1.set_high().unwrap();
        timer0.delay_ms(500);
        row1.set_low().unwrap();
    }
}

Run this code with cargo run --release --example timer-blinky and time it with a stopwatch. You'll find that it is exactly one second for each on-off cycle.

Things you might notice:

We need to use the embedded_hal::Delay trait to get the delay_ms() method we're using.
As before, we dig the peripheral out of the PAC peripherals struct and give it to the HAL.

Now we have a production-quality blinky. Let's talk a bit about the implications of all this.

Discovery

Timers