Fix mono timer

[TheZoq2]

Closes #252

Unfortunately this a breaking change as we now need to take the DCP peripheral when starting the mono timer.

I also added a note from one of the linked issues about mono_timer being stopped when hprintln is running.

For reference, here is a code sample that does not work currently, but does with this fix

#![deny(unsafe_code)]
#![no_main]
#![no_std]

use panic_halt as _;

use cortex_m_semihosting::hprintln;
use cortex_m_rt::entry;
use stm32f1xx_hal::{
    pac,
    prelude::*,
    serial::{Config, Serial},
};
use embedded_hal::digital::v2::OutputPin;

use core::fmt::Write;

#[entry]
fn main() -> ! {
    let mut cp = cortex_m::peripheral::Peripherals::take().unwrap();
    let p = pac::Peripherals::take().unwrap();

    let mut flash = p.FLASH.constrain();
    let mut rcc = p.RCC.constrain();
    let clocks = rcc.cfgr.freeze(&mut flash.acr);

    let mut afio = p.AFIO.constrain(&mut rcc.apb2);
    let mut gpioc = p.GPIOC.split(&mut rcc.apb2);

    let mut led = gpioc.pc13.into_push_pull_output(&mut gpioc.crh);


    // Needed in order for MonoTimer to work properly
    // cp.DCB.enable_trace();

    let mut delay = stm32f1xx_hal::delay::Delay::new(cp.SYST, clocks);
    let mono_timer = stm32f1xx_hal::time::MonoTimer::new(cp.DWT, cp.DCB, clocks);

    let start = mono_timer.now();

    let mut led_state = false;
    let mut prev = start.elapsed();
    loop {
        // hprintln!("{}", new);
        let new = start.elapsed();
        if prev + mono_timer.frequency().0 < new {
            if led_state {
                led.set_high();
            }
            else {
                led.set_low();
            }
            led_state = !led_state;
            prev = new;
        }
    }
}

[TheZoq2]

I should make cargo fmt run automatically before I commit to this repo 😅

[therealprof]

LGTM

[therealprof]

Oh, but since it's a breaking change it should be mentioned in the CHANGELOG. DOH!

[TheZoq2]

Oops, I'll just do that manually with another commit :)

[TheZoq2]

Maybe we should add a CONTRIBUTING.md that just says "Oh, and don't forget to modify the changelog"

[therealprof]

Yeah.