stm32g0 rtfm::Monotonic

stm32_rtfm_monotonic.rs

//! Using STM32G0 TIM2 as monotonic timer
use core::u32;
use core::{
    cmp::Ordering,
    convert::{Infallible, TryInto},
    ops,
};

use defmt::Format;
use hal::stm32;
use hal::timer::*;
use rtic::{Fraction, Monotonic};

/// A measurement of the counter. Opaque and useful only with `Duration`
///
/// # Correctness
///
/// Adding or subtracting a `Duration` of more than `(1 << 31)` cycles to an `Instant` effectively
/// makes it "wrap around" and creates an incorrect value. This is also true if the operation is
/// done in steps, e.g. `(instant + dur) + dur` where `dur` is `(1 << 30)` ticks.
#[derive(Format, Clone, Copy, Eq, PartialEq)]
pub struct Instant {
    inner: i32,
}

impl Instant {
    /// Returns an instant corresponding to "now"
    pub fn now() -> Self {
        unsafe {
            let tim = &*stm32::TIM2::ptr();
            let cnt = tim.cnt.read().bits() as i32;

            Instant {
                inner: cnt,
            }
        }
    }

    /// Returns the amount of time elapsed since this instant was created.
    pub fn elapsed(&self) -> Duration {
        Instant::now() - *self
    }

    /// Returns the underlying count
    pub fn counts(&self) -> u32 {
        self.inner as u32
    }

    /// Returns the amount of time elapsed from another instant to this one.
    /// NOTE: it does not handle timer wraping correctly!
    pub fn duration_since(&self, earlier: Instant) -> Duration {
        let diff = self.inner - earlier.inner;
        assert!(diff >= 0, "second instant is later than self");
        Duration { inner: diff as u32 }
    }
}

impl ops::AddAssign<Duration> for Instant {
    fn add_assign(&mut self, dur: Duration) {
        // NOTE this is a debug assertion because there's no foolproof way to detect a wrap around;
        // the user may write `(instant + dur) + dur` where `dur` is `(1<<31)-1` ticks.
        debug_assert!(dur.inner < (1 << 31));
        self.inner = self.inner.wrapping_add(dur.inner as i32);
    }
}

impl ops::Add<Duration> for Instant {
    type Output = Self;

    fn add(mut self, dur: Duration) -> Self {
        self += dur;
        self
    }
}

impl ops::SubAssign<Duration> for Instant {
    fn sub_assign(&mut self, dur: Duration) {
        // NOTE see the NOTE in `<Instant as AddAssign<Duration>>::add_assign`
        debug_assert!(dur.inner < (1 << 31));
        self.inner = self.inner.wrapping_sub(dur.inner as i32);
    }
}

impl ops::Sub<Duration> for Instant {
    type Output = Self;

    fn sub(mut self, dur: Duration) -> Self {
        self -= dur;
        self
    }
}

impl ops::Sub<Instant> for Instant {
    type Output = Duration;

    fn sub(self, other: Instant) -> Duration {
        self.duration_since(other)
    }
}

impl Ord for Instant {
    fn cmp(&self, rhs: &Self) -> Ordering {
        self.inner.wrapping_sub(rhs.inner).cmp(&0)
    }
}

impl PartialOrd for Instant {
    fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
        Some(self.cmp(rhs))
    }
}

/// A `Duration` type to represent a span of time.
///
/// # Correctness
///
/// This type is *not* appropriate for representing time spans in the order of, or larger than,
/// seconds because it can hold a maximum of `(1 << 31)` "ticks" where each tick is the inverse of
/// the CPU frequency, which usually is dozens of MHz.
#[derive(Clone, Copy, Default, Eq, Ord, PartialEq, PartialOrd)]
pub struct Duration {
    inner: u32,
}

impl Duration {
    /// Creates a new `Duration` from the specified number of clock cycles
    pub fn from_cycles(cycles: u32) -> Self {
        Duration { inner: cycles }
    }

    /// Returns the total number of clock cycles contained by this `Duration`
    pub fn as_cycles(&self) -> u32 {
        self.inner
    }

    /// Returns `Duration` in microseconds
    pub fn micros(&self) -> u32 {
        let frac = Tim2::ratio();
        self.inner * frac.numerator / (64 * frac.denominator)
    }
}

// Used internally by RTFM to convert the duration into a known type
impl TryInto<u32> for Duration {
    type Error = Infallible;

    fn try_into(self) -> Result<u32, Infallible> {
        Ok(self.as_cycles())
    }
}

impl ops::AddAssign for Duration {
    fn add_assign(&mut self, dur: Duration) {
        self.inner += dur.inner;
    }
}

impl ops::Add<Duration> for Duration {
    type Output = Self;

    fn add(self, other: Self) -> Self {
        Duration {
            inner: self.inner + other.inner,
        }
    }
}

impl ops::Mul<u32> for Duration {
    type Output = Self;

    fn mul(self, other: u32) -> Self {
        Duration {
            inner: self.inner * other,
        }
    }
}

impl ops::MulAssign<u32> for Duration {
    fn mul_assign(&mut self, other: u32) {
        *self = *self * other;
    }
}

impl ops::SubAssign for Duration {
    fn sub_assign(&mut self, rhs: Duration) {
        self.inner -= rhs.inner;
    }
}

impl ops::Sub<Duration> for Duration {
    type Output = Self;

    fn sub(self, rhs: Self) -> Self {
        Duration {
            inner: self.inner - rhs.inner,
        }
    }
}

/// Adds the `secs`, `millis` and `micros` methods to the `u32` type
pub trait U32Ext {
    /// Converts the `u32` value as seconds into ticks
    fn secs(self) -> Duration;

    /// Converts the `u32` value as milliseconds into ticks
    fn millis(self) -> Duration;

    /// Converts the `u32` value as microseconds into ticks
    fn micros(self) -> Duration;
}

impl U32Ext for u32 {
    fn secs(self) -> Duration {
        self.millis() * 1_000
    }

    fn millis(self) -> Duration {
        self.micros() * 1_000
    }

    fn micros(self) -> Duration {
        let frac = Tim2::ratio();

        // 64 MHz / fraction / 1_000_000
        Duration {
            inner: (64 * frac.denominator * self) / frac.numerator,
        }
    }
}

/// Implementation of the `Monotonic` trait
pub struct Tim2;

impl Tim2 {
    // consume timer so it cannot be used accidentially elsewhere
    pub fn init(_: Timer<stm32::TIM2>) {
        let tim2 = unsafe { &*stm32::TIM2::ptr() };
        tim2.arr.write(|w| unsafe { w.bits(u32::MAX) });
        // 64 MHz prescaled by 64 (psc is -1) = 1 MHz
        tim2.psc.write(|w| unsafe { w.bits(63) });
        tim2.cnt.write(|w| unsafe { w.bits(0) });
        tim2.egr.write(|w| w.ug().set_bit());
        tim2.cr1.modify(|_, w| w.cen().set_bit().urs().set_bit());
    }
}

impl Monotonic for Tim2 {
    type Instant = Instant;

    fn ratio() -> Fraction {
        // monotonic * fraction = sys clock
        Fraction {
            numerator: 64,
            denominator: 1,
        }
    }

    unsafe fn reset() {
        let ptr = &*stm32::TIM2::ptr();
        ptr.cnt.reset();
    }

    fn now() -> Instant {
        Instant::now()
    }

    fn zero() -> Instant {
        Instant {
            inner: 0,
        }
    }
}