An RTIC monotonic based on RTC for nRF52 series

rtc_monotonic.rs

// RTIC Monotonic impl for the RTCs
use crate::hal::pac::{rtc0, RTC0, RTC1, RTC2};
pub use fugit::{self, ExtU32};
use rtic_monotonic::Monotonic;

pub struct MonoRtc<T: InstanceRtc> {
    overflow: u8,
    rtc: T,
}

impl<T: InstanceRtc> MonoRtc<T> {
    pub fn new(rtc: T) -> Self {
        unsafe { rtc.prescaler.write(|w| w.bits(0)) };

        MonoRtc { overflow: 0, rtc }
    }

    pub fn is_overflow(&self) -> bool {
        self.rtc.events_ovrflw.read().bits() == 1
    }
}

impl<T: InstanceRtc> Monotonic for MonoRtc<T> {
    type Instant = fugit::TimerInstantU32<32_768>;
    type Duration = fugit::TimerDurationU32<32_768>;
    
    const DISABLE_INTERRUPT_ON_EMPTY_QUEUE: bool = false;

    unsafe fn reset(&mut self) {
        self.rtc
            .intenset
            .write(|w| w.compare0().set().ovrflw().set());
        self.rtc
            .evtenset
            .write(|w| w.compare0().set().ovrflw().set());

        self.rtc.tasks_clear.write(|w| w.bits(1));
        self.rtc.tasks_start.write(|w| w.bits(1));
    }

    #[inline(always)]
    fn now(&mut self) -> Self::Instant {
        let cnt = self.rtc.counter.read().bits();
        let ovf = if self.is_overflow() {
            self.overflow.wrapping_add(1)
        } else {
            self.overflow
        } as u32;

        Self::Instant::from_ticks((ovf << 24) | cnt)
    }

    fn set_compare(&mut self, instant: Self::Instant) {
        let now = self.now();
        
        const MIN_TICKS_FOR_COMPARE: u64 = 3;

        // Since the timer may or may not overflow based on the requested compare val, we check
        // how many ticks are left.
        //
        // Note: The RTC cannot have a compare value too close to the current timer value,
        // so we use the `MIN_TICKS_FOR_COMPARE` to set a minimum offset from now to the set value.
        let val = match instant.checked_duration_since(now) {
            Some(x) if x.ticks() <= 0xffffff && x.ticks() > MIN_TICKS_FOR_COMPARE => {
                instant.duration_since_epoch().ticks() & 0xffffff
            } // Will not overflow
            Some(x) => {
                (instant.duration_since_epoch().ticks() + (MIN_TICKS_FOR_COMPARE - x.ticks()))
                    & 0xffffff
            } // Will not overflow
            _ => 0, // Will overflow or in the past, set the same value as after overflow to not get extra interrupts
        };

        unsafe { self.rtc.cc[0].write(|w| w.bits(val)) };
    }

    fn clear_compare_flag(&mut self) {
        unsafe { self.rtc.events_compare[0].write(|w| w.bits(0)) };
    }

    #[inline(always)]
    fn zero() -> Self::Instant {
        Self::Instant::from_ticks(0)
    }

    fn on_interrupt(&mut self) {
        if self.is_overflow() {
            self.overflow = self.overflow.wrapping_add(1);
            self.rtc.events_ovrflw.write(|w| unsafe { w.bits(0) });
        }
    }
}

pub trait InstanceRtc: core::ops::Deref<Target = rtc0::RegisterBlock> {}
impl InstanceRtc for RTC0 {}
impl InstanceRtc for RTC1 {}
impl InstanceRtc for RTC2 {}

Here is an update set_compare that solves the issue.
Unfortunately it does add on the minimal possible delay.

    fn set_compare(&mut self, instant: Self::Instant) {
        let now = self.now();

        const MIN_TICKS_FOR_COMPARE: u64 = 3;

        // Since the timer may or may not overflow based on the requested compare val, we check
        // how many ticks are left.
        let val = match instant.checked_duration_since(now) {
            Some(x) if x.ticks() <= 0xffffff && x.ticks() > MIN_TICKS_FOR_COMPARE => {
                instant.duration_since_epoch().ticks() & 0xffffff
            } 
            Some(x) => {
                (instant.duration_since_epoch().ticks() + (MIN_TICKS_FOR_COMPARE - x.ticks()))
                    & 0xffffff
            } 
            _ => 0, // Will overflow or in the past, set the same value as after overflow to not get extra interrupts
        } as u32;

        unsafe { self.rtc.cc[0].write(|w| w.bits(val)) };
    }

If you come up with a better fix, feel free to ping me!

Also, remember to set this flag as you are using an extended timer:

    const DISABLE_INTERRUPT_ON_EMPTY_QUEUE: bool = false;

Great to have a working solution! Having a minimum spawn-delay of 3 ticks (~10kHz) would work for me. (and not having an app that hangs "for no apparent reason" works for me as well.. 👍 )

Also, remember to set this flag as you are using an extended timer:

    const DISABLE_INTERRUPT_ON_EMPTY_QUEUE: bool = false;

Good catch! I did not even notice this (probably as it has a default value set in the trait). As the run queue in my specific use case is never empty, having this at false didn't manifest a real problem. Still, a foot gun lurking in the deep.. ;)

If you come up with a better fix, feel free to ping me!

Yep.. I'll dive into the Nordic datasheets and errata. I would expect this behavior probably to be documented in there somewhere...

What isn't exactly clear to me is why the added delay "solution" (using asm::delay()) seemed to solve the issue as well... (as the actual time-to-interrupt only becomes shorter by adding delay)

The delay solution works because RTIC's timer queue checks if the time has expired and side steps the interrupt handler. So if it's too short RTIC catches that :)

In reaction to

Here is an update set_compare that solves the issue.

There seems to be a logic error that can result in an overflow condition... consider

(instant.duration_since_epoch().ticks() + (MIN_TICKS_FOR_COMPARE - x.ticks()))

The match arm above that is conditional, so our code is executed only if the following evaluates to false:

Some(x) if x.ticks() <= 0xffffff && x.ticks() > MIN_TICKS_FOR_COMPARE => {

It is, however, executed when x.ticks() > 0xffffff, when at the same time x.ticks() > MIN_TICKS_FOR_COMPARE, the subtraction MIN_TICKS_FOR_COMPARE - x.ticks() results in an overflow. Shouldn't just checking for x.ticks() > MIN_TICKS_FOR_COMPARE in the first match arm condition be enough?