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Add implementation note for embassy-stm32's time-driver

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This is a detail I didn't originally understand when hoping to use TIM16/17 as alternative embassy-time driver providers.

Adding my note here to hopefully save the next person a little time.
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James Munns 2023-12-04 15:57:46 +01:00 committed by GitHub
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commit 18e89d7410
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embassy-stm32/src/time_driver.rs
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@ -18,6 +18,17 @@ use crate::rtc::Rtc;
use crate::timer::sealed::{Basic16bitInstance as BasicInstance, GeneralPurpose16bitInstance as Instance};
use crate::{interrupt, peripherals};
// NOTE regarding ALARM_COUNT:
//
// As of 2023-12-04, this driver is implemented using CC1 as the halfway rollover interrupt, and any
// additional CC capabilities to provide timer alarms to embassy-time. embassy-time requires AT LEAST
// one alarm to be allocatable, which means timers that only have CC1, such as TIM16/TIM17, are not
// candidates for use as an embassy-time driver provider.
//
// The values of ALARM_COUNT below are not the TOTAL CC registers available, but rather the number
// available after reserving CC1 for regular time keeping. For example, TIM2 has four CC registers:
// CC1, CC2, CC3, and CC4, so it can provide ALARM_COUNT = 3.
#[cfg(not(any(time_driver_tim12, time_driver_tim15)))]
const ALARM_COUNT: usize = 3;