AccurateWaiter allows threaded applications to have threads wait for a precise amount of time, without sending the CPU into a permanent spinlock.
Dependents: AccurateWaiter-Test
AccurateWaiter
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AccurateWaiter allows threaded applications to have threads wait for a precise amount of time, without sending the CPU into a permanent spinlock.
In standard Mbed OS, certain types of threaded applications are limited by the precision of ThisThread::sleep_for(), which only allows sleeping for a whole number of milliseconds. This limitation comes from the underlying RTX RTOS, which uses a 1 ms scheduling interrupt.
What a lot of people don't know is that there is actually a way around this limitation, using the RTOS's EventFlags objects. Unlike most other RTOS objects, EventFlags are usable from interrupts. By configuring the Mbed us ticker interrupt to set EventFlags, a waiting thread can be woken immediately after an exact number of ticks on the us ticker.
The result is something that combines the best features of wait_us() and ThisThread::sleep_for(). Other threads may run during the wait period (like ThisThread::sleep_for()), but the wait time can be specified to the microsecond (like wait_us()). The only downside is a bit more overhead when starting the wait operation (to trigger the timer interrupt).
I tested this code on my NUCLEO_F429ZI board, and found that it was able to handle any time duration from 1us-1s accurately with exactly 14us of delay between the set time and the time the wait function returns. Overhead of calling the wait function also is around the 15-25us range.
Note 1: Since it uses the Mbed us ticker, this class allows waiting for >250,000 years before rollover.
Note 2: Each AccurateWaiter object may only be safely used by one thread at a time.
Diff: AccurateWaiter.cpp
- Revision:
- 0:85e56bb98f6e
diff -r 000000000000 -r 85e56bb98f6e AccurateWaiter.cpp --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/AccurateWaiter.cpp Thu Nov 12 22:18:14 2020 -0800 @@ -0,0 +1,36 @@ +// +// Created by jamie on 11/12/2020. +// + +#include "AccurateWaiter.h" + +AccurateWaiter::AccurateWaiter(): +TimerEvent(get_us_ticker_data()) +{ + +} + + +void AccurateWaiter::handler() +{ + // This signals the RTOS that the waiting thread is ready to wake up. + flags.set(1); +} + +void AccurateWaiter::wait_for(std::chrono::microseconds duration) +{ + // set up timer event to occur + insert(duration); + + // wait for event flag and then clear it + flags.wait_all(1); +} + +void AccurateWaiter::wait_until(TickerDataClock::time_point timePoint) +{ + // set up timer event to occur + insert_absolute(timePoint); + + // wait for event flag and then clear it + flags.wait_all(1); +}