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.
AccurateWaiter.cpp
- Committer:
- Jamie Smith
- Date:
- 2020-11-12
- Revision:
- 0:85e56bb98f6e
File content as of revision 0:85e56bb98f6e:
// // 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); }