Make multiticker test more reliable when scheduling very early interrupts.
When very early interrupt is scheduled (e.g now + 2[ticks]) then it's likely
that it won't be fired in some circumstances and as a result there is overdue
event in queue. That overdue event can be mistakenly scheduled again by
`Ticker::detach` (detach calls schedule if head was removed). That's why we
should check interrupts counter immediately after wait period and before detach loop
This test fails sometimes while testing 1 ms timeout on NRF51_DK board.
This board is very slow and lp ticker is inaccurate.
I noticed that measured results are very close to the tolerance value. For 1ms delay we measure in failure case 1560 us and tolerance is set to 550.
Since measurement method is also significantly flawed (extra time for setting up ticker and execution of a ticker irq handler) I recommend to increase tolerance by least +100 us for now.
Since this change concerns new ticker driver for NRF51_DK it goas on ticker feature branch.
On master NRF51_DK Ticker driver uses 32kHz RTC for lp and us ticker. Test uses us ticker to measure lp ticker interrupt scheduling (LowPowerTicker.attach), but since the same hardware is used for both tickers the measurement error is constant. On this branch us ticker uses 1 MHz higher precision clock and the results are different - measurement error grows linearly and shows inaccuracy of the RTC.
Test implements constant delta for measured time equal to 2000 us.
Change delta so it depends on lp ticker tested timeout value:
500us for measurement inaccuracy + 5% tolerance for LowPowerTicker
Martin Kojtal
Maciej Bocianski
Przemyslaw Stekiel
Bartek Szatkowski