The test sometimes fails on NRF51_DK (test case: "Test: LowPowerTimer - time measurement 1 ms.") in morph tests.
The test verifies if LowPowerTimer class correctly counts elapsed time. Sometimes we got measured ~1600 us for delay 1000 us (delta 550 us).
The delay is performed using `wait_us()` function which for delays greater than or equal to 1 ms (our case) calls `Thread::wait((uint32_t)ms);`. This causes rescheduling and potentially can put board into sleep (deep sleep mode is disabled by `wait_us()`). For our test purposes we don't need rescheduling/sleep since this actions takes extra time and have influence on the time measurement accuracy.
The solution is to implement function for delay which is based on busy loop and uses us ticker. It has been verified that this solves the problem. With this fix when measurement of 1 ms is repeated 1000 times we got usually measured time equal to ~1080 us, and sometimes ~1300us (checked that this is caused by systick interrupt handling). Since this is test for drivers layer and the results are acceptable I decided to not disabling systick in the test).
LowPoterTimer test gives sometimes failed result while testing measured time accumulation. The check which verifies if total number of elapsed milliseconds is valid fails. Test assumes that delta value equal to 1 ms is sufficient for all test cases, which is not true since in case where time measurement is performed few times in sequence the measurement error also accumulates and 1 ms might be not enough. To solve this problem delta value for milliseconds tests must be updated.
Martin Kojtal
Przemyslaw Stekiel
Bartek Szatkowski
jeromecoutant