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[ion] Documenting micros() implementation

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This commit is contained in:
Damien Nicolet
2018-11-07 01:30:21 +01:00
parent 4c520cab97
commit bc8d9746c8
1 changed files with 18 additions and 7 deletions
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25
ion/src/device/device.cpp
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@@ -46,13 +46,24 @@ uint32_t Ion::millis() {
}
uint32_t Ion::micros() {
uint32_t c1 = CM4.SYST_CVR()->getCURRENT();
uint32_t ms1 = millis_elapsed;
uint32_t c2 = CM4.SYST_CVR()->getCURRENT();
uint32_t ms2 = millis_elapsed;
uint32_t load = CM4.SYST_RVR()->getRELOAD();
return ((c1 > c2) ? ms1 : ms2) * 1000 + ((load - c2) * 1000) / (load + 1);
// Here, we can use the current value of the systick counter to get a
// microsecond resolution counter. The problem is that the systick interrupt
// may occur between reading millis_elapsed and reading the current value
// of the systick counter. To cope with this, we do two consecutive reading
// of both values. Do not forget that the counter counts backwards down to zero.
uint32_t c1 = CM4.SYST_CVR()->getCURRENT();
uint32_t ms1 = millis_elapsed;
uint32_t c2 = CM4.SYST_CVR()->getCURRENT();
uint32_t ms2 = millis_elapsed;
uint32_t load = CM4.SYST_RVR()->getRELOAD();
// If c1 > c2, then no systick interrupt occured between reading c1 and ms1, so ms1
// value is correct, else an interrupt occured and ms2 value is correct. So we
// have the milliseconds part : "((c1 > c2) ? ms1 : ms2) * 1000"
// Then, we just add and scale the value of c2, as it is correct in both case.
// We do "(load - c2)" to convert from a backward count to a forward count,
// and then scale it to microseconds "* 1000) / (load + 1)" (as load is
// the number of ticks per milliseconds minus one)
return ((c1 > c2) ? ms1 : ms2) * 1000 + ((load - c2) * 1000) / (load + 1);
}
uint32_t Ion::crc32(const uint32_t * data, size_t length) {