Start alarm by subSeconds: Avoid overflow during conversion from ms to ticks. #113
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E.g. when having predivSync=255, a value of subSeconds=2^24 (still well below UINT32_MAX and thus using the 32bit computation branch) would be multiplied by 256 and result in an overflow during computation. In fact, 2^24 ms is about 4 hours 40 minutes.
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The issue stm32duino/STM32LoRaWAN#38 describes a bug that occus on LoRaWAN after 4 hours 40 minutes. This PR is the fix for that.
The bug occurs, when an alarm shall be put in place using subseconds for RTC_StartAlarm64(). If the subSeconds value is large enough, a computation in RTC_StartAlarm64() overflows.
E.g. when having predivSync=255, a value of subSeconds=2^24 (still well below UINT32_MAX and thus using the 32bit computation branch) would be multiplied by 256 and result in an overflow during computation. The overflow leads to not putting the right alarm in place.
In fact, 2^24 ms is about 4 hours 40 minutes.