RTC wakeup depends on calibration value ???
Posted: Tue Jan 22, 2019 10:26 am
There is a problem with the RTC wakeup, the timing depends on the calibration value and is therefore incorrect... Sorry for the long text, but I am kinda stuck...
I am working on a code to self-adjust the RTC on the PyBoard.
The code uses a PPS signal from a GPS to calculate the calibration value of the RTC, which is set with rtc.calibration(value).
This value has a range from -511 to +512, and effectively adds or subtracts this number of pulses over a period of 32 seconds from the quarz oscillation, running at 32768Hz.
That works really well, and my development Pyboard currently uses a correction value of -293, and keeps it's time pretty stable (~1-5 seconds/day deviation).
I want the RTC to generate an interrupt precisely every second using rtc.wakeup(milliSeconds, IRQhandler) with a millisecond value of 1000.
The IRQhandler fetches the current time from the RTC and prints it (using the scheduler), and that's how it looks:
22.01.2019 22:51:57:99 0
The 99 represent 1/100 seconds, and the 0 is the subsecond value. Regardless the correction value, this timestamp should never change!
Now look at these values:
22.01.2019 22:52:57:98 4 This is the timestamp after 1 minute - a deviation of 1/64 second.
22.01.2019 23:00:57:85 38 And this is the timestamp after 9 minutes - we are 148 milliseconds off, compared to the start time.
Remember, the RTC is adjusted.
I then tested with another pyboard and also played with the correction value for the RTC - there is no deviation over hours, when the correction value is 0!
In other words, I never get a precise, repeated interrupt with the RTC.
Sidenote:
For another part of the project the RTC should wake up the µC at a given date/time, but this functionality is not implemented. According to the datasheet this is possible, but the Python implementation currently does not support this
.
I am working on a code to self-adjust the RTC on the PyBoard.
The code uses a PPS signal from a GPS to calculate the calibration value of the RTC, which is set with rtc.calibration(value).
This value has a range from -511 to +512, and effectively adds or subtracts this number of pulses over a period of 32 seconds from the quarz oscillation, running at 32768Hz.
That works really well, and my development Pyboard currently uses a correction value of -293, and keeps it's time pretty stable (~1-5 seconds/day deviation).
I want the RTC to generate an interrupt precisely every second using rtc.wakeup(milliSeconds, IRQhandler) with a millisecond value of 1000.
The IRQhandler fetches the current time from the RTC and prints it (using the scheduler), and that's how it looks:
22.01.2019 22:51:57:99 0
The 99 represent 1/100 seconds, and the 0 is the subsecond value. Regardless the correction value, this timestamp should never change!
Now look at these values:
22.01.2019 22:52:57:98 4 This is the timestamp after 1 minute - a deviation of 1/64 second.
22.01.2019 23:00:57:85 38 And this is the timestamp after 9 minutes - we are 148 milliseconds off, compared to the start time.
Remember, the RTC is adjusted.
I then tested with another pyboard and also played with the correction value for the RTC - there is no deviation over hours, when the correction value is 0!
In other words, I never get a precise, repeated interrupt with the RTC.
Sidenote:
For another part of the project the RTC should wake up the µC at a given date/time, but this functionality is not implemented. According to the datasheet this is possible, but the Python implementation currently does not support this
.