Tamper Pin Output for RTC

Question

Posted on July 25, 2013 at 14:15I am using STM32VLDISCOVERY board (with STM32F100RB) to develop a basic real time clock application. Since I do not have a 32kHz crystal for now, I am using LSI clock instead of LSE.When I debug the code, I can see RTC counter increments by 1 in each 1 sec, which means it should be working fine but when I try to get tamper pin output of RTC with the code below*, I get a square wave with 652Hz frequency instead of ~511Hz.*Code:BKP_TamperPinCmd(DISABLE);BKP_RTCOutputConfig(BKP_RTCOutputsource_CalibClock);(By the way,  to get a proper 1 second time, I set prescalar value to 41942 but the frequency of the output from tamper pin does not change even if I make the prescaler value 32767.) #rtc-tamper-pin-output-signal #rtc

Tesla DeLorean · Answer

Posted on July 25, 2013 at 16:49I don't think the pin output has anything to do with the prescaler, it's surely the raw clock source divided down by 64, not 32768 or whatever. Yes, the LSI is in the order of 40 KHz (37?), and it's not particularly stable either. The connection of the clock on the tamper pin suggest it's RTC_CLOCK, but I don't see a diagram, it could conceivably be just LSI, but you'd want to check you have the correct RTC source selected. On the F2/F4 designs is easier to benchmark the LSI/LSE directly against the HSE, which gives you good measurements based on the accuracy of the crystal/source being used there.

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