RTC timing is slower up to 10 seconds every minute.

Question

Hello,I run with eval board RTC_timestamp example:en.stm32cubeh7\STM32Cube_FW_H7_V1.3.0\Projects\STM32H743I_EVAL\Examples\RTC\RTC_TimeStamp\EWARMSurprisingly, there is a shift delay of ~9 to 10 seconds (!) every 60 seconds.I tested it as following:use stop watcher clock reference for measurement
started the example and put a breakpoints just after HAL_RTC_GetDate
static void RTC_CalendarShow(void)
{
 RTC_DateTypeDef sdatestructureget;
 RTC_TimeTypeDef stimestructureget;
 
 /* Get the RTC current Time */
 HAL_RTC_GetTime(&RtcHandle, &stimestructureget, RTC_FORMAT_BIN);
 /* Get the RTC current Date */
 HAL_RTC_GetDate(&RtcHandle, &sdatestructureget, RTC_FORMAT_BIN);

/* Display time Format : hh:mm:ss */
--> nreakpoimt here !!! sprintf((char*)aShowTime,"%.2d:%.2d:%.2d", stimestructureget.Hours, stimestructureget.Minutes, stimestructureget.Seconds);I see that diff time between real time and the time from stm32 RTC is 10 seconds .STM32 RTC time is slower by 10 seconds every 1 minute !Thank you for any suggestion,ran

S.Ma · Accepted Answer

Check the clock source and tolerances. RTC would work ideally with 32.768kHz LSE clock instead of the LSI.

Tesla DeLorean · Answer

Check Prescalers vs clock source, the LSI does not clock at 32.768 KHz, more likely to be 32 KHz. This wouldn't account for such a gross error.

Don't breakpoint the code. The debugger might stop clocks or distort time via DBGMCU settings.

Use the 1ms SysTick against a crystal source to count off 1000 ms

Or use a GPS 1PPS to generate an interrupt

You might also be able to route LSI or LSE out via MCO pins, or against an internal timer.

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