How to use the RTC clock with the STM32 using HSE with PLL - stm32

Question

I am using the stm32F0xx series and am trying to get the RTC to work. I have an external 8MHz crystal connected and using PLL to create a sysclk of 48MHz. Obviously I would like to use this clock with the RTC. I have tried the following:
//(1) Write access for RTC registers
//(2) Enable init phase
//(3) Wait until it is allow to modify RTC register values
//(4) set prescaler,
//(5) New time in TR
//(6) Disable init phase
//(7) Disable write access for RTC registers
RTC->WPR = 0xCA; //(1)
RTC->WPR = 0x53; //(1)
RTC->ISR |= RTC_ISR_INIT; //(2)
while ((RTC->ISR & RTC_ISR_INITF) != RTC_ISR_INITF) //(3)
{
//add time out here for a robust application
}
RCC->BDCR = RCC_BDCR_RTCSEL_HSE;
RTC->PRER = 0x007C2E7C; //(4)
RTC->TR = RTC_TR_PM | 0x00000001; //(5)
RTC->ISR &=~ RTC_ISR_INIT; //(6)
RTC->WPR = 0xFE; //(7)
RTC->WPR = 0x64; //(7)
In the main loop there is an infinite for that turns two led's on and off. Without the RTC config this works fine but as soon as I add in the code above it stops working.
If I do this then the rest of the code breaks. Can I use HSE and if so am I using the prescalar correctly?

Answer 1

This example from actual working code for using HSE for RTC at STM32f429. It uses STM HAL software library, but can gives you a clue to solve.
Please note, that HSE already must be configured and used as frequency source before this code.
Remark: when reading, you should read not just time but also date.
i.e.:
HAL_RTC_GetTime(&RTChandle, &RTCtime, FORMAT_BIN); //first
HAL_RTC_GetDate(&RTChandle, &RTCdate, FORMAT_BIN); //second, even if you dont required
otherwise registers stay frozen (in this case you see ticks only under debugger but not in real run, because debug reads both registers)
// enable access to rtc register
HAL_PWR_EnableBkUpAccess();
// 1. 8Mhz oscillator (Source crystal! Not after PLL!) div by 8 = 1 Mhz
__HAL_RCC_RTC_CONFIG(RCC_RTCCLKSOURCE_HSE_DIV8);
RTChandle.Instance = RTC;
RTChandle.Init.HourFormat = RTC_HOURFORMAT_24;
// 2. (1 Mhz / 125) = 7999 ticks per second
RTChandle.Init.AsynchPrediv = 125 - 1;
RTChandle.Init.SynchPrediv = 8000 - 1;
RTChandle.Init.OutPut = RTC_OUTPUT_DISABLE;
RTChandle.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
RTChandle.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;
// do init
HAL_RTC_Init(&RTChandle);
// enable hardware
__HAL_RCC_RTC_ENABLE();