CODEWITHSUNDEEP
stm32stm32f4discoveryreal-time-clock
TracyB
TracyB

Reputation: 31

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

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?

Upvotes: 3

Views: 8497

Answers (1)

FlyTekDev
FlyTekDev

Reputation: 31

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();

Upvotes: 3

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