openpilot_comma/panda/board/drivers/rtc.h

#define RCC_BDCR_OPTIONS (RCC_BDCR_RTCEN | RCC_BDCR_RTCSEL_0 | RCC_BDCR_LSEON)
#define RCC_BDCR_MASK (RCC_BDCR_RTCEN | RCC_BDCR_RTCSEL | RCC_BDCR_LSEMOD | RCC_BDCR_LSEBYP | RCC_BDCR_LSEON)

#define YEAR_OFFSET 2000U

typedef struct __attribute__((packed)) timestamp_t {
  uint16_t year;
  uint8_t month;
  uint8_t day;
  uint8_t weekday;
  uint8_t hour;
  uint8_t minute;
  uint8_t second;
} timestamp_t;

uint8_t to_bcd(uint16_t value){
    return (((value / 10U) & 0x0FU) << 4U) | ((value % 10U) & 0x0FU);
}

uint16_t from_bcd(uint8_t value){
    return (((value & 0xF0U) >> 4U) * 10U) + (value & 0x0FU);
}

void rtc_init(void){
    if(board_has_rtc()){
        // Initialize RTC module and clock if not done already.
        if((RCC->BDCR & RCC_BDCR_MASK) != RCC_BDCR_OPTIONS){
            puts("Initializing RTC\n");
            // Reset backup domain
            register_set_bits(&(RCC->BDCR), RCC_BDCR_BDRST);

            // Disable write protection
            register_set_bits(&(PWR->CR), PWR_CR_DBP);

            // Clear backup domain reset
            register_clear_bits(&(RCC->BDCR), RCC_BDCR_BDRST);

            // Set RTC options
            register_set(&(RCC->BDCR), RCC_BDCR_OPTIONS, RCC_BDCR_MASK);

            // Enable write protection
            register_clear_bits(&(PWR->CR), PWR_CR_DBP);
        }
    }
}

void rtc_set_time(timestamp_t time){
    if(board_has_rtc()){
        puts("Setting RTC time\n");

        // Disable write protection
        register_set_bits(&(PWR->CR), PWR_CR_DBP);
        RTC->WPR = 0xCA;
        RTC->WPR = 0x53;

        // Enable initialization mode
        register_set_bits(&(RTC->ISR), RTC_ISR_INIT);
        while((RTC->ISR & RTC_ISR_INITF) == 0){}
        
        // Set time
        RTC->TR = (to_bcd(time.hour) << RTC_TR_HU_Pos) | (to_bcd(time.minute) << RTC_TR_MNU_Pos) | (to_bcd(time.second) << RTC_TR_SU_Pos);
        RTC->DR = (to_bcd(time.year - YEAR_OFFSET) << RTC_DR_YU_Pos) | (time.weekday << RTC_DR_WDU_Pos) | (to_bcd(time.month) << RTC_DR_MU_Pos) | (to_bcd(time.day) << RTC_DR_DU_Pos);

        // Set options
        register_set(&(RTC->CR), 0U, 0xFCFFFFU);
        
        // Disable initalization mode
        register_clear_bits(&(RTC->ISR), RTC_ISR_INIT);

        // Wait for synchronization
        while((RTC->ISR & RTC_ISR_RSF) == 0){}

        // Re-enable write protection
        RTC->WPR = 0x00;
        register_clear_bits(&(PWR->CR), PWR_CR_DBP);
    }
}

timestamp_t rtc_get_time(void){
    timestamp_t result;
    // Init with zero values in case there is no RTC running
    result.year = 0U;
    result.month = 0U;
    result.day = 0U;
    result.weekday = 0U;
    result.hour = 0U;
    result.minute = 0U;
    result.second = 0U;

    if(board_has_rtc()){
        // Wait until the register sync flag is set
        while((RTC->ISR & RTC_ISR_RSF) == 0){}

        // Read time and date registers. Since our HSE > 7*LSE, this should be fine.
        uint32_t time = RTC->TR;
        uint32_t date = RTC->DR;

        // Parse values        
        result.year = from_bcd((date & (RTC_DR_YT | RTC_DR_YU)) >> RTC_DR_YU_Pos) + YEAR_OFFSET;
        result.month = from_bcd((date & (RTC_DR_MT | RTC_DR_MU)) >> RTC_DR_MU_Pos);
        result.day = from_bcd((date & (RTC_DR_DT | RTC_DR_DU)) >> RTC_DR_DU_Pos);
        result.weekday = ((date & RTC_DR_WDU) >> RTC_DR_WDU_Pos);
        result.hour = from_bcd((time & (RTC_TR_HT | RTC_TR_HU)) >> RTC_TR_HU_Pos);
        result.minute = from_bcd((time & (RTC_TR_MNT | RTC_TR_MNU)) >> RTC_TR_MNU_Pos);
        result.second = from_bcd((time & (RTC_TR_ST | RTC_TR_SU)) >> RTC_TR_SU_Pos);
    }
    return result;
}