RTC Public API

Public functions to interact with the RTC peripheral. More...

Collaboration diagram for RTC Public API:

Functions
ERR_te	rtc_init (void)
	Initializes the RTC peripheral.
void	rtc_set_calendar (CALENDAR_ts const *date)
	Sets the RTC calendar (date and weekday).
void	rtc_set_time (TIME_ts const *time)
	Sets the RTC time (hours, minutes, seconds).
void	rtc_get_time (TIME_ts *time)
	Reads the current time from the RTC.

Detailed Description

Public functions to interact with the RTC peripheral.

Function Documentation

rtc_init()

ERR_te rtc_init

(

void

)

Initializes the RTC peripheral.

Enables the PWR and backup domain access, starts the LSE oscillator, selects LSE as the RTC clock source, enables the RTC, and configures the prescalers for a 1 Hz tick (PREDIV_A = 127, PREDIV_S = 255) with 24-hour format.

If the backup register sentinel pattern is already present, the RTC is assumed to be running with a valid configuration and the function returns immediately without modifying the time or calendar.

Returns

• ERR_OK on success
• ERR_MODULE_ALREADY_INITIALIZED if the RTC was already initialized (either by a prior call in this boot or because the backup domain was preserved across a reset)

Note

To force re-initialization and clear the current time, call rcc_reset_bkpd before calling this function.

Typically called indirectly via init_rtc.

See also

rtc_init

Definition at line 49 of file stm32f401re_rtc.c.

                      {
    if(initialized) {
        return ERR_MODULE_ALREADY_INITIALIZED;
    }
 
    // Enable PWR clock to allow access to the backup domain
    rcc_set_pclk_apb1(RCC_APB1ENR_PWREN, ENABLE);
 
    // Enable backup domain write access
    PWR->PWR_CR |= (1 << PWR_CR_DBP);
 
    // Check sentinel: if present, the RTC was already configured before this reset
    if((RTC->RTC_BKPxR[RTC_BKP_DOMAIN_RST_INDICATOR_BKPxR]
        & RTC_BKP_DOMAIN_RST_INDICATOR_PATTERN)
       == RTC_BKP_DOMAIN_RST_INDICATOR_PATTERN)
    {
        return ERR_MODULE_ALREADY_INITIALIZED;
    }
 
    rtc_set_write_protection(DISABLE);
 
    // Start LSE oscillator and wait for it to stabilize
    RCC->RCC_BDCR |= (1 << RCC_BDCR_LSEON);
    while(!(RCC->RCC_BDCR & (1 << RCC_BDCR_LSERDY)));
 
    // Select LSE as the RTC clock source
    RCC->RCC_BDCR &= ~(0x3 << RCC_BDCR_RTCSEL);
    RCC->RCC_BDCR |=  (0x1 << RCC_BDCR_RTCSEL);
 
    // Enable RTC
    RCC->RCC_BDCR |= (1 << RCC_BDCR_RTCEN);
 
    // Enter INIT mode and wait for INITF confirmation
    RTC->RTC_ISR |= (1 << RTC_ISR_INIT);
    while(!(RTC->RTC_ISR & (1 << RTC_ISR_INITF)));
 
    // 24-hour format, prescalers for 1 Hz tick from 32.768 kHz LSE:
    // f_ck_apre = LSE / (PREDIV_A + 1) = 32768 / 128 = 256 Hz
    // f_ck_spre = f_ck_apre / (PREDIV_S + 1) = 256 / 256 = 1 Hz
    RTC->RTC_CR &= ~(1 << RTC_CR_FMT);
    RTC->RTC_PRER =
        (127 << RTC_PRER_PREDIV_A) |
        (255 << RTC_PRER_PREDIV_S);
 
    // Exit INIT mode
    RTC->RTC_ISR &= ~(1 << RTC_ISR_INIT);
    while(RTC->RTC_ISR & (1 << RTC_ISR_INITF));
 
    // Write sentinel to backup register to mark initialization complete
    RTC->RTC_BKPxR[RTC_BKP_DOMAIN_RST_INDICATOR_BKPxR] =
        RTC_BKP_DOMAIN_RST_INDICATOR_PATTERN;
 
    rtc_set_write_protection(ENABLE);
 
    initialized = true;
 
    return ERR_OK;
}

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rtc_set_calendar()

void rtc_set_calendar

(

CALENDAR_ts const *

date

)

Sets the RTC calendar (date and weekday).

Enters RTC INIT mode, writes the date, month, weekday, and year to RTC_DR in BCD format, then exits INIT mode. Write protection is disabled and re-enabled around the operation.

Parameters

[in]

date

Pointer to the calendar structure to write.

Note

Years must be in the range 2000–2099.

See also

rtc_set_calendar

Definition at line 109 of file stm32f401re_rtc.c.

                                               {
    rtc_set_write_protection(DISABLE);
 
    RTC->RTC_ISR |= (1 << RTC_ISR_INIT);
    while(!(RTC->RTC_ISR & (1 << RTC_ISR_INITF)));
 
    RTC->RTC_DR =
        ((date->date % 10)          << RTC_DR_DU)  |
        ((date->date / 10)          << RTC_DR_DT)  |
        ((date->months % 10)        << RTC_DR_MU)  |
        ((date->months / 10)        << RTC_DR_MT)  |
        ((date->week_days)          << RTC_DR_WDU) |
        (((date->year - 2000) % 10) << RTC_DR_YU)  |
        (((date->year - 2000) / 10) << RTC_DR_YT);
 
    RTC->RTC_ISR &= ~(1 << RTC_ISR_INIT);
    while(RTC->RTC_ISR & (1 << RTC_ISR_INITF));
 
    rtc_set_write_protection(ENABLE);
}

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rtc_set_time()

void rtc_set_time

(

TIME_ts const *

time

)

Sets the RTC time (hours, minutes, seconds).

Enters RTC INIT mode, writes hours, minutes, and seconds to RTC_TR in BCD format using DEC_TO_BCD, then exits INIT mode. Write protection is disabled and re-enabled around the operation.

Parameters

[in]

time

Pointer to the time structure to write.

See also

rtc_set_time

Definition at line 131 of file stm32f401re_rtc.c.

                                       {
    rtc_set_write_protection(DISABLE);
 
    RTC->RTC_ISR |= (1 << RTC_ISR_INIT);
    while(!(RTC->RTC_ISR & (1 << RTC_ISR_INITF)));
 
    RTC->RTC_TR =
        (DEC_TO_BCD(time->hours)   << RTC_TR_HU)  |
        (DEC_TO_BCD(time->minutes) << RTC_TR_MNU) |
        (DEC_TO_BCD(time->seconds) << RTC_TR_SU);
 
    RTC->RTC_ISR &= ~(1 << RTC_ISR_INIT);
    while(RTC->RTC_ISR & (1 << RTC_ISR_INITF));
 
    rtc_set_write_protection(ENABLE);
}

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rtc_get_time()

void rtc_get_time

(

TIME_ts *

time

)

Reads the current time from the RTC.

Reads RTC_TR atomically using the shadow register double-read technique: reads TR, reads DR (to unlock the shadow registers), reads TR again, and repeats if the two TR reads differ. Converts BCD values to decimal using BCD_TO_DEC before storing in time.

Parameters

[out]

time

Pointer to the time structure that will receive the current hours, minutes, and seconds.

See also

rtc_get_time

Definition at line 149 of file stm32f401re_rtc.c.

                                 {
    uint32_t tr1, tr2;
 
    // Double-read to ensure consistency: reading DR after TR unlocks the shadow
    // registers and allows TR to be read again safely
    do {
        tr1 = RTC->RTC_TR;
        (void)RTC->RTC_DR;
        tr2 = RTC->RTC_TR;
    } while(tr1 != tr2);
 
    time->seconds = BCD_TO_DEC(tr1 & 0x7F);
    time->minutes = BCD_TO_DEC((tr1 >> RTC_TR_MNU) & 0x7F);
    time->hours   = BCD_TO_DEC((tr1 >> RTC_TR_HU)  & 0x7F);
}

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