STM32F107VC/Using the RTC Real Time Clock

Preparing for programming the STM32F107VC Real Time Clock.

//Define where in the memory the RTC start address peripheral is located
#define RTC_BASE         0x40002800 // See reference manual page 52

//Define the RTC register map. See reference manual section 18.4.7 page 474
unsigned short int volatile * const rtc_crh  = (unsigned short int *) RTC_BASE + 0x0;
unsigned short int volatile * const rtc_crl  = (unsigned short int *) RTC_BASE + 0x4;
unsigned short int volatile * const rtc_prlh = (unsigned short int *) RTC_BASE + 0x8;

//Initialize the RTC peripheral. See reference manual section 18 page 463
void rtc_init(void) {
 // Your code here
}

Initializing the RTC

Goals

Enabling the RTC

One seconds TICK

No interrupts enable

Initializing second counter to present time (Epoch 1/1-1970 00:00:00)

Using external XTAL

Write protect against accidental writes

Enabling the RTC

1. To enable the RTC it is necessary to enable the Power interface in the RCC->APB1ENR register by setting the PWREN bit to 1.
2. To disable write protection to the Backup domain control register - enabling configuration of the RTC in the PWR->CR register setting the DBP bit to 1.
3. To select LSE (Low Speed External XTAL) as clocksource in the RCC->BDCR register bits RTCSEL[1:0] = 01
4. To turn on the LSE in the RCC->BDCR register bit LSEON = 1.
5. Wait in while loop (timed out for error check) for the LSE to be ready in RCC->BDCR register bit LSERDY.
6. Setting the prescaler of the RTC counter - assuming a 32,768 KHz XTAL - in register RTC->PRLH = 0 and RTC->PRLL = 0x7fff.
7. Setting the Counter to the current time in seconds since epoch. (Set the RTC->CNTH before the RTC->CNTL avoiding RTC->CNTL = 0xffff incrementing RTC->CNTH before writing to it.)
8. To enable write protection to the Backup domain control register - disableing configuration of the RTC in the PWR->CR register setting the DBP bit to 0.

Links

• Const and volatile type qualifiers