RM0316 Datasheet, PDF (783/1141 Pages) STMicroelectronics – This reference manual targets application developers

RM0316
Real-time clock (RTC)
27.3.3
Note:
Clock and prescalers
The RTC clock source (RTCCLK) is selected through the clock controller among the LSE
clock, the LSI oscillator clock, and the HSE clock. For more information on the RTC clock
source configuration, refer to Section 9: Reset and clock control (RCC).
A programmable prescaler stage generates a 1 Hz clock which is used to update the
calendar. To minimize power consumption, the prescaler is split into 2 programmable
prescalers (see Figure 291: RTC block diagram):
• A 7-bit asynchronous prescaler configured through the PREDIV_A bits of the
RTC_PRER register.
• A 15-bit synchronous prescaler configured through the PREDIV_S bits of the
RTC_PRER register.
When both prescalers are used, it is recommended to configure the asynchronous prescaler
to a high value to minimize consumption.
The asynchronous prescaler division factor is set to 128, and the synchronous division
factor to 256, to obtain an internal clock frequency of 1 Hz (ck_spre) with an LSE frequency
of 32.768 kHz.
The minimum division factor is 1 and the maximum division factor is 2²².
This corresponds to a maximum input frequency of around 4 MHz.
f_{ck_apre} is given by the following formula:
f _{CK_APRE}
=
---------^f--^R----^T---^C----^C---^L---^K----------
PREDIV_A + 1
The ck_apre clock is used to clock the binary RTC_SSR subseconds downcounter. When it
reaches 0, RTC_SSR is reloaded with the content of PREDIV_S.
f_{ck_spre} is given by the following formula:
f _{CK_SPRE}
=
-------------------------------------^f--^R---^T----^C---^C----^L---^K--------------------------------------
(PREDIV_S + 1) × (PREDIV_A + 1)
27.3.4
The ck_spre clock can be used either to update the calendar or as timebase for the 16-bit
wakeup auto-reload timer. To obtain short timeout periods, the 16-bit wakeup auto-reload
timer can also run with the RTCCLK divided by the programmable 4-bit asynchronous
prescaler (see Section 27.3.6: Periodic auto-wakeup for details).
Real-time clock and calendar
The RTC calendar time and date registers are accessed through shadow registers which
are synchronized with PCLK (APB clock). They can also be accessed directly in order to
avoid waiting for the synchronization duration.
• RTC_SSR for the subseconds
• RTC_TR for the time
• RTC_DR for the date
Every two RTCCLK periods, the current calendar value is copied into the shadow registers,
and the RSF bit of RTC_ISR register is set (see Section 27.6.4: RTC initialization and status
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