Figure 237. Rtc Block Diagram; Rtc Functional Description - STMicroelectronics STM32F405 Reference Manual

RM0090
1. On STM32F4xx devices, the RTC_AF1 and RTC_AF2 alternate functions are connected to PC13 and PI8,
respectively.
26.3

RTC functional description

26.3.1 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
and STM32F415xx/17xx(RCC)
and STM32F43xxx
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
•
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.
Note: 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.

Figure 237. RTC block diagram

Section 7: Reset and clock control for STM32F405xx/07xx
and Section 6: Reset and clock control for STM32F42xxx
(RCC).
Figure 237: RTC block
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diagram):
Real-time clock (RTC)
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