Rtc Functional Description; Clock And Prescalers - ST STM32F205 series Reference Manual

Real-time clock (RTC)
AFI_TIMESTAMP
RTCCLK
LSE (32.768 Hz)
HSE (1 MHz max) RTC_PRER
LSI
Asyn ch.
7-bit prescaler
(default = 128)
RTC_50Hz
WUCKSEL[1:0]
Prescaler
/2,4,8,16
AFI_TAMPER
1. On STM32F20x and STM32F21x devices, the RTC_AF1 and RTC_AF2 alternate functions are connected
to PC13 and PI8, respectively.
22.3

RTC functional description

22.3.1

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
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 13-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.
The minimum division factor is 2 and the maximum division factor is 2
This corresponds to a maximum input frequency of around 1 MHz.
572/1378
Figure 215. RTC block diagram
ck_apre
(default 256 Hz)
Coarse
RTC_PRER
calibration
Synchronous
13-bit prescaler
(default = 256)
RTC_WUTR
16-bit wakeup
WUCKSEL[2]
auto-reload timer
Section 5: Reset and clock control
Figure 215: RTC block
Time stamp
registe rs
Alarm
(RTC_ALRMAR
=
register)
ck_spre
(default 1 Hz)
Calendar
Shadow registers
(RTC_TR,
ck
RTC_DR)
Alarm
=
(RTC_ALRMBR
register)
Periodic wakeup flag
Backup registe rs
and RTC tamper
control registers
diagram):
RM0033 Rev 8
Time stamp flag
AFO_CALIB
(512 Hz)
Output
RTC_AFO
control
ALRAF
RTC_AFI
AFO_ALARM
RTC_AFI
ALRBF
WUTF
TAMPE
TSE
(RCC).
20
.
RM0033
RTC_AF1
RTC_AF2
ai17724c