Table 211. Rtc Functions Over Modes - ST STM32L4 5 Series Reference Manual

RM0351
Pin
PC13
PA0
PE6
PB2
PB15
38.3.3 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 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.
The minimum division factor is 1 and the maximum division factor is 2
This corresponds to a maximum input frequency of around 4 MHz.
f is given by the following formula:
ck_apre
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 is given by the following formula:
ck_spre

Table 211. RTC functions over modes

Functional in all low-
power modes except
RTC functions
Standby/Shutdown
RTC_TAMP1
RTC_TS
RTC_OUT
RTC_TAMP2
RTC_TAMP3
RTC_OUT
RTC_REFIN
Section 6: Reset and clock control
Figure 374: RTC block
f
=
CK_SPRE
DocID024597 Rev 5
Standby/Shutdown
modes
YES
YES
YES
YES
YES
diagram):
f
RTCCLK
f
-------------------------------------- -
=
CK_APRE
PREDIV_A
f
RTCCLK
-----------------------------------------------------------------------------------------------
( ) × (
PREDIV_S 1 PREDIV_A
+
Real-time clock (RTC)
Functional in
Functional in
mode
YES
YES
YES
NO
NO
(RCC).
22
.
1
+
)
1
+
V mode
BAT
YES
YES
YES
NO
NO
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