AN3133 datasheet(13/43 Pages) STMICROELECTRONICS

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AN3133

Real-time clock overview

Doc ID 16956 Rev 5

13/43

The RTC remains active in Low Power Run, Low Power Wait and Active-halt mode only if

the clock source is LSI or LSE. If the RTC clock is HSI or HSE, and the HALT instruction is

executed, the RTC is stopped (since the HSI and HSE clocks are stopped in Halt mode) and

cannot wake up the device.

Refer to the low power modes section of the STM8L05xx, STM8L15xx, STM8L162x,

STM8AL31xx and STM8AL3Lxx microcontroller family reference manual (RM0031) for more

details about low power modes.

1.7

Signals generated by RTC

The RTC peripheral has 2 outputs:

●

RTC_CALIB: it can be used to generate an external clock.

●

RTC_ALARM: unique output resulting from the multiplexing of the RTC alarm and

wakeup events.

1.7.1

RTC_CALIB output

The RTC_CALIB output is used to generate a variable-frequency signal. Depending on the

user application, this signal can play the role of a reference clock to calibrate an external

device, or be connected to a buzzer to generate a sound.

The signal frequency is configured through the 6 LSB bits (PREDIV_A [5:0]) of the

Asynchronous prescaler register, RTC_APRER.

When COSEL=0 (512Hz output), RTC_CALIB is the output of the 5th stage of the 6-bit

asynchronous prescaler. So if PREDIV_A[5]=0, no signal is output on RTC_CALIB.

When RTCCLK frequency is 32.768kHz and PREDIV_A[6:0] = 0x7F, RTC_CALIB frequency

is 512Hz.

When COSEL=1(1Hz output). RTC_CALIB is the output of the 8th stage of the 15-bit

synchronous prescaler. So if PREDIV_A[6:0] = 0x7F and PREDIV_S[15:0] = 0xFF,

RTC_CALIB frequency is 1Hz.

Table 3.

Low power modes where RTC is actor

Mode

Entry

Oscillator

CPU

Peripherals

status

wake up

Wait mode

WFI/WFE(1)

There is no event associated to the RTC. As a consequence, the interrupt is served both in WFE and WFI

modes.

Internal or external

event, reset

Low power run

mode

Software

sequence

LSI or LSE clock

Software sequence,

reset

Low power wait

mode

Software

sequence

+ WFE

LSI or LSE clock

OFF

Internal or external

event, reset

Active-halt

mode

HALT(2)

Before executing the HALT instruction, the application must clear all pending peripheral interrupts by

clearing the corresponding interrupt bit in the peripheral configuration register. Otherwise, the HALT

instruction is not executed and program execution continues.

Off except LSI

or LSE clock

OFF

OFF except RTC

and possibly LCD

External interrupts,

RTC interrupt, reset

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