AN3371 datasheet(14/45 Pages) STMICROELECTRONICS

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Overview of the STM32 advanced RTC

AN3371

14/45

Doc ID 018624 Rev 5

1.3

RTC periodic wakeup unit

Like many STMicroelectronics microcontrollers, the STM32 provides several low power

modes to reduce the power consumption.

The STM32 features a periodic timebase and wakeup unit that can wake up the system

when the STM32 operates in low power modes. This unit is a programmable downcounting

auto-reload timer. When this counter reaches zero, a flag and an interrupt (if enabled) are

generated.

The wakeup unit has the following features:

●

Programmable downcounting auto-reload timer.

●

Specific flag and interrupt capable of waking up the device from low power modes.

●

Wakeup alternate function output which can be routed to RTC_ALARM output (unique

pad for alarm A, alarm B or Wakeup events) with configurable polarity.

●

A full set of prescalers to select the desired waiting period.

1.3.1

Programming the Auto-wakeup unit

Table 7 describes the steps required to configure the Auto-wakeup unit.

Table 7.

Steps to configure the Auto-wakeup unit

Step

What to do

How to do it

Comments

Disable the RTC registers Write protection

Write "0xCA" and

then "0x53" into the

RTC_WPR register

RTC registers can be

modified

Disable the wakeup timer.

Clear WUTE bit in

RTC_CR register

Ensure access to Wakeup auto-reload

counter and bits WUCKSEL[2:0] is allowed.

Poll WUTWF until it

is set in RTC_ISR

It takes approximately

2 RTCCLK clock cycles

Program the value into the wakeup timer.

Set WUT[15:0] in

RTC_WUTR register See Section 1.3.2:

Maximum and minimum

RTC wakeup period

Select the desired clock source.

Program

WUCKSEL[2:0] bits

in RTC_CR register

Re-enable the wakeup timer.

Set WUTE bit in

RTC_CR register

The wakeup timer

restarts downcounting

Enable the RTC registers Write protection

Write "0xFF" into the

RTC_WPR register

RTC registers can no

more be modified

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