Wake Up Timer

The wakeup timer is a programmable 16 bit auto-reload down-counter.
The wakeup timer range can be extended to 17 bits.

The wakeup timer can be used to generate a periodic interrupt through the wakeup timer flag (WUTF). Additionally the WUTF flag can be output (through signals WKUP, RTC_ALARM and on to RTC_AF1) to pin PC13.

The wakeup timer clock input can be

RTCCLK (usually 32.768 kHz) divided by 2, 4, 8 or 16.
As a result this allows wakeup interrupt periods from 122 us to 32 s, with a resolution down to 61 us.
ck_spre (usually 1 Hz internal clock).
As a result this allows wakeup interrupt periods from 1 s to around 36 hours.

Programming Instructions

RTC register write protection

After a reset the backup domain is write protected. The backup domain encompasses: RTC, LSE oscillator, backup SRAM, I/O pins PC13 to PC15 and PI8. The backup domain is powered through VBAT even if VDD is turned off.
Unlock access to backup domain:

Set the DBP bit (disable backup protection) in PWR→CR.

After a backup domain reset (not a system reset) most of the RTC registers are write protected.
Disable the write protection by writing the unlock key to RTC→WPR:

Write 0xCA to RTC→WPR.
Write 0x53 to RTC→WPR.

Enable RTC clock

The RTC clock is disabled by default. Select a clock source and enable the RTC clock.
Make sure that the clock source is available while in power down mode.

Setup / enable an appropriate clock source, e.g. LSI.
Select clock source for RTCCLK in RCC→BDCR.
Enable RTCCLK in RCC→BDCR.

Setup wakeup timer

To configure the auto wakeup timer proceed as follows:

Disable wakeup timer in RTC→CR.
Wait until configuration of RTC is allowed.
Check corresponding bit in RTC→ISR.
Program prescaler value in RTC→CR.
Program reload value in RTC→WUTR.
Enable wakeup interrupt in RTC→CR.

To use the wakeup interrupt, you need to configure the EXTI channel
and enable it in the NVIC:

Clear any pending wakeup interrupt flag in RTC→ISR.
Configure the dedicated EXTI interrupt channel for the RTC.
I.e. select edge and enable.
Enable the RTC wakeup interrupt in the NVIC.

Start the wakeup timer:

Enable wakeup timer in RTC→CR.

Reenable RTC write protection:

Write 0xff to RTC→WPR.

Configuration Registers

RTC_CR - Configuration register

WUCKSEL	000	f_WUT ⇒ RTCCLK/16 (reset state)	10x	f_WUT ⇒ ck_spre (1Hz)
	001	f_WUT ⇒ RTCCLK/8
	010	f_WUT ⇒ RTCCLK/4	11x	f_WUT ⇒ ck_spre (1Hz), 2¹⁶ added to WUTR
	011	f_WUT ⇒ RTCCLK/2
WUTE	0	Wake up timer disabled (reset state)
	1	Wake up timer enabled
WUTIE	0	Wake up timer interrupt disabled (reset state)
	1	Wake up timer interrupt enabled

RTC_WUTR - Wakeup timer register

Status Registers

RTC_ISR - Initialization and status register

WUTWF*	0	Wake up timer configuration not allowed
	1	Wake up timer configuration allowed
WUTF		Wake up timer flag is set by hardware when wakeup counter reaches 0 and cleared by software by writing 0.

* This bit is set, after the WUTE bit (control register) has been set to 0, and reset by hardware.

Back domain control register

RCC_BDCR - Back domain control register

RTCEN*	0	RTC clock disabled
RTCEN*	1	RTC clock enabled
RTCSEL	00	No clock
	01	LSE oscillator clock used as the RTC clock
	10	LSI oscillator clock used as the RTC clock
	11	HSE oscillator clock divided by a programmable prescaler(selection Through RTCPRE[4:0] bits in the RCC clock configuration register (RCC_CFGR)) used as the RTC clock