AN3371 datasheet(11/45 Pages) STMICROELECTRONICS

11 / 45 page

AN3371

Overview of the STM32 advanced RTC

Doc ID 018624 Rev 5

11/45

Configuring the alarm behavior using the MSKx bits

The alarm behavior can be configured using the MSKx bits (x = 1, 2, 3, 4) of the

RTC_ALRMAR register for alarm A (RTC_ALRMBR register for alarm B).

Table 5 shows all the possible alarm settings. As an example, to configure the alarm time to

23:15:07 on Monday (assuming that the WDSEL = 1), MSKx bits must be set to 0000b.

When the WDSEL = 0, all cases are similar, except that the Alarm Mask field compares with

the day number and not the day of the week, and MSKx bits must be set to 0000b.

Table 4.

Steps to configure the alarm

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 alarm A

Clear ALRAE(1) bit in

RTC_CR register.

Respectively ALRBE bit for alarm B.

Check that the RTC_ALRMAR

Poll ALRAWF(2) bit until

it is set in RTC_ISR.

Respectively ALRBWF bit for alarm B.

It takes approximately two

RTCCLK clock cycles (clock

synchronization).

Configure the alarm

Configure

RTC_ALRMAR(3)

Respectively RTC_ALRMBR register for alarm B.

The alarm hour format must be

the same(4) as the RTC

Calendar in RTC_ALRMAR.

As an example, if the alarm is configured to occur at 3:00:00 PM, the alarm will not occur even if the

calendar time is 15:00:00, because the RTC calendar is 24-hour format and the alarm is 12-hour format.

Re-enable alarm A

Set ALRAE(5) bit in

RTC_CR register.

Respectively ALRBE bit for alarm B.

RTC alarm registers can only be written when the corresponding RTC alarm is disabled or during RTC

Initialization mode.

Enable the RTC registers Write

protection

Write "0xFF" into the

RTC_WPR register

RTC registers can no longer be

modified

Table 5.

Alarm combinations

MSK3 MSK2 MSK1 MSK0

Alarm behavior

All fields are used in alarm comparison:

Alarm occurs at 23:15:07, each Monday.

Seconds do not matter in alarm comparison

The alarm occurs every second of 23:15, each Monday.

Minutes do not matter in alarm comparison

The alarm occurs at the 7th second of every minute of 23:XX, each

Monday.

Minutes and seconds do not matter in alarm comparison

Hours do not matter in alarm comparison

Hours and seconds do not matter in alarm comparison

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