AN3133 datasheet(17/43 Pages) STMICROELECTRONICS

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AN3133

Programming the RTC

Doc ID 16956 Rev 5

17/43

Programming the RTC

2.1

Initializing the calendar

Table 4 describes the steps required to correctly configure the calendar time and date.

2.2

Programming the alarm

Table 5 describes the steps required to configure the alarm.

Table 4.

Steps to initialize the calendar

Step

What to do

How to do it

Comments

Enter Initialization mode.

Set INIT bit to ‘1’ in

RTC_ISR1 register.

The calendar counter is

stopped to allow update.

Wait for the confirmation of

Initialization mode (clock

synchronization).

Poll INITF bit of in

RTC_ISR1 until it is set.

It takes approximately 2

RTCCLK clock cycles for

medium density devices.

Program the 3 prescaler

registers if needed.

Registers RTC_APRER

and RTC_SPRERx.

Load time and date values in

the shadow registers.

Set RTC_TRx and

RTC_DRx registers.

Configure the time format (12h

or 24h).

Set FMT bit in

RTC_CR1 register.

Exit Initialization mode.

Clear the INIT bit in

RTC_ISR1 register.

The current calendar counter is

then automatically loaded and

the counting restarts after 4

RTCCLK clock cycles.

Table 5.

Steps to configure the alarm

Step

What to do

How to do it

Comments

Disable the alarm.

Clear ALRAE bit in

RTC_CR2 register.

Check that the RTC_ALRMARx

registers can be accessed.

Poll ALRAWF bit until it

is set in RTC_ISR1.

It takes approximately 2

RTCCLK clock cycles (clock

synchronization).

On medium+ and high density,

there is no synchronization

time to wait for.

Configure the alarm.

Configure

RTC_ALRMARx

registers.

The alarm hour format must be

the same as the RTC Calendar

in RTC_ALARM3(1).

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 the alarm.

Set ALRAE bit in

RTC_CR2 register.

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