AN3371 datasheet(7/45 Pages) STMICROELECTRONICS

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AN3371

Overview of the STM32 advanced RTC

Doc ID 018624 Rev 5

7/45

the month, day of the week, month and year. This data can be converted to BCD format and

displayed on a standard LCD, which is useful in countries that use the 12-hour format with

an AM/PM indicator (see Figure 2). Conversion routines use significant program memory

space and are CPU-time consuming, which may be critical in certain real-time applications.

When using the STM32 RTC calendar, software conversion routines are no longer needed

because their functions are performed by hardware.

The STM32 RTC calendar is provided in BCD format. This avoids binary to BCD software

conversion routines, which use significant program memory space and a CPU-load that may

be critical in certain real-time applications.

Figure 2.

Example of calendar display on an LCD

1.1.1

Initializing the calendar

Table 2 describes the steps required to correctly configure the calendar time and date.

11:15:28:09 PM

WED OCT 26 2011

Table 2.

Steps to initialize the calendar

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

Enter Initialization mode

Set INIT bit to ‘1’ in

RTC_ISR register

The calendar counter is

stopped to allow update

Wait for the confirmation of

Initialization mode (clock

synchronization)

Poll INITF bit of in

RTC_ISR until it is set

It takes approximately 2

RTCCLK clock cycles for

medium density devices

Program the prescalers register

if needed

RTC_PRER register:

Write first the

synchronous value and

then write the

asynchronous

By default, the RTC_PRER

prescalers register is initialized

to provide 1Hz to the Calendar

unit when RTCCLK = 32768Hz

Load time and date values in

the shadow registers

Set RTC_TR and

RTC_DR registers

Configure the time format (12h

or 24h)

Set FMT bit in RTC_CR

FMT = 0: 24 hour/day format

FMT = 1: AM/PM hour format

Exit Initialization mode

Clear the INIT bit in

RTC_ISR register

The current calendar counter is

automatically loaded and the

counting restarts after 4

RTCCLK clock cycles

Enable the RTC Registers

Write Protection

Write "0xFF" into the

RTC_WPR register

RTC Registers can no longer

be modified

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