Using the STM32 hardware real-time clock (RTC).pdf

AN3371

Application note

Using the STM32 hardware real-time clock (RTC)

Introduction

A real-time clock (RTC) is a computer clock that keeps track of the current time. Although

RTCs are often used in personal computers, servers and embedded systems, they are also

present in almost any electronic device that requires accurate time keeping. Microcontrollers

supporting RTC can be used for chronometers, alarm clocks, watches, small electronic

agendas, and many other devices.

This application note describes the features of the real-time clock (RTC) controller

embedded in Ultra Low Power Medium-density and F-2 series devices microcontrollers,

and the steps required to configure the RTC for use with the calendar, alarm, periodic

wakeup unit and tamper detection.

Examples are provided with configuration information to enable you to quickly and correctly

configure the RTC for calendar, alarm, periodic wakeup unit, tamper detection, time stamp

and calibration applications.

Note: 1 All examples and explanations are based on the STM32L1xx and STM32F2xx firmware

libraries and reference manuals of STM32L1xx (RM0038) and STM32F2xx (RM0033).

2 STM32 refers to Ultra Low Power Medium-density devices and F-2 series devices

microcontrollers in this document.

3 Ultra Low Power Medium (ULPM) density devices are STM32L151xx and STM32L152xx

microcontrollers where the Flash memory density ranges between 64 and 128 Kbytes.

4 F-2 series devices are STM32F205xx, STM32F207xx, STM32F215xx and STM32F217xx

microcontrollers.

May 2011

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Contents

AN3371

Contents

Overview of the STM32 advanced RTC . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1

RTC calendar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1.1

Initializing the calendar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1.2

Delivering a 1 Hz signal to the calendar using different clock sources . . 6

1.2

RTC alarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2.1

Programming the alarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.2.2

Configuring the alarm behavior using the MSKx bits . . . . . . . . . . . . . . . . 9

1.3

RTC periodic wakeup unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.3.1

Programming the Auto-wakeup unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.3.2

Maximum and minimum RTC wakeup period . . . . . . . . . . . . . . . . . . . . 12

1.4

RTC coarse calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.5

RTC reference clock detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.6

Time-stamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.7

Back up registers and tamper detection . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.8

RTC and low-power modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.9

Alternate function RTC outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.9.1

RTC_CALIB output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.9.2

RTC_ALARM output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.10 RTC security aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1.10.1 RTC Register write protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1.10.2 Enter/Exit initialization mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1.10.3 RTC clock synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

RTC features summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

RTC firmware driver API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1

Start with RTC driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1.1

Time and Date configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1.2

Alarm configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.1.3

RTC Wakeup configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.1.4

Outputs configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.1.5

Coarse calibration configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.1.6

TimeStamp configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

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Contents

3.1.7

Tamper configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.1.8

Backup data registers configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2

Function groups and description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Application examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

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Overview of the STM32 advanced RTC

AN3371

Overview of the STM32 advanced RTC

The RTC embedded in STM32 microcontrollers can be used to provide a full-featured

calendar, alarm and periodic wakeup unit, digital calibration and advanced tamper detection.

Refer to Section 2: RTC features summary for the complete list of features available on Ultra

Low Power Medium-density and F-2 series devices.

1.1

RTC calendar

A calendar keeps track of the time (hours, minutes and seconds) and date (day, week,

month, year). The STM32 RTC calendar offers several features to help you easily configure

and display the calendar data fields:

●

Calendar with:

– seconds

–mits

– hours in 12-hour or 24-hour format

– day of the week (day)

– day of the month (date)

–mth

– year

●

Calendar in binary-coded decimal (BCD) format

●

Automatic management of 28-, 29- (leap year), 30-, and 31-day months

●

Daylight saving time adjustment programmable by software

Figure 1. RTC calendar fields

DATE

12h or 24h

format

TIME

Date

Week

date

Month

Year

RTC_DR

RTC_TR

MS19524V1

1. RCT_DR, RTC_TR are RTC Date and Time registers.

A software calendar can be a software counter (usually 32 bits long) that represents the

number of seconds. Software routines convert the counter value to hours, minutes, day of

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

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Overview of the STM32 advanced RTC

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.

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

11:15:28 PM

WED NOV 19 2010

1.1.1

Initializing the calendar

Ta b l e 1 describes the steps required to correctly configure the calendar time and date.

Table 1.

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

Write first the

synchronous value and

then write the

asynchronous

By default, the prescalers

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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