ST AN1780 Application note

AN1780

APPLICATION NOTE

STR71X REAL TIME CLOCK APPLICATION EXAMPLE

INTRODUCTION

The purpose of this application note is to explain how to use the STR71x Real Time Clock (RTC) peripheral. As an application example, it demonstrates how to setup the RTC peripheral, in terms of prescaler and interrupts, to be used to keep the time and date and to generate Alarm interrupts.The STR71x RTC has a 32-bit binary counter register. The 32-bit binary counters are designed to continuously count time in seconds. This application note addresses how to convert the 32-bit time value into a date and time value that can be put in the followingform MM/DD/YYYY, HH:MM:SS.

The date used in this application note is the same used with the UNIX operating system, the reference date January 1, 1970, often referred to as Unix Epoch.

This document in organized in two sections, the first presents the STR71x RTC peripheral and how to configure it and generate a fixed time base, the second part describes the related software required for the application.

This application note deals with various techniques for keeping time with the STR71x.

For more information about the STR71x RTC peripheral refer to STR71x Reference Manual.

Note: Because of the reference date used and the use of a 32-bit counter, this algorithm rolls over on Tuesday, January 19, 03:14:07, 2038.

The following application note developed using RVDK Toolchain V1.6.1 and the STR710 Evalboard.

You must have the ARM Real View Developer Kit to get the most out of this Application Note.

AN1780/1204

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STR71X REAL TIME CLOCK APPLICATION EXAMPLE

1 WHAT IS A REAL TIME CLOCK?

A real time clock is a clock that keeps track of the time even when the system is turned off. In contrast, clocks that are not real-time do not function when the system is off. Most of the realtime clocks operate at 32.768KHz.

2 APPLICATIONS—TIME AND ATTENDANCE

Real Time Clocks are usually found in portable systems such as a data collection terminals and smart card readers which are required to keep track of the day and time of certain tasks taking place. After the tasks are completed, usually the portable system will return to a standby mode to conserve power. The alarm can be set to wake up the system at certain time intervals to perform other tasks or to repeat the process. For example, in an access control application, when someone tries to access the building through certain doors, the day and time of the entry is recorded and this information can be used for accounting and security purposes, etc.

3 STR71X RTC PERIPHERAL

The RTC peripheral implemented in the STR71x family is a 32-bit continuously running counter that can be used, with a few configuration parameters, to provide a precise clock-cal- endar function.

The RTC peripheral is mapped on the APB2 bridge and clocked by an external 32.768 kHz oscillator through a 32-bit prescaler register, this prescaler can be used to vary the RTC clock from 32.768 kHz to 0.5 Hz.

Figure 1. RTC Peripheral

32.768 kHz

20-bit Prescaler register

RTC (32-bit) programmable Counter Registers

In this application, the RTC counter registers have to be incremented every second, this can be configured by using a prescaler value of 0x8000 (32768) to slow down the RTC clock from 32768 Hz to 1 Hz. This allows the RTC to keep time in seconds in the 32-bit counter.

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STR71X REAL TIME CLOCK APPLICATION EXAMPLE

4 CONFIGURATION MODE

To write in the RTC_PRL, RTC_CNT, RTC_ALR registers, the peripheral must enter Configuration mode. This is done by setting the CNF bit in the RTC_CRL register.

In addition, writing to any RTC register is only enabled if the previous write operation is finished.

To enable the software to detect this situation, the RTOFF status bit is provided in the RTC_CR register to indicate that an update of the registers is in progress. A new value can be written to the RTC counters only when the RTOFF status bit value is ’1’.

Configuration Procedure:

1.Poll RTOFF, wait until its value goes to ‘1’

2.Set CNF bit to enter configuration mode

2.Write to one or more RTC registers

3Clear CNF bit to exit configuration mode

The write operation only executes when the CNF bit is cleared and it takes at least two Clock32 cycles to complete.

5 32-BIT COUNTER TIME CONVERSION

This application note explains how to convert the 32-bit time value into a date and time value that can be put in the form of MM/DD/YYYY, HH:MM:SS. Many functions provided by the ARM compiler used to convert from a date and time to seconds are also described.

These functions are modified in order to explore the STR71x RTC to deliver time and date.

5.1 C LIBRARY FUNCTIONS USED

5.1.1 RTC prescaler Initialisation:

Before start-up we must adjust the STR71x RTC prescaler register value in order to have the RTC counter registers incremented every second, this can be configured by using a prescaler value of 0x8000 (32768) to slow down the RTC clock from 32768 Hz to 1 Hz.

5.1.2 Origin Date Adjust

Since the date used in this application note is the same used with the UNIX operating system, the reference date January 1, 1970, often referred to as Unix Epoch, we must add this number 0x3FF36300 to the value obtained from the STR71x RTC Counter register with intent to have a start date = 01/01/2004.

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