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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 real-

time 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 stand-

by mode to conserve power. The alarm can be set to wake up the system at certain time inter-

vals to perform other tasks or to repeat the process. For example, in an access control appli-

cation, 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 os-

cillator 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

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.

20-bit Prescaler register

RTC (32-bit) programmable Counter Registers

32.768 kHz

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