STLED325 datasheet(19/62 Pages) STMICROELECTRONICS

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STLED325

Functional description

Doc ID 17576 Rev 1

19/62

3.11

Real time clock (RTC)

The STLED325 integrates a low power Serial RTC with a built-in 32.768 kHz oscillator

(external crystal controlled). Eight bytes of the SRAM are used for the clock/calendar

function and are configured in binary coded decimal (BCD) format. An additional 12 bytes of

SRAM provide status/ control of alarm and watchdog functions. Addresses and data are

transferred serially via a two line, bi-directional I2C interface. The built-in address register is

incremented automatically after each WRITE or READ data byte. Note that all 4 digits must

be enabled before using the RTC display.

Functions available to the user include a non-volatile, time-of-day clock/calendar, alarm

interrupts and watchdog timer. The eight clock address locations contain the century, year,

month, date, day, hour, minute, second and tenths/hundredths of a second in 24 hour BCD

format. Corrections for 28, 29 (leap year - valid until year 2100), 30 and 31 day months are

made automatically.

The RTC operates as a slave device through the slave address of the STLED325 on the

serial bus. Access is obtained by implementing a start condition followed by the correct

device slave address. The 16 bytes contained in the device can then be accessed

sequentially in the following order:

–1. Reserved

– 2. Seconds register

– 3. Minutes register

– 4. Hours register

– 5. Day register

– 6. Date register

– 7. Century/month register

– 8. Year register

– 9. Calibration register

– 10. Watchdog register

– 11 - 16. Alarm registers

The RTC keeps track of the date and time. Once the date and time are set, the clock works

when the STLED325 is in normal operation and standby operation. Wake-up alarm feature

is also included in the RTC module. The accuracy of the RTC is approximately 20 ppm

(±50secs/month). How-ever this much depends on the accuracy of the external crystal

used.

The wake-up alarm is programmed to wake up once the date and time set are met. This

feature is present in normal and standby mode of operation. Only one date and time is

available for setting.

The real time clock (RTC) uses an external 32.768 kHz quartz crystal to maintain an

accurate internal representation of the second, minute, hour, day, date, month, and year.

The RTC has leap-year correction. The clock also corrects for months having fewer than 31

days.

3.11.1

Reading the real time clock

The RTC is read by initiating a Read command and specifying the address corresponding to

the register of the real time clock. The RTC registers can then be read in a sequential read

mode. Alarms occurring during a read are unaffected by the read operation.

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