User manual

Detailed hardware description

Issue E 31

Real time clock

The ZEUS uses an external real time clock (RTC) (Intersil ISL1208) to store the date and

time, and provide power management events. The RTC is connected to the I

C bus of

the PXA270 processor and is accessible through I

C bus address 0x6F. The RTC is

battery backed.

The accuracy of the RTC is based on the operation of the 32.768 KHz watch crystal. The

calibration tolerance is ±20ppm, which provides an accuracy of +/-1 minute per month

when the board is operated at an ambient temperature of +25°C (+77°F). When the

board is operated outside this temperature the accuracy may be degraded by

-0.035ppm/

C² ±10% typical. The watch crystal’s accuracy will age by ±3ppm max in the

first year, then ±1ppm max in the year after, and logarithmically decrease in subsequent

years.

The Intersil ISL1208 RTC provides the following basic functions:

• Real time clock/calendar:

- Tracks time in hours, minutes and seconds.

- Day of the week, day, month and year.

• Single alarm:

- Settable to the second, minute, hour, day of the week, day or month.

- Single event or pulse interrupt mode.

• 2 bytes battery-backed user SRAM.

• I

C interface.

PXA270 has an internal real time clock, which doesn’t keep time after hardware reset,

and should only be used as a wake-up source from deep-sleep.