ZEUS Technical Manual

Detailed hardware description

 

 

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 I2C bus of the PXA270 processor and is accessible through I2C 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.

•I2C 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.

© 2007 Eurotech Ltd Issue D

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