VIPER Technical Manual

Detailed hardware description

 

 

Real time clock

There are two RTCs on the VIPER. Under embedded Linux and VxWorks the internal RTC of the PXA255 should only be used for power management events, and an external Dallas DS1338 RTC should be used to keep the time and date. Under Windows CE the time and date stamps are copied from the external RTC to the internal RTC of the PXA255, to run the RTC internally.

The accuracy of the DS1338 RTC is based on the operation of the 32.768KHz watch crystal. Its calibration tolerance is ±20ppm, which provides an accuracy of +/-1 minute per month if the board is in an ambient environment of +25°C. When the board is operated outside this temperature then 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 decreasing in subsequent years.

The following PXA255 GPIO pins are used to emulate the I²C interface to the DS1338 RTC:

PXA255 Pin

Function

GPIO84

 

 

Clock (100kHz max)

 

 

GPIO83

Data

 

 

 

 

The DS1338 RTC also contains 56 bytes of RAM, which can be used for any user data that needs to be recoverable on power-up.

To ensure the DS1338 RTC doesn’t lose track of the date and time when the

5V supply is powered-down, the onboard battery must be fitted. See the section

Battery339Hbackup, page 73340H, for details.

Watchdog timer

The PXA255 contains an internal watchdog timer, which can be used to protect against erroneous software. Timeout periods can be adjusted from 271ns to 19 minutes 25 seconds. When a timeout occurs the board is reset. On reset the watchdog timer is disabled until enabled again by software.

For further details see the Eurotech Operating System Technical Manual and the PXA255 Developer’s Manual on the Development Kit CD.

© 2007 Eurotech Ltd Issue E

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