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VIPER Technical Manual Detailed hardware description
© 2007 Eurotech Ltd Issue E 26
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
339HBattery backup, page 340H73, 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.