VIPER
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 DS1307 RTC should be used to keep the time and date. Under
Windows CE .NET 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 DS1307 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 DS1307
RTC:
PXA255 Pin
Function
GPIO84
Clock (100MHz max)
GPIO83 Data
The DS1307 RTC also contains 64Bytes of RAM, which can be used for any user data
that needs to be recoverable on power-up.
To ensure the DS1307 RTC doesn’t lose track of the date and time when the 5V
supply is powered-down, an external battery must be fitted. See the section
, for details.
© 2004 Arcom Issue H
21
