in a datalogger application we are using:

PIC 16C715
CCS C-compiler
PCF8583 RTC
crystal oscillator at 4 MHz

To save battery lifetime the device is mostly in sleep mode. We use the WDT
to wake it up periodically. When a wakeup happens we check for a RTC alarm
(e.g. the RTC pulls down a port pin of the PIC), do something (and reset the
RTC alarm condition) and then go back to sleep mode. If there is no RTC
alarm we go to sleep immediately. This approach works great most of the
time, but some devices do very seldom stop to work. In this cases the RTC
alarm pin is down (e.g. the RTC still worked and did set the alarm
condition) but the PIC does not react on the alarm any more. There are two
ways to restart it: 1. Disconnect the power supply, 2. Touch the oscillator
input.

At the moment we have no idea whether it's a software or hardware problem.
We think that there are the following possibilities:

1. The oscillator doesn't restart upon WDT wakeup.
2. The WDT stops to work and therefor does not generate a wakeup any more
3. The program counter gets corrupted

Any ideas

Frank Schmidberger