Thank you Skimask,

I have now put a "SLEEP 10" command in my main loop to make some tests.

When I change the prescaler values in OPTION_REG, nothing changes; the SLEEP time stays around 10 seconds.

When I now activate the WDTCON register (with OPTION_REG) and put the prescaler to 1:32, my LED blinks almost every half second. Setting the rate at 1:64 doubles the 'pause' delay.

A.- Why is the prescaler in OPTION_REG not affecting the SLEEP time?

B.- According to all datasheets, it is not reliable to use WDT since the time may vary in an important scale. What is then the best way to make long delays with the PIC?