I'm working on an application where the current and space budgets are extremely tight. There is an MCU in there which has to keep track of time. That is, it has to wake up every 2 seconds or so, do its task and go back to sleep. We are currently at the 90S1200 because the 20-pin ssop is actually smaller than an 8-pin soic. There is an external crystal running at a locked frequency of 4MHz.
The easiest way to do the timing control is to use the 8-bit timer/counter with a prescale. Unfortunately, the 0.4mA drawn in idle mode is something we can't live with. So the next candidate is the watchdog. It is probably OK to shut off the crystal, but we still need to make sure that the sleep time does not deviate enormously.
So my question is: Could anyone please help me out with some info on how to calibrate the watchdog timer? Any tips on keeping track of time while saving current are also very welcome.