My experience recently with sleeping, using maximum watchdog of about 8 seconds, on a Atmega 2560 is that it is very roughly somewhere between 7.6 and 8.1 seconds !!! Very inaccurate.
Easy but more expensive solution would be a RTC. Or if power isn't an issue leave the external crystal on, which is far more accurate. Or use a crystal to trigger a counter - which is really how an RTC works anyway.
BTW. The way I solved my issue, where I needed to sleep for 2.5 hours, was to wake up after about 2.1 hours, and recheck the GPS, adjust my 8 seconds time and re-sleep until closer. This meant on average I only had to check my GPS 2 or 3 times each 3 hours period, and due to the VBAT connection (about 100 uA) it would often only take about 10 seconds to get an accurate fix.
On 06/09/2012, at 10:11 AM, James Muraca <jmur...@gmail.com> wrote: