We've been using a software calendar clock with several QE[32/128] designs (w/ 4MHz crystal) and the overall experience is great. In our case, to support the clock, the MCU is powered from a CR2032 battery during power loss. The system wakes up every second when in this mode (it could be every minute if clock accuracy is less important than power consumption). Under normal operation, it wakes up every 1msec to support the RTOS task switching, and various other timing functions. Total PCB power consumption for clock maintenance alone is somewhere around 2uA (if I remember correctly, it's been a while), which for our application(s) is perfectly fine.
There are so many factors that affect acceptable (clock battery) consumption for each application, it's not easy to give specific recommendations. For example, clock power consumption is far more important in e-agenda type devices, where they spend most of the time in low-power mode keeping time but you want battery replacements/recharges to be as few as possible, than in always-on systems which only need protection from occasional and temporary (and, usually, no more than few-hour long) power outages.
Message Edited by tonyp on 2009-10-01 05:23 PM
