Observation reveals that, if you trim a digital wrist watch crystal oscillator carefully you can get long term accuracies of well under 1 second per day drift. This is a low cost 32 KHz crystal which hasn't been chosen for its precision! This is a lot better than the specifications for the crystal that the manufacturer would claim. Importantly, even if it drifts, my experience is that it drifts consistently and in the same direction. Of course, in a professional design you are obliged to do a worst case calculation based on the manufacturer's worst case specs :-). The watch has the advantage of being attached to a very good "oven" (your wrist, maintained at about 98 degrees F). An article for a "micropower crystal oven" appears in Electronics and Wireless World, August 1997 page 660. The circuit uses 2 '555 oscillators which apply a variable mark space signal to a heater resistor with the overall mark space being controlled by feedback from an NTC thermistor located with the crystal in the "oven" compartment. Power consumption is not specified but can be minimised by running the oven at "just" higher than the highest external temperature liable to be experienced. More insulation will require less wattage (and produce a slower thermal response). -----Original Message----- From: Bob Nelson To: PICLIST@MITVMA.MIT.EDU Date: Monday, 9 February 1998 16:52 Subject: Timing Questions >If you were to take any 4 of the shelf 20Mhz 16C54 PIC chips, put them in >different locations using good quality Xtals and battery power but the >ambient temperature varied by 50 degrees F, run identical code in them for a >timing function, would they all have the same accuracy? > >If so,what is the shortest time span that one could measure? > >I have an idea for a project but need to know if it is feasable. > >Bob >