At 10:32 AM 7/14/2005, Spehro Pefhany wrote: >At 07:07 AM 7/14/2005 -0500, you wrote: > >>Can I assume in the crystal mfg's literature that "Load >>capacitance" specifies the recommended values for c1 & c2? That >>would mean I'm using capacitors that are 83% bigger than recommended. > >No, you're using about the correct value, from that pov. The capacitors >are effectively in series in a Pierce oscillator and there is some >stray/input capacitance, so 33pF is about right for the 18pF load >capacitance. That means that the oscillator will be nominally on the marked >frequency, not a bit off. This is not your main problem. Sounds ok to me, as long as the oscillator is on-frequency. >HOWEVER, drive power and starting are different requirements from getting >the frequency spot-on... most crystal MANUFACTURERS (the guys who you >will complain to if you start getting 1% failures in 1E6 crystals) >recommend that you use a (expensive) 'scope current probe and calculate the >actual drive power of the crystal with different series resistors (plot it >on a graph). And you look at what maximum value of resistor will >cause the oscillator to fail to start, over temperature and allowing >for aging, unit to unit variation, power supply variations and so on. >Then you pick a value (if possible) that allows reliable and fast >starting and keeps the drive power within the recommended range. >Too much drive power can lead to accelerated aging, deterioration of >characteristics, and outright failure. You got it. Overdriving the crystal is easy to do, and will cause some level of failures off the bat, with more coming later.. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist