On Thu, 19 Feb 2009 04:25:39 -0500 solarwind wrote: > I see two resistors and two capacitors. What is the purpose for them? > Why those specific values for the capacitors? I thought the crystal is > supposed to be the only thing there that determines frequency. The 15M feeds DC back from the output of the gate, and puts it in a linear state (so it amplifies linearly - more or less). The 330k reduces de current from the output to the capacitor - the largest power consumption in the circuit is charging and discharging it. The two capacitors have several functions, but one of the specs of the crystal is that it needs a certain C in parallel to be exactly at 32768 Hz. (That's why one of the caps is variable - the total cap in parallel to the crystal is 10pF in series with 1/2 of the 39 pF, and the input C of the gate - about 5pF extra). Also, the caps improve the phase difference needed to oscillate, and avoid stray 'parasitic' capacitance oscillation. Sometimes the internal C of a crystal is sufficient to oscillate at another (higher) frequency. John -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist