# Checking out the Pierce oscillator

Posted on Feb 4, 2014

The Pierce oscillator is a very simple circuit and as with the ring oscillator also uses inverters (actually just one inverter will do). The principle of operation, however, is very different. While the ring oscillator exploits the incremental propagation delay through the series of inverters, the Pierce oscillator puts the inverter/gate into the

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linear region of operation and employs it as an analog amplifier. To find out the details of the gate-based Pierce oscillator I recommend reading the references listed below. What follows is just a summary of the tips and formulas culled from the said references, pointers and tidbits which I found useful in getting a crystal-based oscillator up and running. I also did a number of breadboarded experiments and the results are shown below. INV is of course the inverter (buffered or unbuffered). Oscillator signal is taken from the inverter output (Vout). To improve (decrease) its rise/fall time, another inverter or, better yet, a (fast!) Schmitt trigger can be used to spruce up the signal. The signal at the inverter input is a sinusoid and can also be used (some Microchip MCU datasheets show the clock signal being derived from the input of the inverter while other Microchip datasheets show it being taken from the inverter output). Rf is a feedback resistor that puts the gate in linear (as opposed to digital) mode operation. The following table provides a rule of thumb value for Rf given crystal frequency: Among other things, Rs limits the amount of crystal drive-increasing Rs decreases drive. A ballpark figure or first cut value for Rs can be derived by computing for and equating Rs to the reactance of Cb: Rs is typically 40 K ohms or less, but is almost never more than 100 K ohms. If the value for Rs is too high, then the high...

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