[Al Skierkiewicz]

Any oscillator has a few requirements to operate. One is that the loop must have 360 degrees of phase shift (or multiples of 360) and the loop must have unity gain or better to begin and sustain oscillation. An inverter has 180 degree phase shift already, input to output. Adding a feedback network from the output back to the input that adds an additional 180 phase shift will cause the circuit to oscillate since an inverter is (by definition) a unity gain device. A simple crystal connected from the output of an inverter back to the input will cause the crystal to start osciallating and produces the required phase shift. There are other methods that insure that the oscillator will start, run without temperature or capacitive influences changing the output frequency or stability. There are several resources on these designs both on the internet and the library. This type of oscillator produces a square wave if that is what you need. Crystal oscillators generally are high frequency types, i.e. >100kHz. If you are looking for something lower, then the 555 or one of the other IC timers would be your best bet. There are a few cheap crystal modules that are designed to be used for timing applicataions. They have a crystal running at 32kHz and divide down to an accurate 1Hz pulse. Digital watches use this method to keep accurate time over several days-weeks.