3.5 External Oscillator Pins

When the Primary Oscillator (POSC) circuit is used to connect a crystal oscillator, special care and consideration is needed to ensure proper operation. The POSC circuit should be tested across the environmental conditions in which the end product is intended to be used. The load capacitors specified in the crystal oscillator data sheet can be used as a starting point, however, the parasitic capacitance from the PCB traces can affect the circuit, and the values may need to be altered to ensure proper start-up and operation. Excessive trace length and other physical interaction can lead to poor signal quality. Poorly tuned oscillator circuits can have reduced amplitude, incorrect frequency (runt pulses), distorted waveforms and long start-up times that may result in unpredictable application behavior, such as instruction misexecution, illegal opcode fetch, etc. Ensure that the crystal oscillator circuit is at full amplitude and the correct frequency before the system begins to execute code. In planning the application’s routing and I/O assignments, ensure that adjacent port pins, and other signals in close proximity to the oscillator, do not have high frequencies, short rise and fall times, and other similar noise. For further information on the POSC, see Primary Oscillator (POSC).