80C186EB/80C188EB Microprocessor User's Manual - CEUNES

CLOCK GENERATION AND POWER MANAGEMENT5.1.1.1 Oscillator OperationA phase shift oscillator operates through positive feedback, where a non-inverted, amplified versionof the input connects back to the input. A 360° phase shift around the loop will sustain thefeedback in the oscillator. The on-chip inverter provides a 180° phase shift. The combination ofthe inverter’s output impedance and the first load capacitor (see Figure 5-2) provides another 90°phase shift. At resonance, the crystal becomes primarily resistive. The combination of the crystaland the second load capacitor provides the final 90° phase shift. Above and below resonance, thecrystal is reactive and forces the oscillator back toward the crystal’s nominal frequency.Z = Inverter Output Z090˚90˚180˚NOTE:At resonance, the crystal is essentially resistive.Above resonance, the crystal is inductive.Below resonance, the crystal is capacitive.A1125-0AFigure 5-2. Ideal Operation of Pierce OscillatorFigure 5-3 shows the actual microprocessor crystal connections. For low frequencies, crystal vendorsoffer fundamental mode crystals. At higher frequencies, a third overtone crystal is the onlychoice. The external capacitors, C X1 at CLKIN and C X2 at OSCOUT, together with stray capacitance,form the load. A third overtone crystal requires an additional inductor L 1 and capacitor C 1to select the third overtone frequency and reject the fundamental frequency. See “Selecting Crystals”on page 5-5 for a more detailed discussion of crystal vibration modes.5-2