Intel 80C186XL, 80C188XL user manual = Inverter Output Z 90˚ 180˚, Oscillator Operation

CLOCK GENERATION AND POWER MANAGEMENT

5.1.1.1Oscillator Operation

A phase shift oscillator operates through positive feedback, where a non-inverted, amplified ver- sion of the input connects back to the input. A 360° phase shift around the loop will sustain the feedback in the oscillator. The on-chip inverter provides a 180° phase shift. The combination of the 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 crystal and the second load capacitor provides the final 90° phase shift. Above and below resonance, the crystal is reactive and forces the oscillator back toward the crystal’s nominal frequency.

Z0 = Inverter Output Z
90˚	90˚	180˚

NOTE:

At resonance, the crystal is essentially resistive.

Above resonance, the crystal is inductive.

Below resonance, the crystal is capacitive.

A1125-0A

Figure 5-2. Ideal Operation of Pierce Oscillator

Figure 5-3 shows the actual microprocessor crystal connections. For low frequencies, crystal ven- dors offer fundamental mode crystals. At higher frequencies, a third overtone crystal is the only choice. The external capacitors, CX1 at X1 and CX2 at X2, together with stray capacitance, form the load. A third overtone crystal requires an additional inductor L1 and capacitor C1 to 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