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

CLOCK GENERATION AND POWER MANAGEMENTChoose C 1 and L 1 component values in the third overtone crystal circuit to satisfy the followingconditions:• The LC components form an equivalent series resonant circuit at a frequency below thefundamental frequency. This criterion makes the circuit inductive at the fundamentalfrequency. The inductive circuit cannot make the 90° phase shift and oscillations do nottake place.• The LC components form an equivalent parallel resonant circuit at a frequency abouthalfway between the fundamental frequency and the third overtone frequency. Thiscriterion makes the circuit capacitive at the third overtone frequency, necessary for oscillation.• The two capacitors and inductor at OSCOUT, plus some stray capacitance, approximatelyequal the 20 pF load capacitor, C X2 , used alone in the fundamental mode circuit.(a)FundamentalMode Circuit(b)Third OvertoneMode Circuit(c)Third Overtone Mode(Equivalent Circuit)CLKINCLKINC X1OSCOUTOSCOUTL 1C X2L 1C 1C = C = 20pF C L X1 X2 1= 200pF = (See text)1A1126-0AFigure 5-3. Crystal Connections to MicroprocessorChoosing C 1 as 200 pF (at least 10 times the value of the load capacitor) simplifies the circuitanalysis. At the series resonance, the capacitance connected to L 1 is 200 pF in series with 20 pF.The equivalent capacitance is still about 20 pF and the equation in Figure 5-4(a) yields the seriesresonant frequency.To examine the parallel resonant frequency, refer to Figure 5-3(c), an equivalent circuit to Figure5-3(b). The capacitance connected to L 1 is 200 pF in parallel with 20 pF. The equivalent capacitanceis still about 200 pF (within 10%) and the equation in Figure 5-4(a) now yields the parallelresonant frequency.5-3