Intel 80C188XL, 80C186XL user manual Crystal Connections to Microprocessor

CLOCK GENERATION AND POWER MANAGEMENT

Choose C1 and L1 component values in the third overtone crystal circuit to satisfy the following conditions:

•The LC components form an equivalent series resonant circuit at a frequency below the fundamental frequency. This criterion makes the circuit inductive at the fundamental frequency. The inductive circuit cannot make the 90° phase shift and oscillations do not take place.

•The LC components form an equivalent parallel resonant circuit at a frequency about halfway between the fundamental frequency and the third overtone frequency. This criterion makes the circuit capacitive at the third overtone frequency, necessary for oscil- lation.

•The two capacitors and inductor at X2, plus some stray capacitance, approximately equal the 20 pF load capacitor, CX2, used alone in the fundamental mode circuit.

(a)		(b)			(c)
Fundamental		Third Overtone		Third Overtone Mode
Mode Circuit		Mode Circuit		(Equivalent Circuit)
X1		C X1
		X1
				C X2	L1
X2		L	1
		X2
C X2	C X1	C X2			C1
	CX1 = CX2 = 20pF			C1 = 200pF	L1 = (See text)
					A1520-0A

Figure 5-3. Crystal Connections to Microprocessor

Choosing C1 as 200 pF (at least 10 times the value of the load capacitor) simplifies the circuit analysis. At the series resonance, the capacitance connected to L1 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 series resonant frequency.

5-3