80C186EC/80C188EC Microprocessor User's Manual

CLOCK GENERATION AND POWER MANAGEMENT
Choose C 1 and L 1 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 oscillation.
• The two capacitors and inductor at OSCOUT, plus some stray capacitance, approximately
equal the 20 pF load capacitor, C X2 , used alone in the fundamental mode circuit.
(a)
Fundamental
Mode Circuit
(b)
Third Overtone
Mode Circuit
(c)
Third Overtone Mode
(Equivalent Circuit)
CLKIN
CLKIN
C X1
OSCOUT
OSCOUT
L 1
C X2
L 1
C 1
C = C = 20pF C L X1 X2 1
= 200pF = (See text)
1
A1126-0A
Figure 5-3. Crystal Connections to Microprocessor
Choosing C 1 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 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 series
resonant frequency.
To examine the parallel resonant frequency, refer to Figure 5-3(c), an equivalent circuit to Figure
5-3(b). The capacitance connected to L 1 is 200 pF in parallel with 20 pF. The equivalent capacitance
is still about 200 pF (within 10%) and the equation in Figure 5-4(a) now yields the parallel
resonant frequency.
5-3