Crystal Connections To Microprocessor - Intel 80C186XL User Manual

Choose C and L component values in the third overtone crystal circuit to satisfy the following
1 1
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, C
(a)
Fundamental
Mode Circuit
X1
X2
C X2 C X1
C
X1
Figure 5-3. Crystal Connections to Microprocessor
Choosing C as 200 pF (at least 10 times the value of the load capacitor) simplifies the circuit
1
analysis. At the series resonance, the capacitance connected to L
The equivalent capacitance is still about 20 pF and the equation in Figure 5-4(a) yields the series
resonant frequency.
CLOCK GENERATION AND POWER MANAGEMENT
, used alone in the fundamental mode circuit.
X2
(b)
Third Overtone
Mode Circuit
X1
X2
C X2
= C = 20pF
X2
Third Overtone Mode
(Equivalent Circuit)
C X1
C X2
L 1
C 1 = 200pF
is 200 pF in series with 20 pF.
1
(c)
L
1
C 1
L = (See text)
1
A1520-0A
5-3