Intel 80C186EA, User Manual

Page: 145 / 417

CLOCK GENERATION AND POWER MANAGEMENT
5-4
Figure 5-4. Equations for Crystal Calculations
The equation in Figure 5-4(b) yields the equivalent capacitance C
eq
at the operation frequency.
The desired operation frequency is the third overtone frequency marked on the crystal. Optimiz-
ing equations for the above three criteria yields Table 5-1. This table shows suggested standard
inductor values for various processor frequencies. The equivalent capacitance is about 15 pF.
Table 5-1. Suggested Values for Inductor L
1
in Third Overtone Oscillator Circuit
CLKOUT
Frequency (MHz)
Third-Overtone Crystal
Frequency (MHz)
Inductor L
1
Values (µH)
13.04 26.08 6.8, 8.2, 10.0
16 32 3.9, 4.7, 5.6
20 40 2.2, 2.7, 3.3
(a) Series or Parallel Resonant Frequency (b) Equivalent Capacitance
f
1
2
π
L
1
C
1
------------------------- =
C
eq
ω
2
C
1
C
x2
L
1
C
1
– C
x2
–
ω
2
C
1
L
1
1
–
----------------------------------------------------------- =