80C186EC/80C188EC Microprocessor User's Manual

CLOCK GENERATION AND POWER MANAGEMENT
(a) Series or Parallel Resonant Frequency
(b) Equivalent Capacitance
f
1
ω 2 C 1 C x2 L 1 – C 1 – C x2
= ------------------------
C eq = -----------------------------------------------------------
2π L 1 C 1
ω 2 C 1 L 1 – 1
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. Optimizing
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
5-4