Equations For Crystal Calculations - Intel 80C186XL User Manual

CLOCK GENERATION AND POWER MANAGEMENT
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
tance is still about 200 pF (within 10%) and the equation in Figure 5-4(a) now yields the parallel
resonant frequency.
(a) Series or Parallel Resonant Frequency
1
f
= ------------------------ -
2 L C
1
The equation in Figure 5-4(b) yields the equivalent capacitance C
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
CLKOUT
Frequency (MHz)
10
12
16
20
5-4
is 200 pF in parallel with 20 pF. The equivalent capaci-
1
1
Figure 5-4. Equations for Crystal Calculations
Third-Overtone Crystal
Frequency (MHz)
(b) Equivalent Capacitance
2
C C L
1 x2
C
= -----------------------------------------------------------
eq
2
C
1
at the operation frequency.
eq
in Third Overtone Oscillator Circuit
1
20
25
32
40
– C – C
1 1 x2
L – 1
1
Inductor L
1
Values (µH)
10.0, 12.0, 15.0
6.8, 8.2, 10.0
3.9, 4.7, 5.6
2.2, 2.7, 3.3