Intel 80C186XL Clock Generation And Power Management, 4.Equations for Crystal Calculations

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 L1 is 200 pF in parallel with 20 pF. The equivalent capaci- 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		(b) Equivalent Capacitance
f	1	Ceq	ω2 C1 Cx2 L1 – C1 – Cx2
	= ------------------------		= -----------------------------------------------------------
	2π L1 C1		ω2 C1 L1 – 1

Figure 5-4. Equations for Crystal Calculations

The equation in Figure 5-4(b) yields the equivalent capacitance Ceq 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 L1 in Third Overtone Oscillator Circuit

CLKOUT	Third-Overtone Crystal	Inductor L1
Frequency (MHz)	Frequency (MHz)	Values (µH)
10	20	10.0, 12.0, 15.0
12	25	6.8, 8.2, 10.0
16	32	3.9, 4.7, 5.6
20	40	2.2, 2.7, 3.3

5-4