ICS1892 datasheet(31/148 Pages) ICST

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Chapter 6

Interface Overviews

ICS1892, Rev. D, 2/26/01

February 26, 2001

ICS1892

6.7.2

Clock Source: Crystal

Figure 6-1 shows the recommended configuration when a crystal is used to supply the ICS1892 clock

source. As shown, connect the two leads of the crystal between the ICS1892 pins REF_IN and REF_OUT.

To properly load the crystal, also add two capacitors (C1 and C2 of Figure 6-1): one connected between

REF_IN and ground (digital domain) and one connected between REF_OUT and ground (digital domain).

Note:

Because a crystal is a tuned RLC circuit, crystal loading has a significant impact on the clock

source accuracy.

As revealed by an impedance analysis of the recommended crystal configuration circuit, capacitors C1 and

C2 are in series. In addition, the circuit has stray capacitance, which Figure 6-1 shows as CS3 and CS4.

This stray capacitance is the collective result of board layout and pad capacitance.

Stray capacitance CS3 is in parallel with C1, depicted cumulatively as CL1. Stray capacitance CS4 is in

parallel with C2, depicted cumulatively as CL2. Therefore, the total capacitive load as viewed by the crystal

is the series sum of the two capacitors CL1 and CL2. (To add capacitors in series, add their inverse.)

If the capacitors C1 and C2 have the same value (which is recommended), then CL1 = CL2. In this case,

each capacitance CL1 and CL2 equals twice the rated load capacitance of the crystal. For example, if

CS3 = CS4 = 5 pF, and the rated capacitive load of the crystal is 25 pF, then C1 = C2 = 45 pF.

(CL1 = CL2 = 50 pF. Therefore, CL1 in parallel with CL2 equals 25 pF.)

Crystal accuracy is affected by load capacitance. The following factors also affect the crystal accuracy and

must be considered when selecting a crystal for a design:

• The crystal cut. The crystal must be cut for accuracy. In some cases, this cut can require using a fixture

that has equivalent capacitive loading characteristics as the final application.

• The crystal temperature characteristics

• The crystal aging characteristics

• CL1 and CL2, that is, the specific capacitive loading that occurs as a result of the particular printed circuit

board that is used and the board layout

Figure 6-1.

Recommended Configuration for a Crystal Clock Source

ICS1892

REF_IN

REF_OUT

CS3

CL1

CS4

CL2

ICS1892 Datasheet(PDF) 31 Page - Integrated Circuit Systems