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Echelon LonWorks Router User Manual

Page 43

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Figure 13. Series 5000 Chip Clock Generator Circuit

To ensure proper oscillator startup, the equivalent series resistance specification

for the crystal should be ≤50 Ω, and the crystal shunt capacitance should be no

greater than 7 pF.
Using a 33 pF capacitor for C2 (in Figure 13), the Series 5000 router chip’s

XOUT pin cannot be used to drive an external CMOS load. However, if you

maintain the required capacitance for the XOUT pin, you can drive an external

clock, for example, for another Series 5000 half-router.
If your Series 5000 router device requires a common clock signal for both router

halves, you can adjust the value for C2 (in Figure 13), add a buffer, and leave

the B Side XOUT unconnected, as shown in Figure 14.Clock traces should be

kept short (≤2 cm, ≤0.8 inch). Keep the crystal circuit close to the Router 5000

chips and isolated from communications lines. In addition, a logic ground guard

must be added for the clock trace to minimize clock noise and to help keep EMI

levels low. However, this ground guard should not be used as a ground source for

digital circuitry.
In addition, the connection between A Side XOUT pin and the B Side XIN pin

includes standard (inverting or non-inverting) bus buffer/line driver.
Important: Because the Series 5000 router A Side XOUT pin drives an input

buffer, the values of the external capacitors are not equal. The value for A Side

XOUT is specified as 30 pF based on an internal input capacitance of 4.5 pF of

the XIN/XOUT pins and internal input capacitance for the buffer/line driver of 3

pF at 25 ºC (so that the total capacitance for the A Side XOUT pin is 33 pF). For

some bus buffer/line drivers, input capacitance can vary over temperature, up to

10 pF. If your device is likely to experience extreme temperatures, consider

changing the value for the A Side XOUT capacitor to 27 pF to allow for the

change in capacitance over temperature.

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Router User’s Guide

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