UR5HCFJ8-FB 查看數據表（PDF） - Semtech Corporation

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UR5HCFJ8-FB

Low-Power Keyboard Encoder for Portable Systems

Semtech Corporation 

IMPLEMENTATION NOTES FOR THE UR5HCFJ8

The following notes pertain to the suggested schematics found on the next

pages.

The Built-in Oscillator on the UR5HCFJ8 requires the attachment of the 4.00

MHz Ceramic Resonators with built-in Load Capacitors.. You can use either

an AVX, part number PBRC-1.00 BR; or a Murata part number

CSTCC4.00MG ceramic resonator.

It may also be possible to operate with the 4.00 MHz Crystal, albeit with

reduced performance. Due to their high Q, the Crystal oscillator circuits

start-up slowly. Since the LapCoderTM constantly switches the clock on and

off, it is important that the Ceramic Resonator is used (it starts up much

quicker than the Crystal). Resonators are also less expensive than Crystals.

Also, if Crystal is attached, two Load Capacitors (33pF to 47pF) should be

added, a Capacitor between each side of the Crystal and ground.

In both cases, using Ceramic Resonator with built-in Load Capacitors, or

Crystal with external Load Capacitors, a feedback Resistor of 1 Meg should

be connected between OSCin and OSCout.

Troubleshoot the circuit by looking at the Output pin of the Oscillator. If the

voltage it half-way between Supply and Ground (while the Oscillator should

be running) --- the problem is with the Load Caps / Crystal. If the voltage it

all the way at Supply or Ground (while the Oscillator should be running) ---

there are shorts on the PCB.

NOTE: when the Oscillator is intentionally turned OFF, the voltage on the

Output pin of the Oscillator is High (at the Supply rail).

Copyright Semtech, 1997-2001

9

DOC5-FJ8-DS-107

www.semtech.com

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