UR5HCFJL Datasheet, PDF (14/20 Pages) Semtech Corporation – Zero-PowerTM Keyboard Encoder for Portable Systems

IMPLEMENTATION NOTES FOR THE UR5HCFJL
The following notes pertain to the suggested schematics found on the next
pages.
The Built-in Oscillator on the UR5HCFJL 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
CSTCC2.00MG ceramic resonator.
It may also be possible to operate with the 2.00 MHz Crystal, albeit with
reduced performance. Due to their high Q, the Crystal oscillator circuits
start-up slowly. Since the GreenCoder^TM 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
14
DOC5-FJL-DS-106
www.semtech.com