UR5HC418 Datasheet, PDF (7/14 Pages) Semtech Corporation – AT / PS/2 - Compatible Keyboard Encoder

IMPLEMENTATION NOTES FOR THE UR5HC418
The following notes pertain to the suggested schematics found on the next
pages.
The built-in oscillator on the UR5HC418 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 DeskCoder^TM constantly switches the clock on and
off, it is important that a 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 one MegaOhm
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 ©1997-2001 Semtech Corporation
7
DOC5-418-DS-109
www.semtech.com