IMPLEMENTATION NOTES FOR THE UR5HC418

Copyright ©1997-2001 Semtech Corporation

DOC5-418-DS-109

www.semtech.com

7

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

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).