crystal
I have a design with a W65C51S ACIA IC, which uses a 1,8 crystal MHz for serial communication. The clock generation example in the datasheet shows an external 1 MOhms resistor in parallel to the crystal.
It seems that the crystal doesn´t produce any clock signal if the resistor is missing. So what does this resistor do?
With the details provided, it does seem like the crystal may be damaged - it's the most sensitive component on the board, and there's a good chance the pulser has been dropped a few times during it's long life.
The circuit seems reasonable and if it was originally designed with the resistor (rather than the capacitor), then I'd leave it like that. There are similar circuits out there that do use a capacitor as you say:
I'd go for replacing the crystal - choose a crystal designed for series operation such as the FOXLF160 (as opposed to the FOXLF160-20) While you're grabbing the crystal, getting a couple of 74LS38s wouldn't be a bad idea either, given they are only around 50 pence/cents each.
Also, given the price of decent CMOS oscillators nowadays, the other option is just to buy a 16MHz CMOS oscillator (5V supply tolerant) and replace the first stage with that.
I'm confused. I just looked at the PL2303HX datasheet (http://www.adafruit.com/datasheets/PL2303HX.pdf) and I see no sign that a crystal is required, or that it is even possible to use one! AFAICT, the clock generator is internal to the chip.
I'd say that your problem lies elsewhere.
Best Answer
The datasheet of the W65C51S doesn't provide enough details to be sure but my guess is that the crystal oscillator circuit inside the IC doesn't have a 1 M ohm feedback resistor.
This is a typical example of a commonly used crystal oscillator circuit:
This is a Pierce oscillator circuit.
My guess is that on the W65C51S the feedback resistor \$R_f\$ isn't implemented. It might have been impossible or too costly to do so. So an external resistor is needed.
The function of the resistor is to DC bias the inverter such that it has a high gain (large amplification) which is needed to make the oscillator work.