PIC gurus, a little help...

[Carl]

I pulled the ceramic resonator off my SandShark PIC, wanted to take the frequency up a little to pull in the sample delay. I don't have any other resonators, so I thought I would try simply tacking in an external signal, namely my function generator, which would allow me to vary the frequency on the fly.

OK, reading about the 16c73b, it appears that I should be able to feed the signal into pin 9. I get nothing. Any ideas?

- Carl

[Jan]

Re: PIC gurus, a little help...

Carl,

I do not think this is going to work.
You need to put a Xtal (with 2x 27pF) or Resonator between pin 9 and 10.

Apart from that, you will need to tell the software the frequency you are running.
This because the PIC gets its timings from this clock.

So shifting the frequency in flight will not work.

Perhaps it helps to know that the standard frequency for PICS is often 4 MHz... (this one can do up to 20 MHz)

Have a look at the datasheet below

Best regards, Jan



Datasheet

[Norbert]

Re: PIC gurus, a little help...

Of course it should work. Pin 9 is okay.
Maybe your configuration bits are set for RC-oscillation?

Maybe you don't get the signal to the controller. (cable from function generator to PIC)

Remove a capacitor if you have used one with the ceramic resonator.

[Carl]

Re: PIC gurus, a little help...

I realize that there is supposed to be a resonator/crystal across pins 9/10, but all that does is establish oscillation... I can measure a nice, strong (0 to 5v) sine wave at pin 10. I'm wondering why I can't remove the resonator, and apply a sine, or a square wave.

The PIC timing is done in multiples of clock cycles. I don't think it cares what the actual frequency is, as long as it's not too high. This particular PIC is 4MHz, and I want to overdrive it by 25%, to 5MHz. But to start with, I applied 4MHz.

- Carl

[Carl]

Re: PIC gurus, a little help...

Config bits are for using a resonator, which I removed (had internal caps). I measured a good signal at pin 9, and even a poor-looking square wave at pin 10.

I've now ordered some additional ceramic resonators.

- Carl

[Jan]

Re: PIC gurus, a little help...

The frequency is not really the problem.

The frequency shift is.. or perhaps only with the types I use ... who knows.

Best regards, Jan