Keran wrote..

> I have a PIC 16F874 sitting in a socket on my PCB, and as far as I am aware,
> it's simply refusing to start up.
>
> I've checked the power and ground connections, and it programs (in circuit)
> correctly. It's driven by a 20MHz crystal (parallel resonant), with 15pF
> capacitors to ground on either side of the the crystal. When I programmed
> it, the HS oscillator configuration bit was set.
>
> I've tried changing the crystal loading caps - I've doubled them up to 30pF,
> but still no joy. I don't have a 'scope handy, but I had thought that the
> oscillator just wasn't starting up - is this likely?
>
>
> Any ideas? What should I try next? I can post a circuit diagram if that's
> any help to anyone.
>
The first thing you MUST establish is if the xtal is
oscillating.
The oscillator may be not working for a variety of reasons
but its usually that the wrong oscillator type has been
selected at the programming stage.

If a scope is not handy then you could try a simple signal
detector connected to a voltmeter.
A detector could be a signal diode (1n914), cap(1nF),
Resistor to ground (1 Meg). See hideous ascii art.

  To osc pin   -----l>l---l l---+------- to meter
                                      l
                                            1Meg
                                                    l
                                   Gnd

This will yield some voltage above ground but less than 5
volts if the oscillator is working (2.5 volt in theory). No
oscillation will pretty much be ground or 0volts.  Its a
start anyway.
_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

www.psych.auckland.ac.nz

_____________________________