> Hi, all,
>     I used a 4 Mhz (14,8,7,1) crystal can with a 16f84-04/P and
> experimented with a 74HC164. The result was displayed on a 7 segments LED.
> All goes well. But when I saw the over clocking postings, I tried to use a
> 6 Mhz can with no display. I then tried a 3.84Mhz, but the output was
> messed up. However, a 7.68Mhz and a 12Mhz crystal can worked well. What is
> the catch? Is the crystals at fault?
>     I saw some schematic using a two leads crystal across the OSC1 and
> OSC2 pins of the PIC and then grounded both pins thru two 15pf Cap.
> Compare with the 14,8,7,1 can crystal, Which method is better?
>     Thanks in advance.
> Gary
>
Sounds to me like the 6Mhz can was not working at all.  Unless you were
using time critical circuitry outside of the PIC (such as a monostable) I
would have thought that the 3.84 MHz device would have worked fine.  Maybe a
poor connection?

Remember that there is no guarantee that every 4MHz PIC you buy will be
capable of running at 12 MHz.  Some will, some won't.

Canned oscillators are simpler to use, start reliably and have a full TTL
level output which could be tapped off for other parts of the circuit.  A
discreet crystal is cheaper and available in more frequencies than a canned
oscillator, it also consumes much less power.

Regards

Mike Rigby-Jones