Speculation...

The maximum speed of the PIC is likely due most to propogation delay.
The device is fully static, so there aren't any dram type capacitors
that discharge (or require a charging time) which would limit the speed
of the chip.

Since the pipeline is fairly short, then the only thing that would
affect propogation is how they built the devices on board and the
process used to fab the chip.  The maximum clock speed of the chip
likely corresponds to a conservative propogation rate of the entire
pipeline throughtout the entire chip, regardless of whether portions of
the chip are actually active during a given instruction or not.

This means that if you use instructions that only actively use half the
pipeline, you could likely double the speed of the PIC with few adverse
effects.  However, other instructions and many peripherals would result
in garbage data, since they are not fully switched by the time their
data is required.

So, in answer to your question, there is no way for anyone without
intimate knowledge of the chip's design and manufacturing parameters to
know how far beyond the rated clock one can safely go before hitting the
absolute minimum propogation delay of the entire chip.  I'm certian,
however, Microchip has set the max clockspeed somewhere close to the
maximum propogation delay of the entire chip, so there should be a *lot*
of room to maneuver there.

Speculation off...

This topic has been discussed thouroughly in the archive - please visit
there for any further speculation and anecdotal evidence.  Nothing new
has happened since the last time this was hashed out to warrant a new
discussion  on the topic.

-Adam

Ishaan Dalal wrote:

>I was playing around with an 18F452 recently, and all I had at hand was a 50
>MHz Xtal oscillator. The PIC seemed to work fine with it -- a crude delay
>program also showed that clock speed was indeed around 12.5 MHz. The next
>day I switched to a 40 MHz, and the delay increased. The PIC seemed to work
>OK at 50 MHz (analog peripherals/SPI/I2C not tested).
>
>Does anybody know how far you can take a PIC beyond the rated clock?
>Microchip must surely leave some amount of overhead in there. Purely as an
>academic exercise, of course... :-)
>
>Cheers,
>Ishaan
>
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