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Thread: Rated PIC speed ?
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face BY : Mark Willis email (remove spam text)



My understanding is that when the needed quantities of higher speed
rated testing chips are done, the rest of the same identical parts are
tested at the lower clock speed and then sold as the slower part.  Most
all probably *would* test OK at the higher speeds.  All are identically
constructed on the same equipment, no electrical or physical differences
(the occasional die shrink happens occasionally, but that happens to ALL
the parts, not just the higher speed ones <G>)  They may test at slower
speed first, know how to look it up so I don't store that info <G>

Remember: this testing is done over *all* guaranteed supply voltages and
*all* temperatures, not just the nominal power supply voltage and
temperature you probably use the chip at - So yes, you can overclock.
Do so with care, though.

Works for prototypes and "play projects", I certainly would not do this
for ANY life-critical projects, too much liability for saving a few
dollars;  OTOH, by adding a heat sink, there are reportedly people who
can run a 20MHz part WAY overclocked, and reportedly don't have problems
(in their commercial application, that their livelihood depends on.)
Something like a 20MHz part at 40-50 MHz speeds, which sounds way
overclocked to me.  I haven't tested this;  A Microchip FAE said it, and
I DO believe him! <G>

Moral of the story:  Test your own chips, individually, at the desired
speed and over the expected variety of power supply voltages and
temperatures it'll run at, if you're going to do this, and you want good
reliability;  Microchip does make pretty robust chips.  Add a nice heat
sink if over the max rated speed for that IC, probably.  Expect some
chips not to work at higher speeds, if rarely.  Worry less about a chip
run within it's rated speed range (i.e. an F84-04 run at 10MHz) than
outside the available speed range (i.e. an F84-04 run at 20MHz.)

And, think safety - We all know Murphy's Laws.  (My fathers' pellet
stove feeds pellets happily, even if the stove's gone out - we've had to
dig the entire firebox free of literally gallons unburned pellets, a few
times, not a lethal problem, but really QUITE annoying!)  If your
overclocked PIC fails, what'll the consequences be?  Annoying or
catastrophic?

Probably best to do the same job in a sneakier way with lower speeds, or
with different hardware (FPGA/CPLD/other front end) instead of pushing
hardware beyond it's limits, if you can.  The "Principle of least
astonishment" is here on Earth for a good reason, folks <G>

 Mark

Tobie Horswill wrote:
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