On Wed, Jun 18, 2008 at 6:38 AM, Olin Lathrop wrote: > David Meiklejohn wrote: >> I'm just wondering how much "safety factor" >> is in the specs, in practice. > > No amount of your own testing can give you a reliable answer other than > "none". And yet some companies build on the idea that they can. ZWorld, for instance, started off by selling SBCs with overclocked Z80s in the 1980's. They went through fairly exhaustive testing on many chips to verify that it would work generally, and then they tested each board that came off the line fairly extensively, and had a liberal return policy. IIRC they never had a failure that was due to overclocking, but the overclocking was less than 2x the rated max. As far as current chip manufacturers, though, even though they fabricate the chip they still have to test them to determine, statistically, what the maximum speed is - Microchip doesn't produce chips that stop working at 4, 20, 32, 40, etc MHz, they produce batches of chips that will work 99.995% of the time below their rated max. If you want to run your chip at 2x, 5x, or 10x the rated max you'll find that a certain percentage of chips will run at those speeds. Further, the silicon isn't different for the -20 and the -4, and depending on market demand they may be marking -20 capable chips as -4. Lastly, I haven't yet met an engineer or company that doesn't under-rate their parts to some degree to add some margin. So, I disagree - one can do enough testing to guarantee that a statistical percentage of your product is going to be fine over the product's normal life. The manufacturer of the product itself does this. The big caveat here is that now you are working outside the manufacturer's recommended maximums you cannot rely on them for support and you must start to deal with issues that are very silicon specific - which you may not have much information for. It's a lot of extra work, and given the wide range of available, cheap, fast 32 bit processors I have to wonder if there's ever any financial justification for overclocking a PIC. -Adam -- EARTH DAY 2008 Tuesday April 22 Save Money * Save Oil * Save Lives * Save the Planet http://www.driveslowly.org -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist