On Fri, 19 Jan 2001 19:54:39 +1100, you wrote: >Bob Ammerman wrote: >> >> > Barry, >> > >> > I am trying to avoing the adding of a pull-up. Till now, the pin = had no >> > purpose. It is not connected to any circuitry except for a header. = Now I >> > want to use that pin as an input to allow the user to select a = setting (by >> > shorting or not shorting the header to power or ground. >> >> At worst, you are only driving it for an instruction time or two = against the >> opposite value. > > >I'm surprised at this! With all the professionals >on this list, I have become used to the "never go >outside the spec" attitude, like never exceeding >clock frequency etc. I really don't like the idea >of driving a pin against a voltage source or sink, >at 5v or 0v, this is as bad as driving a pin into >a short circuit - which I wouldn't do either! When you've seen a JW pic with bond wires GLOWING and it still works afterwards you have a lot of faith in the PICs robustness.=20 >Doing it only for "an instruction or two" doesn't >seem to make it any better, it is still so far >out of spec to be scary.=20 It does make a big difference - the only damage mechanism is thermal heating. If you limit it to a few microseconds there isn't much chance of heating even something as small as an on-chip transistor enough to burn it.=20 Also, a short pulse will be supplied by the decoupling cap, so you don;t risk causing a brownout. >I would much prefer to >overclock a PIC at 1.5x its clock freqency than to >do something like this! I would disagree - overclockability will be more dependent on part-to-part differences than the heating effect of a few microseconds' output short.=20 I would still only use it for 'very occasional' use - things like production test though. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.