I believe that the series resistor is a valuable addition. Values of 10k or higher appear to work fine on slow signals. Obviously when you do want to detect a rapid transition then some tradeoff must be made but for detecting a switch there is no problem. The capacitor is a good idea, there are two possible problems though. If you slow the transition down you lengthen the time the pin will be between defined levels. This has already been discussed elsewhere and you can usually ignore it. If you connect too large a capacitor to an I/O pin a drop in supply could cause current to be pumped into the pin, causing latchup. I believe that 10n will not cause a problem (no evidence though) but for higher values another series resistor might be needed. Oliver. ----- Original Message ----- From: Lorick To: Sent: Sunday, October 08, 2000 2:24 PM Subject: Re: [PIC]: Length of digital input lines... > > if your > > 'low speed' signal has fast rise and fall times you may have to slow them > > down in order to avoid odd effects. > > The way it is set up, the switches at the far end of the cable are a rotary > switch with one always grounding an input pin and the others just floating, > letting the pull up on the board keep the inputs high. So the rise and fall > times are the result of someone rotating the switch and grounding another > pin from remote. > > > the switch could dump a 50ma current for a short time (nanoseconds). If > the > > edge of the pulse is not attenuated when it gets to the PIC the pin would > > try to go to -5v, and suck that 50ma through the protection diode until > the > > line settles. > > If I have a resistance in series with the pic input and then do everything > else from that resistor, like adding the pull up at that point for a > resistance of the pull up plus the serial R, and then a capacitor from that > same R junction to ground, does this adequately arm me for attenuation and > slowdown? > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu