There is (unfortunatley) no hard fast rule or easy way to calculate a good= =20 value for pull-lups. I use 4.7K everywhere. Why? Because that was a good value 30 years ago=20 when everything was DTL and I see no reason to change. Too low is bad because the drive ha= s=20 to work too hard. That's OK on the older PIC stuff, it had plenty of muscle. The newer parts= =20 are wimps. It also makes for a waste of power. Higher values get you slower rise times when the driver goes to tri-state.= =20 That can be a factor when the wire goes to a lot of places or goes a long way. You kind of answered= =20 your own question by running some tests and finding out that 20K seemed a bit high. If I was trying to= =20 pinch the power down as far as possible I would probably go to 10K. Some of my co-workers use that=20 value in their designs and it behaves OK. This is another area where there is just much less science than we would=20 like to have. ----- Original Message -----=20 From: "Justin Richards" To: "Microcontroller discussion list - Public." Sent: Wednesday, April 03, 2013 2:20 PM Subject: Re: [PIC] Pull Up Resistors > Hi Allen, > > this does not directly answer you question but it is an interesting > discussion about pullups on I2C previously on this list > > http://microcontrollers.2385.n7.nabble.com/I2C-pullups-td84418.html > > Justin > > > On 4 April 2013 01:46, Allen Mulvey wrote: > >> Sorry, this is my first post. I forgot to tag the subject >> line. >> >> >> >> Is there a way to compute the optimum value of pull up (or >> >> pull down) resistors on PIC digital inputs? >> >> >> >> Most example circuits I have seen seem to use 10k. My >> >> EasyPIC7 board uses 4k7. I have been experimenting and 20k >> >> resistors produce unreliable results. I don't see anything >> >> in the data sheet to help with this. >> >> >> >> I am using a pic16f877a. The leakage current is one >> >> micro-amp. Sampling the port must draw considerably more. My >> >> current project is not battery operated but, if it were, I >> >> would like to use the largest resistors possible. >> >> >> >> Thanks, >> >> >> >> Allen >> >> >> >> -- >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> > --=20 > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist=20 --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .