At 10:36 AM 3/25/2006 -0500, you wrote: >On 3/25/06, Olin Lathrop wrote: > > > > mrgizmo wrote: > > > I have 2 projects one using a 16F628 and another using a 12F675 both > > > running on 5V. Is there an exact value to use as a pull up resistor, > > > or is it even nessasary? > > > > Pullups on Vdd and Vss are either pointless or not recommended. Other > > pins depend on the application. > > >I didn't get that he was talking about pullups on the power supply pins. >Those would be entirely pointless. Sometimes it's useful to have a resistor from Vdd to Vss, just to ensure that the power supply voltage drops low enough for a good reset to take place within a reasonable length of time (at the cost of wasted power, of course), but I don't think this is in the spirit of the original question. You could *calculate* the maximum pullup/pulldown value based on worst-case thresholds and leakages (DC), and capacitances and required response times (AC) but generally you would want to use a lower value resistor for various reasons (eg. noise). The internal pullups (where available) are essentially current sources rather than resistors (which isn't a bad thing) but they have a very loose tolerance typical of untrimmmed on-chip components. If you're after the lowest power consumption you could use a high-value off-chip 5% resistor to advantage. If it's a line-voltage powered 5V circuit and you just want resistors to pull up for switch inputs or whatever, something in the 1K-10K range is usually appropriate. >Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com ->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist