Post by thread:[OT] Pulldown size

David Olson wrote: > I've got some switches externally located off my PIC board (about 2 > feet) and I'm purposely driving my port pins high when the switches > are closed (due to the fact that I don't want a short to give me an > unwanted trigger) and I'm not sure how to size the pulldown resistors. This is rather confusing. What I *think* you're saying is that you are using low-value pull-up resistors in case of cable/ switch leakage which you pull up with another port pin *only* when you wish to poll the inputs. This is a current saving method often proposed (by Mark) here. Fine. PIC can drive 20mA, so you can use 270 ohm multiplied by the number of resistors paralleled to the various sense lines. What it will obviously *not* do, is distinguish between a contact closure of the remote switch and a cable short. Obviously it cannot. There are ways of distinguishing a valid signal from a short, but if we're talking microcontrollers, the obvious one is to put a 12C508 at the end of the cable to monitor the switch closure, and have it send a unique serial code on signal closure (or various codes to signal various actuations). If all switches are together, but remote from the main board, this makes eminent sense. There are many delightful variations of this; power for the remote PIC can be derived from a diode and capacitor like the DOW (Dallas One Wire) protocols. In fact, you can use a DOW encoder and not bother developing your own. (That said, a long cable is going to have far too much capacitance for a real DOW device - you'll probably want a much lower baudrate.) -- Cheers, Paul B.

[Long post snipped...] > > I've got some switches externally located off my PIC board (about 2 > > feet) and I'm purposely driving my port pins high when the switches > > are closed (due to the fact that I don't want a short to give me an > > unwanted trigger) and I'm not sure how to size the pulldown resistors. > > This is rather confusing. What I *think* you're saying is that you > are using low-value pull-up resistors in case of cable/ switch leakage > which you pull up with another port pin *only* when you wish to poll the > inputs. Sorry if I was confusing - where I got off track was in saying "sizing" which I think people assumed to be resistance. What I'm interested in is the power rating. 1/4 watts are huge. 1/8 to 1/16 is better but, how do I determine what power rating I should start with. If I have to place 8 on the board - even with a network, it takes up more space than I'd like. I was going the route of 10k ohms for size. I don't have to worry about power conservation in this since it's not battery dependant. I've read through a bunch of the archived posts on the subject and there's lots of talk about everybody's favourite resistance value - not a lot about power rating. -DO

<x-flowed>At 08:01 AM 1/26/00 -0800, David wrote: >Sorry if I was confusing - where I got off track was in saying "sizing" >which I think people assumed to be resistance. What I'm interested in is the >power rating. 1/4 watts are huge. 1/8 to 1/16 is better but, how do I >determine what power rating I should start with. If I have to place 8 on the >board - even with a network, it takes up more space than I'd like. > >I was going the route of 10k ohms for size. I don't have to worry about >power conservation in this since it's not battery dependant. With 10k and 5 volts, power = 5*5/10k = 0.0025 W. Use a 10k SIP resistor pack for small size and easy placement. For maximum current sourcing (e.g., RA3 open collector), you would need 220 ohms, with a power of 5*5/220 = 0.114, or a 1/4 W resistor. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: spam_OUTralTakeThisOuTlcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causae scire" ================================================================ </x-flowed>