Post by thread:[PIC] Unused inputs left to float

Wouter's email gave me pause for thought. On all of our PIC applications we have left unused pins to float and normally set to input without any apparent ill effect. However in the old days of CMOS this was bad news as the high impedance could lead to oscillating inputs and high current drain. Does anyone know if this applies to PIC's or has the input technology long moved on? Robin Abbott Forest Electronics - Home of WIZ-C ANSI C Compiler for PIC's with RAD Front end spam_OUTrobin.abbottTakeThisOuTfored.co.uk http://www.fored.co.uk {Original Message removed}

So this implies unused inputs should be set to outputs and held low or high I guess. Thanks for that, although a quick look at some app notes seems to show the MC don't necessarily do this ! Robin Abbott Forest Electronics - Home of WIZ-C ANSI C Compiler for PIC's with RAD Front end .....robin.abbottKILLspam@spam@fored.co.uk http://www.fored.co.uk -----Original Message----- From: piclist-bouncesKILLspammit.edu [.....piclist-bouncesKILLspam.....mit.edu] On Behalf Of Dario Greggio Sent: 17 November 2007 10:24 To: Microcontroller discussion list - Public. Subject: Re: [PIC] Unused inputs left to float - was PIC looses it's programmemory Robin Abbott wrote: > Does anyone know if this applies to PIC's or has the input technology > long moved on? Definitely it still applies! -- Ciao, Dario

On Nov 17, 2007 5:14 PM, Robin Abbott <EraseMErobin.abbottspam_OUTTakeThisOuTfored.co.uk> wrote: > Wouter's email gave me pause for thought. On all of our PIC applications we > have left unused pins to float and normally set to input without any > apparent ill effect. However in the old days of CMOS this was bad news as > the high impedance could lead to oscillating inputs and high current drain. > > Does anyone know if this applies to PIC's or has the input technology long > moved on? > If you are developing real application and not demo boards, you should add a pull-up or pull down to set the input to a defined state. Or you can set it to output and output either low or high. All in all, to set the pin to a defined state. As for why Microchip ANs have some unused pin not following the recommendations, there are some bad ANs from Microchip. Anyway, they are meant for demo usage, not real product... Xiaofan

> Does anyone know if this applies to PIC's or has the input technology > long moved on? I don't recall any mention by Microchip of what to do with unused inputs. There may be an AN for it, but AFAIK it's not in any PIC manual. The focus is on pins that are being used, not aren't It's definitely A Thing To Be Aware Of "What should I do with pins that I am not using?" http://www.piclist.com/techref/logic/xtrapins.htm

Robin Abbott wrote: > Wouter's email gave me pause for thought. On all of our PIC applications we > have left unused pins to float and normally set to input without any > apparent ill effect. However in the old days of CMOS this was bad news as > the high impedance could lead to oscillating inputs and high current drain. > > Does anyone know if this applies to PIC's or has the input technology long > moved on? > > Robin Abbott > Forest Electronics - Home of WIZ-C ANSI C Compiler for PIC's with RAD Front > end > robin.abbottspam_OUTfored.co.uk > http://www.fored.co.uk > > > {Original Message removed}

MC writes poorly-thought-out app notes. Many are wrong, and use bad engineering practice as well. Don't pay too much attention to what MC does. --Bob A Robin Abbott wrote: {Quote hidden}> So this implies unused inputs should be set to outputs and held low or high > I guess. > > Thanks for that, although a quick look at some app notes seems to show the > MC don't necessarily do this ! > > > Robin Abbott > Forest Electronics - Home of WIZ-C ANSI C Compiler for PIC's with RAD Front > end > @spam@robin.abbottKILLspamfored.co.uk > http://www.fored.co.uk > > > {Original Message removed}

> If you are developing real application and not demo boards, you should > add a pull-up or pull down to set the input to a defined state. Is it considered ok to connect inputs directly to Vdd or Vss, or should a pull up/down resistor always be used? I can see that using a resistor is a good idea in case the PIC somehow ends up with what is supposed to be an input, set to output, in which case the resistor protects against e.g. grounding a high output, but is there any other reason to use them? E.g. I note that MCLR is often held high through a resistor, instead of directly to Vdd. What's the danger in connecting it (an input-only pin) to Vdd? David Meiklejohn http://www.gooligum.com.au

Using a resistor makes in-field changes (such as adding an extra output pin) easier then if the pin was hard wired to Vdd or Vss on the PCB. The resistor can be de-soldered and make the pin available for other uses. Jan-Erik. David Meiklejohn wrote: {Quote hidden}>> If you are developing real application and not demo boards, you should >> add a pull-up or pull down to set the input to a defined state. > > Is it considered ok to connect inputs directly to Vdd or Vss, or should a > pull up/down resistor always be used? I can see that using a resistor is > a good idea in case the PIC somehow ends up with what is supposed to be an > input, set to output, in which case the resistor protects against e.g. > grounding a high output, but is there any other reason to use them? E.g. > I note that MCLR is often held high through a resistor, instead of > directly to Vdd. What's the danger in connecting it (an input-only pin) > to Vdd? > > > David Meiklejohn > http://www.gooligum.com.au >

CMOS technology has generally improved to the point where a floating pin in very unlikely to damage the chip (a very real problem in the early days of CMOS - the chip could fry itself). However, a floating input is still an input. It can oscillate due to a variety of inputs (EM, static, nearby traces via capacitance, etc), or it may sit at a particular voltage. If you measure the current of a CMOS device, and drive its inputs in the linear region (rather than digital) you'll find that it draws more current because the high transistor and low transistor are both conducting, drain VCC straight to ground. The current I've seen (atmel part) was in the range of 1-3mA, which is enough to cause erratic operation on a watch battery type application. Also note that if the input is oscillating you end up with a nice transmitter. If you have room, put in the resistors and leave it an input - this is the safest option. If you don't have room, set it as an output and leave it unconnected, but it's possible that this will cause you problems if/when the code goes off into lala land for some other reason. -Adam On 11/17/07, Robin Abbott <KILLspamrobin.abbottKILLspamfored.co.uk> wrote: {Quote hidden}> Wouter's email gave me pause for thought. On all of our PIC applications we > have left unused pins to float and normally set to input without any > apparent ill effect. However in the old days of CMOS this was bad news as > the high impedance could lead to oscillating inputs and high current drain. > > Does anyone know if this applies to PIC's or has the input technology long > moved on? > > Robin Abbott > Forest Electronics - Home of WIZ-C ANSI C Compiler for PIC's with RAD Front > end > RemoveMErobin.abbottTakeThisOuTfored.co.uk > http://www.fored.co.uk > > > {Original Message removed}