Post by thread:[PIC]: Can PIC's inputs be left floating?

In designing circuits with CMOS chips, the basic rule is that you should not leave any unused inputs floating. They are tied either to +V or Ground or there is a pull-up / pull-down resistor. Is the same rule applicable when you design circuits using PIC micros? If you define a port or a pin as input, are there internal pull-up / pull-down resistors that are enabled automatically? Thanks for helpful responses. Regards, Bala Chandar -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use spam_OUTlistservTakeThisOuTmitvma.mit.edu?body=SET%20PICList%20DIGEST

This has been discussed several times: You will find the following answers, choose 1 (or more): - Make your unused pins outputs, and don't connect anything to them - Make your unused pins inputs, and .... --- Tie them directly to Vcc --- Tie them directly to GND --- Pull them up to Vcc --- Pull them down to GND --- Rely on the built in pull-up resistors on PORTB Bob Ammerman RAm Systems (contract development of high performance, high function, low-level software) {Original Message removed}

> In designing circuits with CMOS chips, the basic rule is that you should not > leave any unused inputs floating. They are tied either to +V or Ground or > there is a pull-up / pull-down resistor. Is the same rule applicable when > you design circuits using PIC micros? If you define a port or a pin as > input, are there internal pull-up / pull-down resistors that are enabled > automatically? PICs generally don't have pullups except for MCLR and optionally on port B. The best thing to do with unconnected pins is to set them as outputs. ***************************************************************** Olin Lathrop, embedded systems consultant in Devens Massachusetts (978) 772-3129, .....olinKILLspam@spam@cognivis.com, http://www.cognivis.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use listservKILLspammitvma.mit.edu?body=SET%20PICList%20DIGEST

> In designing circuits with CMOS chips, the basic rule is that you should not > leave any unused inputs floating. They are tied either to +V or Ground or > there is a pull-up / pull-down resistor. Is the same rule applicable when > you design circuits using PIC micros? If you define a port or a pin as > input, are there internal pull-up / pull-down resistors that are enabled > automatically? > The rule is indeed applicable to PIC processors with I/O configured as inputs. The same sorts of high currents flow when a PIC's inputs start wavering near the bias points of the input transistors. There are no internal pull-ups or pull-downs that enable automatically. There *are* however, internal pull-ups on PortB in most PICs. You can use these to pull up any pins configured as inputs and left floating. Personally, if I don't use a pin in a design, I will configure it as an output in software. -d -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use .....listservKILLspam.....mitvma.mit.edu?body=SET%20PICList%20DIGEST

internal pullup on MCLR? news to me. > PICs generally don't have pullups except for MCLR and optionally on port B. > The best thing to do with unconnected pins is to set them as outputs. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu?body=SET%20PICList%20DIGEST

On Mon, 6 Nov 2000 21:36:01 +0900, you wrote: >In designing circuits with CMOS chips, the basic rule is that you should not >leave any unused inputs floating. They are tied either to +V or Ground or >there is a pull-up / pull-down resistor. Is the same rule applicable when >you design circuits using PIC micros? If you define a port or a pin as >input, are there internal pull-up / pull-down resistors that are enabled >automatically? There are no automatic pullups. There are about 4 ways to do it, there us no 'best' way - it depends on the application. 1) tie the pin high or low and set it as an input. This can be handy if PCB layout is tight as you can route power/ground through the pin. 2) Leave it open and set it as an output. This has the advantage that you can use the pin to output debugging signals or data, and also simplifies any 'last minute pcb alterations' if you suddenly find you need another pin. This is not such a good idea if ultra-low power consumption is required over reset or WDT reset as the pins will float briefy before the code starts, but it is not normally a problem. 3) Leave it open, set as input and use internal pullups (usually port B only) if the pullups will not cause problems for other pins (e.g. due to power draw) 4) Tie it to another pin (input or output) and set it as an input. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use listservspam_OUTmitvma.mit.edu?bodyT%20PICList%20DIGEST

The unused PIC pin question comes around a lot! Only portB has internal pull-ups that can be enabled. Without them enabled, or on other ports, input pins should not be allowed to float. I always leave unused pins floating and set them as outputs. I can easily modify my circuit later if I need those pins. Harold On Mon, 6 Nov 2000 21:36:01 +0900 Bala Chandar <@spam@Bala.ChandarKILLspamAVENTIS.COM> writes: {Quote hidden}> In designing circuits with CMOS chips, the basic rule is that you > should not > leave any unused inputs floating. They are tied either to +V or > Ground or > there is a pull-up / pull-down resistor. Is the same rule applicable > when > you design circuits using PIC micros? If you define a port or a pin > as > input, are there internal pull-up / pull-down resistors that are > enabled > automatically? > > Thanks for helpful responses. > > Regards, > Bala Chandar > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > use KILLspamlistservKILLspammitvma.mit.edu?body=SET%20PICList%20DIGEST > > > > FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ YOU'RE PAYING TOO MUCH FOR THE INTERNET! Juno now offers FREE Internet Access! Try it today - there's no risk! For your FREE software, visit: dl.http://www.juno.com/get/tagj. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use RemoveMElistservTakeThisOuTmitvma.mit.edu?body=SET%20PICList%20DIGEST

On Mon, 6 Nov 2000 14:13:16 -0000, you wrote: >internal pullup on MCLR? news to me. PIC12C508/9 - and a real pain it is too if you use simple zener-resistor brownout ccts!! >> PICs generally don't have pullups except for MCLR and optionally on port >B. >> The best thing to do with unconnected pins is to set them as outputs. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! use TakeThisOuTlistservEraseMEspam_OUTmitvma.mit.edu?bodyT%20PICList%20DIGEST