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'[PIC]: Can PIC's inputs be left floating?'
2000\11\06@073627 by Bala Chandar

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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

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2000\11\06@082425 by Bob Ammerman

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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}

2000\11\06@090445 by Olin Lathrop

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> 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, .....olinKILLspamspam@spam@cognivis.com, http://www.cognivis.com

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2000\11\06@091108 by David Kott

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> 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

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2000\11\06@091322 by Kevin Blain

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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.

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2000\11\06@100927 by mike

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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.
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2000\11\06@115927 by Harold M Hallikainen

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       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.ChandarKILLspamspamAVENTIS.COM>
writes:
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2000\11\06@120150 by jamesnewton

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www.piclist.com/../logic/xtrapins.htm

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2000\11\06@143544 by mike

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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.

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