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'unused pins'
1997\10\21@180140 by stephen mcalonan

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I am developing a small application around the PIC16C73.  Is there
anything that I need to do with the unused pins (pullups, etc)?
Thanks in advance,
Steve McAlonan

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1997\10\21@182958 by ndie Ohtsji [4555]

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Hi Steve,

I'm not too familiar with the 16C73 so whatever I say may be false
.... but I'm sure others will flame me if I'm wrong.

I assume you mean all the unused port pins.....if so then, make them
outputs and set them to a know state, then you don't have
to do anything......err you may want to check your data sheet
as some may have an open drain.  In this case you may want to set
them low instead of high....then you don't have to do anything.

If you want the outputs high (for whatever reason), some have internal
pullups you can enable.  For the open drain pin(s), you need to connect
a pullup.

If you make them inputs, then use pullups (10K) to prevent them from
floating around.

Regards,

-Randie

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{Quote hidden}

1997\10\21@191758 by DREITEK

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In a message dated 97-10-21 18:02:42 EDT, you write:

<<
I am developing a small application around the PIC16C73.  Is there
anything that I need to do with the unused pins (pullups, etc)?
Thanks in advance,
Steve McAlonan >>

Steve,
It is a good idea to make unused I/O pins into outputs.  If they are left as
inputs they can oscillate causing your power consumption to go up.  If you
want, you can tie them to either +5 or GND through a high value resistor (10K
- 100K).  Don't tie them directly to either rail.  If your software goes nuts
or you forget and set a pin to an output and it is driven hard to the oposite
rail you can damage the I/O pin.

Hope this helps
Dave Duley
V.P. DreiTek Inc.
http://www.dreitek.com

1997\10\21@194109 by John Payson

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> Steve,
> It is a good idea to make unused I/O pins into outputs.  If they are left as
> inputs they can oscillate causing your power consumption to go up.  If you
> want, you can tie them to either +5 or GND through a high value resistor (10K
> - 100K).  Don't tie them directly to either rail.  If your software goes nuts
> or you forget and set a pin to an output and it is driven hard to the oposite
> rail you can damage the I/O pin.

I prefer to leave them unconnected and program them for outputs (or enable
the PORTB/PORTC pullups); this saves parts versus resistors, and it's more
versatile as well.  During debugging, it's often useful to have unused pins
available for "test" outputs (e.g. if you want to know how much time the PIC
is spending in an interrupt, you can set a pin at interrupt entry and clear
it on exit).  You should also note that wiring many I/O pins together to a
common pullup resistor is not necessarily safe, since the PIC may try to out-
put high on some and low on others.  In addition, doing that makes it much
harder to use spare pins for "debug output".

1997\10\25@094353 by paulb

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John Payson quoted:

>> If you want, you can tie them to either +5 or GND through a high
>> value resistor (10K - 100K).  Don't tie them directly to either rail.
>> If your software goes nuts or you forget and set a pin to an output
>> and it is driven hard to the oposite rail you can damage the I/O pin.

 And wrote:

> You should also note that wiring many I/O pins together to a common
> pullup resistor is not necessarily safe, since the PIC may try to out-
> put high on some and low on others.

 Which should be self-evident.  I am sure the first poster had every
intention that separate terminating resistors be used.

 What this comment brought to mind was a curious practice way back in
the days of raw TTL, of pulling unused (NAND) inputs high via a resistor
which may have been commoned to many devices.  The supposed reason was
a defect in some early chips with inputs which might break down if the
supply voltage was applied directly.  It never really made sense, and
was largely forgotten by the time LS arrived.

 A comment on this thread suggested that unterminated inputs might
"oscillate".  This seems equally implausible, the correct reason (and
quoted in the data sheets) being that inputs floating in the "no man's
land" between valid logic states induce significant bias current in the
CMOS input stage, defeating most user's efforts to minimise current
consumption.

 And you wouldn't leave a port B line to float with RBIE enabled, would
you now?

 Cheers,
       Paul B.

1997\10\25@174612 by Harold M Hallikainen

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       On PIC's, how about just leaving unused pins open and programming
them to be outputs?

Harold

1997\10\27@122242 by David W. Duley

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In a message dated 97-10-25 17:47:02 EDT, you write:

<<
        On PIC's, how about just leaving unused pins open and programming
them to be outputs?

Harold >>

Harold,
That works great and is a common technique.

Dave Duley


'Unused pins'
1999\02\18@204941 by Biswanath Dutta
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Hi Everyone,

I can understand that unused input pins can pick up signals if left
floating.

But how is it that outputs and the working of the software affected if
the inputs are unused in the software ?.

Biswanath

1999\02\19@121502 by John Payson

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|I can understand that unused input pins can pick up signals if left
|floating.

|But how is it that outputs and the working of the software affected if
|the inputs are unused in the software ?.

The software isn't likely to care about floating inputs if it
never reads them.  From my experience with the PICs, the only
bad effect of leaving an input floating is that it will cause
a slight increase in device power consumption; even this will
probably not be noticeable except in LP-mode "fleapower" app-
lications (when trying to run forever off a battery, there's a
big difference between 50uA and 250uA; if the PIC is taking 3mA,
however, adding another 0.2mA won't make much difference.

1999\02\19@213122 by Biswanath Dutta

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I was actually thinking about the problem recently posted by John Esposito
"Hardware or Software Problem ???"
The problem was solved gounding all unused input pins.

John Payson wrote:

{Quote hidden}

1999\02\20@023538 by Quentin

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Biswanath Dutta wrote:
>
> I was actually thinking about the problem recently posted by John Esposito
> "Hardware or Software Problem ???"
> The problem was solved gounding all unused input pins.
>
Posible the problem was somewhere else. I also use a breadboard for
prototyping and I never had a problem with floating pins. Sometimes I
only use PortA and set Port B as outputs. No problem.
Another thing I can think of is where your Tris instruction is in the
program. My first instruction is to set the unused pins. I don't touch
them again after that.

Quentin

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