grounding unused inputs
Dan Michaels email (remove spam text)
Olin Lathrop wrote:
>> To be "safe", it is best to tie unused input pins to ground [or Vcc,
>> doesn't much matter which] *ONLY* through a resistor, rather than
>> directly with a short. This is not superstition, as during code
>> development it is all too easy to accidentally configure a pin
>> intended as input to output. You then run the risk of
>> short-circuiting an output.
>Perhaps in a low volume product. I wouldn't want to burden a high volume
>product with unnecessary parts for a minor debugging convenience.
Even in a high volume product, personally I would never simply
(a) configure a pin as output and then leave it unterminated, or
(b) set a pin to input and then short straight it to gnd. There are
a number of possible "disastrous" scenarios this way:
1 - with (a), an ESD spike might modify the TRIS register, set an
output pin to input, and then it would float and pick up noise,
and possibly affect proper operation.
2 - with (b), an ESD spike might modify the TRIS resgister, set an
input pin to output, and fry your product.
3 - with (b), on the next go round with code development, you might
do the TRIS thing yourself by mistake.
4 - with (b), it takes more work if you want to use the pin
differently in the future.
5 - other circumstances may accidentally mod the TRIS register in
the field, eg brownouts, lightning crashes, screwy things related
to general susceptibility of the /MCLR pin, who knows what.
Better to spend a few pennies and tie the unused pins to gnd via
a bussed SIP resistor net, and improve the reliability of your
product. Also, as mentioned several times, it helps saves the butt
of those of us in the subset of non-perfect coders when they screw
up during development. Good practice for code dev, good practice
for field reliability.
In a nice "guaranteed" noise-free field environment, you might be
ok doing it the "quick-and-dirty" way. But, all in all, better
safe than sorry. Experience, yes. Superstition, no.
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See also: www.piclist.com/techref/microchip/ios.htm?key=input
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