Dan M wrote:
>Hi Brendan,
>That is a pretty fair question to ponder.
>
>Port Pins as INPUTS
>If as INPUTS, set either to ground or Vdd .  Floating INPUT is a NO NO
>in CMOS.  Floating the input gates of a CMOS devices will allow them to
>charge to the surrounding fields.  This could  cause the floating input
>to hanging in between a logic 1 or 0 (1/2 Vdd) where the CMOS device
>drains its highest current and heats it up.   (I guess you know that)

Yes, this is correct.

>Port Pins as OUTPUTS
>Well if you have your unused Pins set as OUTPUTS, and floating, a few
>things can happen.  Accidental shorts to these pins can happen.  You'll
>have to check the data sheets on what kind of protection there is
>against this.  Shorts from a voltage source higher then the device's Vdd
>may damage them.  Out side of shorts there is no real problem setting
>them to OUTPUTS.  If I may add, as OUTPUT, the programming if it sends
>logic changes to this unused pin, current during transit still normally
>occur internally to the output gate.  Also if your supply source is from
>a batt, then you might take this current into consideration, and not set
>your unused pins to output.
>
>On the other hand, you can still get away with the OUTPUT setting while
>using a batt, just as long as there is no logic changes take place on
>these outputs.
>
>I personally I like to set the I/O Pins to INPUT and ground them.  Yet
>there are times I may make them OUTPUTS and let them float and chance
>the problems of shorts.  Mainly if I make the unused Pins as output,
>generally this will occur in my projects because the USED PINS on their
>common port is also outputs (it is less of a pain in the neck to program
>around).

I guess you and I will have to disagree on this Dan. Newbies just don't need
the added complication of having port pins grounded and accidently setting
them to output killing the chip in the process. The ideal way for them is to
use pull-up/down resistors I guess but not many will actually do it.  :-)

>In the real world, unused and used OUTPUTS are prone to taking a short,
>and can be damaged, no more differently.

In the real world, your unused pins won't get shorted because they don't
go anywhere near the customer interface. (usually not connected at all)
Regards...
___________________________________________
David Duffy        Audio Visual Devices P/L
U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362   Fax: +61 7 38210281
New Web: www.audiovisualdevices.com.au
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