Hi Olin,

I do understand the importance of not floating the pins but I am
not sure of your suggestion:

> If you have a little extra space, bring the unused pins out
> to pads and
> still make sure they are driven.  =


Would the pin be tied to ground? If it's just a pad by itself I am not sure
why would this be any advantage. Any suggestion?

Thanks,
John


--- On Sun, 9/13/09, Olin Lathrop <olin_piclist@embedinc.com> wrote:

> From: Olin Lathrop <olin_piclist@embedinc.com>
> Subject: Re: [PIC] Unused port lines question
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Date: Sunday, September 13, 2009, 4:34 PM
> Roger Weichert wrote:
> > I'm building a project (using a PIC16F627) and have
> reached the stage
> > where I'm ready to design the board layout, so need to
> decide what
> > I'm going to do with the unused port lines. ...
> >
> > I'm aiming for a tiny pcb with no wasted space, so am
> trying to avoid
> > any components I don't absolutely need to have.
> =

> In that case make them outputs and make sure the firmware
> drives them.=A0 If
> any are input-only pins, then tie them to Vss or Vdd
> directly or thru a
> resistor.
> =

> If you have a little extra space, bring the unused pins out
> to pads and
> still make sure they are driven.=A0 If you have a little
> more extra space you
> can add 100Kohm pullup or pulldown resistor pads.=A0 You
> don't actually have
> to load the parts, so it's only a board area issue.=A0
> Any changes to make use
> of the pins would require rework and firmware change
> anyway.=A0 The parts can
> be added to the pads and the firmware changed at that time
> to make pins
> inputs if that's what's needed.
> =

> If you were able to fit the resistor pads then the next
> step up is to
> install at least a few of the resistors and make the pins
> inputs.=A0 Unless
> this is a very high volume project, a few cents for the
> extra resistors
> won't matter.=A0 The advantage here is the same firmware
> can then auto-detect
> different hardware versions.=A0 In future versions or
> reworked versions of
> this board the input lines would be pulled differently.
> =

> This topic comes up occasionally, so from experience I
> expect you'll hear
> voodoo electronics about how lines should always have
> resistors on them, not
> driven by the micro in case of short, etc.=A0 All these
> come down to
> protecting the hardware from firmware bugs.=A0 That may
> make sense for the
> development prototype, but is just silly superstition for
> volume production.
> =

> =

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> (978) 742-9014.=A0 Gold level PIC consultants since
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