Floating inputs on unused GPIO pins

[Laurent Pinchart]

Hi Leon,

On Monday 13 October 2008, Leon Woestenberg wrote:
> Hello Laurent,
> 
> On Mon, Oct 13, 2008 at 11:56 AM, Laurent Pinchart
> <laurentp at cse-semaphore.com> wrote:
> > our hardware engineer asked me to make sure all unused GPIO pins are
> > configured as outputs to avoid floating inputs. He got theory on his side
> > (floating inputs can lead to higher current consumption, metastability or
> > even permanent damage), but I'd like to ask the list for practical
> > feedback.    
> >
> Ideally, configure them as inputs or tri-state (both mean high
> impedance), and do use pull-down or -up resistors.

As I explained in my mail to Geert, that's not really an option. The GPIO pins are connected to an optional expansion module. When the module is plugged in the pins have dedicated functions. When the module is not present they are by default floating inputs. That's the case our hardware engineer is concerned about.

> Now, the answer might be different when your pins are not connected.
> Check on the processor if internal pull resistors are present. If so,
> use as input. If not, then your h/w engineer *may* have a point, not
> sure on that though.

There are no internal pull-up or pull-down resistors on the MPC8248 GPIO pins. I know our hardware engineer has a valid point theoretically. Does the point stand practically, or does the MPC8248 "state-of-the-art"(tm)(c)(whatever) technology make floating inputs safe ?

-- 
Laurent Pinchart
CSE Semaphore Belgium

Chaussee de Bruxelles, 732A
B-1410 Waterloo
Belgium

T +32 (2) 387 42 59
F +32 (2) 387 42 75
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