Floating inputs on unused GPIO pins

[Laurent Pinchart]

Hi Geert,

On Monday 13 October 2008, Geert Uytterhoeven wrote:
> On Mon, 13 Oct 2008, Laurent Pinchart 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.    
> > 
> > I haven't been able to find any mention of floating input protection in
> > the MPC8248 documents. Has anyone here experienced issues with floating
> > inputs in real-life designs ? Do you setup your board with all unused GPIO
> > pins configured as outputs ?   
> 
> I always thought configurable GPIO pins default to _inputs_ after power-on,
> so it's safe to tie them to VCC through a pull-up resistor?
> For CMOS, you can tie them to VDD or GND without a resistor.

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.

Best regards,

-- 
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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