unused IO on SPARTAN-IIE

[Amontec Team, Laurent Gau]

HI all,

I am designing on a FG256 package.

I have a large part of unused IOs.
Is that better to fix the unused IOS to VCCO or to GND or can I let all
unused IOs float?

Regards,
Laurent

 

[Austin Lesea]

Laurent,

Unused IOs can be left floating, unless you would prefer to use them to
help reduce ground bounce. By programming an unused IO pin next to a
group of output IO pins to a '0' (like a LVCMOS 24 mA) and connecting it
to ground, one can reduce the ground bounce by about 20% (by reducing
the inductance of the ground return). Pins past the ones nearest those
IO pins switching do not provide a further benefit.

Leaving some pins floating is always a good idea for testing, as these
pins can be used to probe internal nodes during debug (if you do not use
Chipscope, which captures everything through the JTAG port).

Connecting pins to Vcco and programming them to a '1' will also reduce
Vcco bounce, but as Vcco bounce is hardly ever a concern, it is hardly
ever used that way.

Austin

Amontec Team, Laurent Gauch wrote:

HI all,

I am designing on a FG256 package.

I have a large part of unused IOs.
Is that better to fix the unused IOS to VCCO or to GND or can I let all
unused IOs float?

Regards,
Laurent

 

[Chris Cheung]

Leave them float.


"Amontec Team, Laurent Gauch" <laurent.gauch@amontecDELETEALLCAPS.com> wrote
in message news:409ff3eb$1@news.vsnet.ch...

HI all,

I am designing on a FG256 package.

I have a large part of unused IOs.
Is that better to fix the unused IOS to VCCO or to GND or can I let all
unused IOs float?

Regards,
Laurent

 

[Amontec Team, Laurent Gau]

Chris Cheung wrote:

Leave them float.


"Amontec Team, Laurent Gauch" <laurent.gauch@amontecDELETEALLCAPS.com> wrote
in message news:409ff3eb$1@news.vsnet.ch...

HI all,

I am designing on a FG256 package.

I have a large part of unused IOs.
Is that better to fix the unused IOS to VCCO or to GND or can I let all
unused IOs float?

Regards,
Laurent




I think leaving them float is not so good idea.

Since I am using FG256, the ball will be not able for debug pin.

If I route the pcb ground to the unused pin I can let the unused FPGA
IOs Float or drive the unused FPGA IOs as an output '0' -> to reduce
ground bouncing !

Laurent
www.amontec.com

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[E.S.]

Amontec Team, Laurent Gauch wrote:

If I route the pcb ground to the unused pin I can let the unused FPGA
IOs Float or drive the unused FPGA IOs as an output '0' -> to reduce
ground bouncing !

There was a longer discussion of this very subject few month ago.
Peter or Austin explained it in a longer post, you should find
it in google, looking for "virtual grounds"