unused IO on SPARTAN-IIE

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.

Aust> HI all,

Leave them float.

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

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-- See

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"