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Altera_Forum's avatar
Altera_Forum
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18 years ago

Is "Output, Driving Ground" same as out_pin <='0';

I have a design which uses the unused pins are set to "Output, Driving Ground" setting. I thought I would prefer to explicitly set them to ground in the VHDL. I did this and did a compare of the...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
18 years ago

I did a test using tabular text files (.ttf) for a Cyclone III device. I back annotated a fit result so that the only difference would be how I handled an unused pin.

When I used

set_instance_assignment -name RESERVE_PIN "AS OUTPUT DRIVING GROUND" -to unused_output

I got exactly the same .ttf file as when I drove the pin unused_output with a low in the source file.

If you did not back annotate the fit for the first thing you tried, the change to the source file when you changed between having the pin there and using a reserve-pin setting might have had the effect of a different seed for the Fitter, changing the placement and routing.