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I've got a CPLD design which has one spare (Debug) pin. I'm trying to find out if it's possible to use the UCF file to select which output port (NET) that pin becomes.

The problem is however, that I've got two different nets I may want to connect that pin to, depending on what I'm debugging, but the fitter always wants to assign every output port to a pin, and fails because there isn't enough.

For example, I want to connect SIGNAL1 at present

NET SIGNAL1 LOC=P114;
NET SIGNAL2 LOC=Don't connect it please;

But then I might want SIGNAL2 on it:

NET SIGNAL1 LOC=Don't connect it please;
NET SIGNAL2 LOC=P114;

I know I can go and modify the design to remove the output I don't want as an output at that particular time, but, sheesh, this just seems like a lot of hassle. There must be an easier way!

Matthew Millman
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  • You could parameterize a single net to be assigned to the pin in the ucf file. Then you just change the parameter from 0 to 1, for example. Then you could use a generator to assign the correct net to the port. Or you could use if/else wherever you drive that port – johnnymopo Nov 08 '15 at 21:01

1 Answers1

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What you are requesting is not possible.

Every port of your top-level design entity needs a physical pin. You can't tell the tool: "don't connect it."

Why don't you use just one pin and change the assignment in your top-level entity?

Paebbels
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  • I suspected that'd be the case. Guess I'll just have to change the top level design each time. Slight bummer as editing the UCF is just so much easier! – Matthew Millman Nov 08 '15 at 22:34