Xilinx ISE 'feature': forcing a DUT signal

Oct 01, 2007 3 Replies
E

Has anyone managed to get a Verilog testbench to force an internal DUT signal using ISE (9.2i)? This seems to be impossible.



A DUT force has to be, pretty much by definition, hierarchical. If you try something like



force top.mod.x = y; // or... assign top.mod.x = y;



you get a message about an unsupported hierachical expression. Tweaking the assignments eventually gives you the catch-all unsupported constructs message, with details in #24068 and an accompanying pdf doc. The suggested work-around - to add new ports to the DUT - isn't helpful.



The ISE docs claim to support 'procedural continuous assignments', but this seems to be a rather wide definition of 'supported'.



Thanks -



Evan


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M

Hmm. Is x also declared as a testbench wire? is assign top.mod.x = y; outside of blocks?

-- Mike Treseler

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E

The syntax is valid; (this) assign is a procedural statement which can only be used on variables (and the test code works on two other sims). The procedural assign isn't particularly useful; I only mentioned it because it shows the same problem as the 'force' statement, which *is* very useful.

X's error message and docs claim that it's the hierarchical reference on the LHS ('top.mod.x') which is unsupported, but ISE actually has no problem with hierarchical lvalues. It only appears to be procedural force/assign which is affected.

It can be pretty hard to write comprehensive testbenches without a force statement, so I was hoping to find someone who may have a workaround, short of wiring up extra ports in the DUT.

Evan

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M

I get it. Thanks.

See slide 4 here

formatting link
claims a related Xilinx synthesis limitation.

"A procedural continuous assignment (PCA) creates a dynamic binding to a register variable when the statement executes. It uses "=" as in procedural assignment with the keyword assign. WARNING: The Xilinx synthesis engine does not accept this Verilog construct"

-- Mike Treseler

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