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TAnde27's avatar
TAnde27
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6 years ago
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Why won't Quartus give me an error or warning for this obvious Verilog error during compilation?

Hi, In my Verilog HDL projects, it seems Quartus provides no notification (errors, warnings, or messages in the compilation log) if I accidentally declare a port as an input but then assign it with...
  • ak6dn's avatar
    ak6dn
    6 years ago

    This is why such tools as 'VeriLint' were developed that try to check the semantics of what your code is doing vs pure syntax.

    Your example has correct language syntax, but the semantics of what you are doing is not (necessarily) meaningful. It could be, or not.

    You can't expect the QuartusII verilog parser to figure these things out. VHDL language is much more strict, Verilog is looser.

AnandRaj_S_Intel's avatar
AnandRaj_S_Intel
Icon for Regular Contributor rankRegular Contributor
6 years ago

Hi Tyler,

Quartus can't report logical error.

Modules can be instantiated from within other modules.And can interface with input / output depending on Logic.

The output "y" of bitwise_negation module is connect to input "y" of bitwise_negation_wrapper module.

IT DOES NOT HAVE ANY SYNTAX ERROR.

Check your RTL viewer.

About my example.

I'm connecting q output of first FF using reg net_1 to d input of second FF.

Regards

Anand

TAnde27's avatar
TAnde27
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6 years ago

Hi Anand,

After doing some research, I now see that the reason the Quartus compiler did not throw an error is because of a Verilog language "feature" called "port coercion". When I look at various FPGA forum discussions related to Verilog port coercion, folks seem to say, "yeah, this is kind of a goofy feature about Verilog, but that's just the way it is." See for instance, the paper here: https://sutherland-hdl.com/papers/2007-SNUG-SanJose_gotcha_again_paper.pdf, where the authors refer to port coercion as a Verilog "gotcha".

Quoting from section 2.7 of the link I reference, the authors say, "Port coercion can occur...if a module writes values to a port that is defined as input."

My example is doing this, but the example you provided does not coerce a port. That's the fundamental difference between your example and my example, and it is the basic issue I was inquiring about. I'd like to know if there is a compiler flag to make Quartus print a warning message like "WARNING: port y of module bit_negation_wrapper is being coerced from input to inout.", or otherwise, if there's something I can look at in the compilation report to tell me that port coercion has occurred.

The example I provide is a simplified version of an error that took me many hours to debug in a much larger project (+10,000 lines of HDL code). This sort of thing is not allowed in VHDL (the compiler would have thrown a syntax error), but is apparently OK in Verilog.

Thank you,

Tyler