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Forums » Usage »

Debug zero-time loop

Added by Guy Hutchison 9 months ago

I've been trying to debug what looks like a zero-time loop. I suspect the problem is in my testbench because the original code (xge mac from opencores) has a verilator testbench as well, but when I simulate my drivers & monitors only see a single clock event and from then on backtrace shows me a path to inside verilator or the systemC core:

#4 0x08056e9f in Vxge_mac::eval() () #0 0x080a801a in sc_core::sc_notify_time_compare(void const*, void const*) () #2 0x080aa100 in sc_core::sc_simcontext::simulate(sc_core::sc_time const&) () #0 0x080ac082 in sc_core::sc_simcontext::crunch(bool) ()

It appears that the last call (::crunch() ) never exits. Any ideas on what's going on here?


Replies (4)

RE: Debug zero-time loop - Added by Wilson Snyder 9 months ago

Verilator should report an error if the eval loop hangs. You can compile your model with "CPPFLAGS += -DVL_DEBUG=1" and then call Verilated::debug(1) and you'll get a number of printouts showing hopefully where it is looping.

RE: Debug zero-time loop - Added by Guy Hutchison 9 months ago

Resolved -- error between seat and keyboard. SC_CTHREAD() only calls method task once and not once per event. Thanks for the help!

RE: Debug zero-time loop - Added by Guy Hutchison 9 months ago

So, having resolved my previous issue, I'm unable to get tracing working. I'm using exactly the same code as from a previous working testbench:

/* start Verilator traces */
if (ec0.dumping) {
  Verilated::traceEverOn(true);
  tfp = new VerilatedVcdC;
  mac0.trace(tfp, 99);
  tfp->open(ec0.dumpfile_name);
}
/* start SC simulator */
sc_start();
// close dumpfile
if (ec0.dumping) {
  printf ("Closing dumpfile\n");
  tfp->close();
}

It creates the dumpfile, but the dump has signal definitions only and no toggle data. I can turn on Verilated debug mode and see that evaluations are happening, it just doesn't seem to get written out.

RE: Debug zero-time loop - Added by Wilson Snyder 9 months ago

If you use VerilatedVcd__C__ you need to call tfp->dump(time) yourself. If you use VarilatedVcd___Sc__ this is done for you.

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