Simulator Support

This page documents any known quirks and gotchas in the various simulators.


Accessing bits in a vector

Accessing bits of a vector doesn’t work:

dut.stream_in_data[2] <= 1

See access_single_bit test in examples/functionality/tests/


To get waveforms in VCD format some Verilog code must be added to the top component as shown in the example below:

module button_deb(
    input  clk,
    input  rst,
    input  button_in,
    output button_valid);

//... Verilog module code here

// the "macro" to dump signals
initial begin
  $dumpfile ("button_deb.vcd");
  $dumpvars (0, button_deb);

Time unit and precision

Setting the time unit and time precision is not possible from the command-line, and therefore make variables COCOTB_HDL_TIMEUNIT and COCOTB_HDL_TIMEPRECISION are ignored.


cocotb supports Verilator 4.020 and above. Verilator converts Verilog code to C++ code that is compiled. It does not support VHDL. One major limitation compared to standard Verilog simulators is that it does not support delayed assignments.

To run cocotb with Verilator, you need verilator in your PATH.

Finally, cocotb currently generates a Verilator toplevel C++ simulation loop which is timed at the highest precision. If your design’s clocks vary in precision, the performance of the simulation can be improved in the same order of magnitude by adjusting the precision in the Makefile, e.g.,

COCOTB_HDL_TIMEPRECISION = 1us # Set precision to 10^-6s

New in version 1.3.


To enable HDL code coverage, add Verilator’s coverage option(s) to the EXTRA_ARGS make variable, for example:

EXTRA_ARGS += --coverage

This will result in coverage data being written to coverage.dat.

Synopsys VCS

Aldec Riviera-PRO

The LICENSE_QUEUE environment variable can be used for this simulator – this setting will be mirrored in the TCL license_queue variable to control runtime license checkouts.

Mentor Questa

Mentor ModelSim

Any ModelSim PE or ModelSim PE derivative (like ModelSim Microsemi, Intel, Lattice Edition) does not support the VHDL FLI feature. If you try to run with FLI enabled, you will see a vsim-FLI-3155 error:

** Error (suppressible): (vsim-FLI-3155) The FLI is not enabled in this version of ModelSim.

ModelSim DE and SE (and Questa, of course) supports the FLI.

Cadence Incisive

Cadence Xcelium


Support is preliminary. Noteworthy is that despite GHDL being a VHDL simulator, it implements the VPI interface.