Simulator Support

This page lists the simulators that cocotb can be used with and documents specifics, limitations, workarounds etc.

Icarus Verilog

In order to use this simulator, set SIM to icarus:

make SIM=icarus

Accessing bits in a vector

Accessing bits of a vector directly was not possible until (including) version 10.3:

dut.stream_in_data[2] <= 1

See also


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);


In order to use this simulator, set SIM to verilator:

make SIM=verilator

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.


To get waveforms in VCD format, add Verilator’s trace option(s) to the EXTRA_ARGS make variable, for example in a Makefile:

EXTRA_ARGS += --trace --trace-structs

To set the same options on the command line, use EXTRA_ARGS="--trace --trace-structs" make .... A VCD file named dump.vcd will be generated in the current directory.

Synopsys VCS

In order to use this simulator, set SIM to vcs:

make SIM=vcs

cocotb currently only supports VPI for Synopsys VCS, not VHPI.

Aldec Riviera-PRO

In order to use this simulator, set SIM to riviera:

make SIM=riviera


On Windows, do not install the C++ compiler, i.e. unselect it during the installation process of Riviera-PRO. (A workaround is to remove or rename the mingw directory located in the Riviera-PRO installation directory.)

Deprecated since version 1.4: Support for Riviera-PRO was previously available with SIM=aldec.

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.

Aldec Active-HDL

In order to use this simulator, set SIM to activehdl:

make SIM=activehdl


cocotb does not work with some versions of Active-HDL (see #1494).

Known affected versions:

  • Aldec Active-HDL 10.4a

  • Aldec Active-HDL 10.5a

Mentor Questa

In order to use this simulator, set SIM to questa:

make SIM=questa

For more information, see Mentor ModelSim.

Mentor ModelSim

In order to use this simulator, set SIM to modelsim:

make SIM=modelsim


In order to use FLI (for VHDL), a vdbg executable from the simulator installation directory needs to be available on the PATH during cocotb installation. This is needed to access the proprietary mti.h header file.

Any ModelSim PE or ModelSim PE derivatives (like the ModelSim Microsemi, Intel, Lattice Editions) do not support the VHDL FLI feature. If you try to use them with FLI, 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) support the FLI.

Cadence Incisive

In order to use this simulator, set SIM to ius:

make SIM=ius

For more information, see Cadence Xcelium.

Cadence Xcelium

In order to use this simulator, set SIM to xcelium:

make SIM=xcelium

The simulator automatically loads VPI even when only VHPI is requested.


In order to use this simulator, set SIM to ghdl:

make SIM=ghdl

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

Tachyon DA CVC

In order to use Tachyon DA’s CVC simulator, set SIM to cvc:

make SIM=cvc

Note that cocotb’s makefile is using CVC’s interpreted mode.