Simulator Support

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

In general, cocotb can be used with any simulator supporting the industry-standard VPI, VHPI or FLI interfaces. However, in practice simulators exhibit small differences in behavior that cocotb mostly takes care of.

If a simulator you would like to use with cocotb is not listed on this page open an issue at the cocotb GitHub issue tracker.

Icarus Verilog

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

make SIM=icarus

Note

A working installation of Icarus Verilog is required. You can find installation instructions in the Icarus Verilog Installation Guide.

Accessing bits in a vector

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

dut.stream_in_data[2].value = 1

See also https://github.com/steveicarus/iverilog/issues/323.

Waveforms

Icarus Verilog can produce waveform traces in the FST format, the native format of GTKWave. FST traces are much smaller and more efficient to write than VCD, but requires the use of GTKWave.

To enable FST tracing, set WAVES to 1.

make SIM=icarus WAVES=1

Issues for this simulator

Verilator

Warning

cocotb supports Verilator 5.022 and later.

Verilator is in the process of adding more functionality to its VPI interface, which is used by cocotb to access the design. Therefore, Verilator support in cocotb is currently experimental, and some features of cocotb may not work correctly or at all. If you encounter an issue using Verilator with cocotb, please try the newest release and check existing issues before opening a new one.

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

make SIM=verilator

Note

A working installation of Verilator is required. You can find installation instructions in the Verilator documentation.

Note

Delayed assignments require the use of Verilator’s timing argument.

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

Added in version 1.3.

Changed in version 1.5: Improved cocotb support and greatly improved performance when using a higher time precision.

Coverage

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.

Waveforms

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.

Verilator can produce waveform traces in the FST format, the native format of GTKWave. FST traces are much smaller and more efficient to write, but require the use of GTKWave.

To enable FST tracing, add --trace-fst to EXTRA_ARGS as shown below.

EXTRA_ARGS += --trace --trace-fst --trace-structs

The resulting file will be dump.fst and can be opened by gtkwave dump.fst.

Issues for this simulator

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.

Issues for this simulator

Aldec Riviera-PRO

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

make SIM=riviera

Note

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.

Issues for this simulator

Aldec Active-HDL

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

make SIM=activehdl

Warning

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

Issues for this simulator

Mentor/Siemens EDA Questa

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

make SIM=questa

Starting with Questa 2022.3 and cocotb 1.7 users with VHDL toplevels can choose between two communication interfaces between Questa and cocotb: the proprietary FLI and VHPI. For backwards-compatibility cocotb defaults to FLI. Users can choose VHPI instead by setting the VHDL_GPI_INTERFACE environment variable to vhpi before running cocotb.

For more information, see Mentor/Siemens EDA ModelSim.

Issues for this simulator

Mentor/Siemens EDA ModelSim

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

make SIM=modelsim

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.

In order to start ModelSim or Questa with the graphical interface and for the simulator to remain active after the tests have completed, set GUI=1. If you have previously launched a test without this setting, you might have to delete the SIM_BUILD directory (sim_build by default) to get the correct behavior.

Issues for this simulator

Cadence Incisive

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

make SIM=ius

For more information, see Cadence Xcelium.

Issues for this simulator

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.

Testing designs with VHDL toplevels is only supported with Xcelium 23.09.004 and newer.

Issues for this simulator

GHDL

Warning

GHDL support in cocotb is experimental. Some features of cocotb may not work correctly or at all. At least GHDL 2.0 is required.

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

make SIM=ghdl

Note

A working installation of GHDL is required. You can find installation instructions in the GHDL documentation.

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

Issues for this simulator

Waveforms

To get waveforms in VCD format, set the SIM_ARGS option to --vcd=anyname.vcd, for example in a Makefile:

SIM_ARGS+=--vcd=anyname.vcd

The option can be set on the command line, as shown in the following example.

SIM_ARGS=--vcd=anyname.vcd make SIM=ghdl

A VCD file named anyname.vcd will be generated in the current directory.

SIM_ARGS can also be used to pass command line arguments related to other waveform formats supported by GHDL.

NVC

Note

NVC version 1.11.0 or later is required.

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

make SIM=nvc

Issues for this simulator

Coverage

To enable code coverage, add --cover to SIM_ARGS, for example in a Makefile:

SIM_ARGS += --cover

Specifying types of coverage is also supported. For example, to collect statement and branch coverage:

SIM_ARGS += --cover=statement,branch

The covdb files will be placed in the RTL_LIBRARY subdirectory of SIM_BUILD. For instructions on how to specify coverage types and produce a report, refer to NVC’s code coverage documentation.

Waveforms

To get waveforms in FST format, set the SIM_ARGS option to --wave=anyname.fst, for example in a Makefile:

SIM_ARGS += --wave=anyname.fst

SIM_ARGS can also be used to set the waveform output to VCD by adding --format=vcd.

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.

Issues for this simulator