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.
In order to use this simulator, set :makeSIM to icarus:
make SIM=icarusAccessing bits of a vector directly was not possible until (including) version 10.3:
dut.stream_in_data[2].value = 1See also steveicarus/iverilog#323.
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
`ifdef COCOTB_SIM
initial begin
$dumpfile ("button_deb.vcd");
$dumpvars (0, button_deb);
#1;
end
`endif
endmoduleWarning
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. Some features of cocotb may not work correctly or at all.
Currently cocotb only supports Verilator 4.106 (no earlier or later version). See also the corresponding cocotb issue 2300 and upstream issue.
In order to use this simulator, set :makeSIM to verilator:
make SIM=verilatorOne major limitation compared to standard Verilog simulators is that it does not support delayed assignments when accessed from cocotb.
To run cocotb with Verilator, you need verilator in your PATH.
1.3
1.5 Improved cocotb support and greatly improved performance when using a higher time precision.
To enable HDL code coverage, add Verilator's coverage option(s) to the :makeEXTRA_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 :makeEXTRA_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 :makeEXTRA_ARGS as shown below.
EXTRA_ARGS += --trace-fst --trace-structs
The resulting file will be dump.fst and can be opened by gtkwave dump.fst.
In order to use this simulator, set :makeSIM to vcs:
make SIM=vcscocotb currently only supports VPI for Synopsys VCS, not VHPI.
In order to use this simulator, set :makeSIM to riviera:
make SIM=rivieraNote
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.)
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.
In order to use this simulator, set :makeSIM to activehdl:
make SIM=activehdlWarning
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
In order to use this simulator, set :makeSIM to questa:
make SIM=questaFor more information, see sim-modelsim.
- All issues with label category:simulators:questa
- Questa 2021.1 and later added experimental support the VHPI interface in addition to the proprietary FLI interface, which can be enabled by setting the
VHDL_GPI_INTERFACEenvironment variable tovhpibefore running cocotb. However, VHPI support in Questa is not complete yet and users of cocotb should continue to use FLI for the time being.
In order to use this simulator, set :makeSIM to modelsim:
make SIM=modelsimNote
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.
In order to start ModelSim or Questa with the graphical interface and for the simulator to remain active after the tests have completed, set :makeGUI=1. If you have previously launched a test without this setting, you might have to delete the :makeSIM_BUILD directory (sim_build by default) to get the correct behavior.
In order to use this simulator, set :makeSIM to ius:
make SIM=iusFor more information, see sim-xcelium.
In order to use this simulator, set :makeSIM to xcelium:
make SIM=xceliumThe simulator automatically loads VPI even when only VHPI is requested.
Warning
GHDL support in cocotb is experimental. Some features of cocotb may not work correctly or at all.
In order to use this simulator, set :makeSIM to ghdl:
make SIM=ghdlNoteworthy is that despite GHDL being a VHDL simulator, it implements the VPI interface.
To get waveforms in VCD format, set the :makeSIM_ARGS option to --vcd=anyname.vcd, for example in a Makefile:
SIM_ARGS+=--vcd=anyname.vcdThe option can be set on the command line, as shown in the following example.
SIM_ARGS=--vcd=anyname.vhd make SIM=ghdlA VCD file named anyname.vcd will be generated in the current directory.
:makeSIM_ARGS can also be used to pass command line arguments related to other waveform formats supported by GHDL <ghdl:export_waves>.
In order to use Tachyon DA's CVC simulator, set :makeSIM to cvc:
make SIM=cvcNote that cocotb's makefile is using CVC's interpreted mode.