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 <= 1
access_single_bit test in
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 endmodule
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.
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.
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
** 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.
Support is preliminary. Noteworthy is that despite GHDL being a VHDL simulator, it implements the VPI interface.