Learning about cocotb framework for RTL simulation
Want to learn
The current stable version 1.7.1 of cocotb requires:
sudo apt-get install make python3 python3-pip
We can use the global python installation for cocotb. However, the better way is to create a separated environment for cocotb to prevent any corruption on main python installation.
Here, we create our own python env inside the cocotb working directory.
cd $PATH_TO_YOUR_COCOTB_WS
python3 -m venv venv
It creates the venv directory. Now, check the current environment
which python3
You will notice that it still uses the global environment usr/bin/python3
Lets activate the working environment
source venv/bin/active
Now, check the current environment
which python3
New environment for cocotb is activated.
pip install cocotb pytest cocotb-bus cocotb-coverage
Make sure pip version 3.x
pip -V
Otherwise, using pip3 instead.
The packages are installed in venv/lib/python3.x/site-packages
You may need to add to PATH and add this line into your .bashrc file
export PATH=/home/tesla/.local/bin:$PATH
Checking whether it is successfull or not by typing cocotb-config
Cocotb supports the following RTL simulators: Synopsys VCS, Intel Questa and Icarus Verilog. Icarus Verilog is free and can be obtained from https://github.com/steveicarus/iverilog. To install Icarus Verilog, follow the instructions from the git repository, or simply:
sudo apt install iverilog
Another way to install iverilog is to recompile and install from source code by following steps. I recommend this way to get the lastest version.
git clone https://github.com/steveicarus/iverilog
cd iverilog
sh ./autoconf.sh
./configure
make
sudo make install
If using Questasim, just need to add the Questasim installation path
export MODELSIM_BIN_DIR="/opt/Intel/questa_fe/bin"
Need to export the LM_LICENSE_FILE of Quartus package before running cocotb
Note that: the questasim here is Intel version. I have not make cocotb worked with Mentor Questasim yet.
Use the @cocotb.test() decorator to mark the test function to be run.
Use .value = value to assign a value to a signal.
Use .value to get a signal’s current value.
============ Learning to write python testbenches
Concurrent Execution
Adding GUI=1 and WAVES=1 in the Makefiles. Or typing following command
make GUI=1 WAVES=1
Switching from GUI=0 to GUI=1 and vice versa requires cleaning sim_build folder by
cd $PATH_TO_TEST_DIR
make clean
make GUI=1
Reason: For Questa, runsim.do file is generated based on the value of the GUI parameter. But if the file has been generated once, its contents is not updated unless one deletes the file. https://github.com/cocotb/cocotb/issues/2734
For Questa/Modelsim, the SCRIPT_FILE is currently run before the simulation is initialized using the vsim command. In the common case where SCRIPT_FILE=wave.do, a file that sets up traces in the GUI, these traces fail to initialize since the underlying signals don’t exist yet.
This commit runs the SCRIPT_FILE before the vsim command, allowing traces to be set up properly.