Quickstart Guide¶
Installing cocotb¶
Pre-requisites¶
Cocotb has the following requirements:
Python 2.7, Python 3.5+ (recommended)
Python-dev packages
GCC 4.8.1+ and associated development packages
GNU Make
A Verilog or VHDL simulator, depending on your RTL source code
Installation via PIP¶
New in version 1.2.
Cocotb can be installed by running
pip3 install cocotb
or
pip install cocotb
For user local installation follow the pip User Guide.
To install the development version of cocotb:
git clone https://github.com/cocotb/cocotb
pip install -e ./cocotb
Native Linux Installation¶
The following instructions will allow building of the cocotb libraries for use with a 64-bit native simulator.
If a 32-bit simulator is being used then additional steps are needed, please see our Wiki.
Debian/Ubuntu-based¶
sudo apt-get install git make gcc g++ swig python-dev
Red Hat-based¶
sudo yum install gcc gcc-c++ libstdc++-devel swig python-devel
Windows Installation¶
Download the MinGW installer from https://osdn.net/projects/mingw/releases/.
Run the GUI installer and specify a directory you would like the environment installed in. The installer will retrieve a list of possible packages, when this is done press “Continue”. The MinGW Installation Manager is then launched.
The following packages need selecting by checking the tick box and selecting “Mark for installation”
Basic Installation
-- mingw-developer-tools
-- mingw32-base
-- mingw32-gcc-g++
-- msys-base
From the Installation menu then select “Apply Changes”, in the next dialog select “Apply”.
When installed a shell can be opened using the msys.bat file located under
the <install_dir>/msys/1.0/
Python can be downloaded from https://www.python.org/downloads/windows/. Run the installer and download to your chosen location.
It is beneficial to add the path to Python to the Windows system PATH variable
so it can be used easily from inside Msys.
Once inside the Msys shell commands as given here will work as expected.
Running your first Example¶
Assuming you have installed the prerequisites as above, the following lines are all you need to run a first simulation with cocotb:
git clone https://github.com/cocotb/cocotb
cd cocotb/examples/endian_swapper/tests
make
Selecting a different simulator is as easy as:
make SIM=vcs
Running the same example as VHDL¶
The endian_swapper example includes both a VHDL and a Verilog RTL implementation.
The cocotb testbench can execute against either implementation using VPI for
Verilog and VHPI/FLI for VHDL. To run the test suite against the VHDL
implementation use the following command (a VHPI or FLI capable simulator must
be used):
make SIM=ghdl TOPLEVEL_LANG=vhdl
Using cocotb¶
A typical cocotb testbench requires no additional HDL code (though nothing prevents you from adding testbench helper code). The Design Under Test (DUT) is instantiated as the toplevel in the simulator without any wrapper code. Cocotb drives stimulus onto the inputs to the DUT and monitors the outputs directly from Python.
Creating a Makefile¶
To create a cocotb test we typically have to create a Makefile. Cocotb provides rules which make it easy to get started. We simply inform cocotb of the source files we need compiling, the toplevel entity to instantiate and the Python test script to load.
VERILOG_SOURCES = $(PWD)/submodule.sv $(PWD)/my_design.sv
# TOPLEVEL is the name of the toplevel module in your Verilog or VHDL file:
TOPLEVEL=my_design
# MODULE is the name of the Python test file:
MODULE=test_my_design
include $(shell cocotb-config --makefiles)/Makefile.inc
include $(shell cocotb-config --makefiles)/Makefile.sim
We would then create a file called test_my_design.py containing our tests.
Creating a test¶
The test is written in Python. Cocotb wraps your top level with the handle you
pass it. In this documentation, and most of the examples in the project, that
handle is dut, but you can pass your own preferred name in instead. The
handle is used in all Python files referencing your RTL project. Assuming we
have a toplevel port called clk we could create a test file containing the
following:
import cocotb
from cocotb.triggers import Timer
@cocotb.test()
def my_first_test(dut):
"""Try accessing the design."""
dut._log.info("Running test!")
for cycle in range(10):
dut.clk = 0
yield Timer(1, units='ns')
dut.clk = 1
yield Timer(1, units='ns')
dut._log.info("Running test!")
This will drive a square wave clock onto the clk port of the toplevel.
Accessing the design¶
When cocotb initializes it finds the top-level instantiation in the simulator
and creates a handle called dut. Top-level signals can be accessed using the
“dot” notation used for accessing object attributes in Python. The same mechanism
can be used to access signals inside the design.
# Get a reference to the "clk" signal on the top-level
clk = dut.clk
# Get a reference to a register "count"
# in a sub-block "inst_sub_block"
count = dut.inst_sub_block.count
Assigning values to signals¶
Values can be assigned to signals using either the
value property of a handle object
or using direct assignment while traversing the hierarchy.
# Get a reference to the "clk" signal and assign a value
clk = dut.clk
clk.value = 1
# Direct assignment through the hierarchy
dut.input_signal <= 12
# Assign a value to a memory deep in the hierarchy
dut.sub_block.memory.array[4] <= 2
The syntax sig <= new_value is a short form of sig.value = new_value.
It not only resembles HDL syntax, but also has the same semantics:
writes are not applied immediately, but delayed until the next write cycle.
Use sig.setimmediatevalue(new_val) to set a new value immediately
(see setimmediatevalue()).
Reading values from signals¶
Accessing the value property of a handle object will return a BinaryValue object.
Any unresolved bits are preserved and can be accessed using the binstr attribute,
or a resolved integer value can be accessed using the integer attribute.
>>> # Read a value back from the DUT
>>> count = dut.counter.value
>>>
>>> print(count.binstr)
1X1010
>>> # Resolve the value to an integer (X or Z treated as 0)
>>> print(count.integer)
42
>>> # Show number of bits in a value
>>> print(count.n_bits)
6
We can also cast the signal handle directly to an integer:
>>> print(int(dut.counter))
42
Parallel and sequential execution¶
A yield will run a function (that must be marked as a “coroutine”, see Coroutines)
sequentially, i.e. wait for it to complete.
If a coroutine should be run “in the background”, i.e. in parallel to other coroutines,
the way to do this is to fork() it.
The end of such a forked coroutine can be waited on by using join().
The following example shows these in action:
@cocotb.coroutine
def reset_dut(reset_n, duration):
reset_n <= 0
yield Timer(duration, units='ns')
reset_n <= 1
reset_n._log.debug("Reset complete")
@cocotb.test()
def parallel_example(dut):
reset_n = dut.reset
# This will call reset_dut sequentially
# Execution will block until reset_dut has completed
yield reset_dut(reset_n, 500)
dut._log.debug("After reset")
# Call reset_dut in parallel with the 250 ns timer
reset_thread = cocotb.fork(reset_dut(reset_n, 500))
yield Timer(250, units='ns')
dut._log.debug("During reset (reset_n = %s)" % reset_n.value)
# Wait for the other thread to complete
yield reset_thread.join()
dut._log.debug("After reset")