More Examples

Apart from the examples covered with full tutorials in the previous sections, the directory cocotb/examples/ contains some more smaller modules you may want to take a look at.


The directory cocotb/examples/adder/ contains an adder RTL in both Verilog and VHDL, an adder_model implemented in Python, and the cocotb testbench with two defined tests ­ a simple adder_basic_test() and a slightly more advanced adder_randomised_test().

Matrix Multiplier

The directory cocotb/examples/matrix_multiplier contains a module for multiplying two matrices together, implemented in both VHDL and SystemVerilog.

The module takes two matrices a_i and b_i as inputs and provides the resulting matrix c_o as an output. On each rising clock edge, c_o is calculated and output. When input valid_i is high and c_o is calculated, valid_o goes high to signal a valid output value.

A reusable DataValidMonitor class is defined. It monitors a streaming data/valid bus, samples the bus when a transaction occurs, and places those transactions into an asynchronous Queue. The queue allows another coroutine to consume monitored transactions at its own pace.

A reusable MatrixMultiplierTester is also defined. It instantiates two of the DataValidMonitors: one to monitor the matrix multiplier input, and another to monitor the output. The MatrixMultiplierTester fork()s a coroutine which consumes transactions from the input monitor, feeds them into a model to compute an expected output, and finally compares the module output to the expected for correctness.

The main test coroutine stimulates the matrix multiplier DUT with the test data. Once all the test inputs have been applied it decides when the test is done.

The testbench makes use of TestFactory and random data generators to test many sets of matrices.

The number of data bits for each entry in the matrices, as well as the row and column counts for each matrix, are configurable in the Makefile.


The example module uses one-dimensional arrays in the port definition to represent the matrices.

Mixed Language

The directory cocotb/examples/mixed_language/ contains two toplevel HDL files, one in VHDL, one in SystemVerilog, that each instantiate an endian_swapper entity in SystemVerilog and VHDL in parallel and chains them together so that the endianness is swapped twice.

Thus, we end up with SystemVerilog+VHDL instantiated in VHDL and SystemVerilog+VHDL instantiated in SystemVerilog.

The cocotb testbench pulls the reset on both instances and checks that they behave the same.

Mixed-signal (analog/digital)

This example with two different designs shows how cocotb can be used in an analog-mixed signal (AMS) simulation, provided your simulator supports this. Such an AMS setup involves a digital and an analog simulation kernel, and also provides means to transfer data between the digital and the analog domain.

The “-AMS” variants of the common digital HDLs (VHDL-AMS, Verilog-AMS and SystemVerilog-AMS) and languages like Spice can be used to express the analog behavior of your circuit.

Due to limitations of the underlying simulator interfaces (VPI, VHPI, FLI), cocotb cannot directly access the analog domain but has to resort to e.g. HDL helper code. Thus, unlike the other examples, part of this testbench is implemented with cocotb and the helper part with HDL.

System Modeling