GF-Cocotb -- Python Testbench Generation

Generate Cocotb testbenches as an alternative to SystemVerilog TBs.

Tool Detection

python3 -c "import cocotb" 2>/dev/null

If not found, return GATEFLOW-RESULT ERROR with: pip install cocotb

Test Template

import cocotb
from cocotb.clock import Clock
from cocotb.triggers import RisingEdge

@cocotb.test()
async def test_reset(dut):
    clock = Clock(dut.clk, 10, units="ns")
    cocotb.start_soon(clock.start())
    dut.rst_n.value = 0
    for _ in range(3):
        await RisingEdge(dut.clk)
    dut.rst_n.value = 1
    await RisingEdge(dut.clk)
    assert dut.count.value == 0

When to Use

Cocotb	SV Testbench
Python (lower barrier)	SystemVerilog
Complex stimulus (Python libs)	Protocol + coverage
Slower (co-simulation)	Faster (native)

Makefile Template

SIM ?= icarus
TOPLEVEL_LANG ?= verilog
VERILOG_SOURCES += $(PWD)/rtl/$(DUT).sv
TOPLEVEL = $(DUT)
MODULE = test_$(DUT)
COCOTB_HDL_TIMEUNIT = 1ns
COCOTB_HDL_TIMEPRECISION = 1ps

ifeq ($(SIM),verilator)
    EXTRA_ARGS += --trace --trace-structs
endif

SIM_BUILD = sim_build/$(SIM)
include $(shell cocotb-config --makefiles)/Makefile.sim

Test Templates

FIFO Test

import cocotb
from cocotb.clock import Clock
from cocotb.triggers import RisingEdge, ClockCycles
import random

async def reset_fifo(dut):
    dut.rst.value = 1
    dut.wr_en.value = 0
    dut.rd_en.value = 0
    await ClockCycles(dut.clk, 5)
    dut.rst.value = 0
    await RisingEdge(dut.clk)

@cocotb.test()
async def test_fifo_write_read(dut):
    clock = Clock(dut.clk, 10, units="ns")
    cocotb.start_soon(clock.start())
    await reset_fifo(dut)

    assert dut.empty.value == 1
    written = []
    for i in range(16):
        data = random.randint(0, 255)
        written.append(data)
        dut.din.value = data
        dut.wr_en.value = 1
        await RisingEdge(dut.clk)
    dut.wr_en.value = 0
    await RisingEdge(dut.clk)
    assert dut.full.value == 1

    read = []
    for i in range(16):
        dut.rd_en.value = 1
        await RisingEdge(dut.clk)
        read.append(int(dut.dout.value))
    dut.rd_en.value = 0
    assert read == written

Valid/Ready Handshake Test

@cocotb.test()
async def test_handshake(dut):
    clock = Clock(dut.clk, 10, units="ns")
    cocotb.start_soon(clock.start())
    # ... reset ...

    async def driver():
        for data in range(20):
            dut.s_data.value = data
            dut.s_valid.value = 1
            while True:
                await RisingEdge(dut.clk)
                if dut.s_ready.value == 1:
                    break
        dut.s_valid.value = 0

    async def backpressure():
        while True:
            dut.m_ready.value = random.randint(0, 1)
            await ClockCycles(dut.clk, random.randint(1, 4))

    cocotb.start_soon(driver())
    cocotb.start_soon(backpressure())
    await ClockCycles(dut.clk, 200)

FSM Test

from enum import IntEnum

class State(IntEnum):
    IDLE = 0
    ACTIVE = 1
    DONE = 2

@cocotb.test()
async def test_fsm(dut):
    clock = Clock(dut.clk, 10, units="ns")
    cocotb.start_soon(clock.start())
    # reset...
    assert int(dut.state.value) == State.IDLE
    dut.start.value = 1
    await RisingEdge(dut.clk)
    dut.start.value = 0
    await RisingEdge(dut.clk)
    assert int(dut.state.value) == State.ACTIVE

Python Runner (pytest)

import os
from pathlib import Path
from cocotb_tools.runner import get_runner

def test_module():
    sim = os.getenv("SIM", "icarus")
    runner = get_runner(sim)
    runner.build(
        sources=[Path("rtl/my_module.sv")],
        hdl_toplevel="my_module",
    )
    runner.test(
        hdl_toplevel="my_module",
        test_module="test_my_module",
    )

cocotbext Protocol Libraries

Package	Protocol	Install
cocotbext-axi	AXI4, AXI-Lite, AXI-Stream	pip install cocotbext-axi
cocotbext-wishbone	Wishbone B4	pip install cocotbext-wishbone
cocotbext-spi	SPI	pip install cocotbext-spi
cocotbext-uart	UART	pip install cocotbext-uart
cocotbext-eth	Ethernet	pip install cocotbext-eth
cocotbext-pcie	PCIe	pip install cocotbext-pcie

AXI-Lite Example

from cocotbext.axi import AxiLiteBus, AxiLiteMaster

axil = AxiLiteMaster(AxiLiteBus.from_prefix(dut, "s_axi"), dut.clk, dut.rst)
await axil.write(0x0000, b'\x42\x00\x00\x00')
data = await axil.read(0x0000, 4)

Decision Matrix: Cocotb vs SV TB

Factor	Use Cocotb	Use SV/UVM
Team knows Python	Yes	-
FPGA project	Yes	-
Large ASIC with UVM infra	-	Yes
Need open-source CI	Yes	-
Standard protocols (AXI, SPI)	Yes (cocotbext)	-
Deep constrained random	-	Yes
Rapid iteration	Yes	-

GATEFLOW-RESULT Integration

---GATEFLOW-RESULT---
STATUS: PASS | FAIL | ERROR
ERRORS: <count>
WARNINGS: <count>
FILES: <test files>
DETAILS: <summary>
---END-GATEFLOW-RESULT---