Coroutines and Tasks

Testbenches built using cocotb use Python coroutines. Tasks are cocotb objects that wrap coroutines and are used to schedule concurrent execution of the testbench coroutines.

While active tasks are executing, the simulation is paused. The coroutine uses the await keyword to block on another coroutine’s execution or pass control of execution back to the simulator, allowing simulation time to advance.

Typically coroutines await a Trigger object which pauses the task, and indicates to the simulator some event which will cause the task to resume execution. For example:

async def wait_10ns():"About to wait for 10 ns")
    await Timer(10, units='ns')"Simulation time has advanced by 10 ns")

Coroutines may also await on other coroutines:

async def wait_100ns():
    for i in range(10):
        await wait_10ns()

Coroutines can return a value, so that they can be used by other coroutines.

async def get_signal(clk, signal):
    await RisingEdge(clk)
    return signal.value

async def check_signal_changes(dut):
    first = await get_signal(dut.clk, dut.signal)
    second = await get_signal(dut.clk, dut.signal)
    if first == second:
        raise TestFailure("Signal did not change")

Concurrent Execution

Coroutines can be scheduled for concurrent execution with fork(), start(), and start_soon().

fork() schedules and executes the new coroutine immediately, returning control to the calling task after the new coroutine finishes or yields control. No other pending tasks are run.

The async function start() schedules the new coroutine to be executed concurrently, then yields control to allow the new task (and any other pending tasks) to run, before resuming the calling task.

start_soon() schedules the new coroutine for future execution, after the calling task yields control.


The preferred way to schedule tasks is with start() and start_soon(). fork() remains for historical reasons, but may be removed in a future version of cocotb.

async def test_act_during_reset(dut):
    """While reset is active, toggle signals"""
    tb = uart_tb(dut)
    # "Clock" is a built in class for toggling a clock signal
    cocotb.start_soon(Clock(dut.clk, 1, units='ns').start())
    # reset_dut is a function -
    # part of the user-generated "uart_tb" class
    # run reset_dut immediately before continuing
    await cocotb.start(tb.reset_dut(dut.rstn, 20))

    await Timer(10, units='ns')
    print("Reset is still active: %d" % dut.rstn)
    await Timer(15, units='ns')
    print("Reset has gone inactive: %d" % dut.rstn)

Other tasks can be used in an await statement to suspend the current task until the other task finishes.

async def test_count_edge_cycles(dut, period_ns=1, clocks=6):
    cocotb.start_soon(Clock(dut.clk, period_ns, units='ns').start())
    await RisingEdge(dut.clk)

    timer = Timer(period_ns + 10, 'ns')
    task = cocotb.start_soon(count_edges_cycles(dut.clk, clocks))
    count = 0
    expect = clocks - 1

    while True:
        result = await First(timer, task)
        if count > expect:
            raise TestFailure("Task didn't complete in expected time")
        if result is timer:
  "Count %d: Task still running" % count)
            count += 1

Tasks can be killed before they complete, forcing their completion before they would naturally end.

async def test_different_clocks(dut):
    clk_1mhz   = Clock(dut.clk, 1.0, units='us')
    clk_250mhz = Clock(dut.clk, 4.0, units='ns')

    clk_gen = cocotb.start_soon(clk_1mhz.start())
    start_time_ns = get_sim_time(units='ns')
    await Timer(1, units='ns')
    await RisingEdge(dut.clk)
    edge_time_ns = get_sim_time(units='ns')
    if not isclose(edge_time_ns, start_time_ns + 1000.0):
        raise TestFailure("Expected a period of 1 us")

    clk_gen.kill()  # kill clock coroutine here

    clk_gen = cocotb.start_soon(clk_250mhz.start())
    start_time_ns = get_sim_time(units='ns')
    await Timer(1, units='ns')
    await RisingEdge(dut.clk)
    edge_time_ns = get_sim_time(units='ns')
    if not isclose(edge_time_ns, start_time_ns + 4.0):
        raise TestFailure("Expected a period of 4 ns")

Changed in version 1.4: The cocotb.coroutine decorator is no longer necessary for async def coroutines. async def coroutines can be used, without the @cocotb.coroutine decorator, wherever decorated coroutines are accepted, including yield statements and cocotb.fork().

Changed in version 1.6: Added cocotb.start() and cocotb.start_soon() scheduling functions.

Async generators

In Python 3.6, a yield statement within an async function has a new meaning (rather than being a SyntaxError) which matches the typical meaning of yield within regular Python code. It can be used to create a special type of generator function that can be iterated with async for:

async def ten_samples_of(clk, signal):
    for i in range(10):
        await RisingEdge(clk)
        yield signal.value  # this means "send back to the for loop"

async def test_samples_are_even(dut):
    async for sample in ten_samples_of(dut.clk, dut.signal):
        assert sample % 2 == 0

More details on this type of generator can be found in PEP 525.

Generator-based coroutines


This style is no longer recommended and support may someday be removed.

Prior to Python 3.5, and the introduction of async and await, coroutines were implemented as wrappers around generators. Coroutine functions would be decorated with coroutine and would use yield to block on other coroutines or triggers. You may see existing code that uses this syntax for coroutines, but do not worry, it is compatible with async coroutines.

Any object that can be used in an await statement can also be used in a yield statement while in a generator-based coroutine; including triggers like Timer.

def simple_clock(signal, half_period, half_period_units):
    signal.value = 0
    timer = Timer(half_period, half_period_units)
    while True:
        # in generator-based coroutines triggers are yielded
        yield timer
        signal.value = ~signal

Likewise, any place that will accept async coroutines will also accept generator-based coroutines; including fork().

def start_clock(clk):
    # generator-based coroutines can still be forked
    cocotb.start_soon(simple_clock(clk, 5, units='ns'))
    yield RisingEdge(clk)

async coroutines can be yielded in generator-based coroutines.

async def detect_transaction(clk, valid):
    await RisingEdge(clk)
    while not valid.value:
        await RisingEdge(clk)

def monitor(clk, valid, data):
    # async coroutines can be yielded
    yield detect_transaction(clk, valid)
    return data.value

Generator-based coroutines can also be awaited in async coroutines.

async def check_incrementing(clk, valid, data):
    # generator-based coroutines can be awaited
    prev_count = await monitor()
    while True:
        count = await monitor()
        assert count == (prev_count + 1)
        prev_count = count

You may also see syntax like yield [trigger_a, trigger_b, ...], which is syntactic sugar for First.

def run_for(coro, time, units):
    timeout = Timer(time, units='ps')
    # block until first trigger fires
    yield [timeout, coro]

Tests can also be generator-based coroutines. Tests are not required to be decorated with coroutine as the test decorator will handle this case automatically.

# just need the test decorator
def run_test(dut):
    yield start_clock(dut.clk)
    checker = check_incrementing(
    yield run_for(checker, 1, 'us')