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.
async def wait_10ns(): cocotb.log.info("About to wait for 10 ns") await Timer(10, units='ns') cocotb.log.info("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()
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")
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.
@cocotb.test() 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.
@cocotb.test() 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: dut._log.info("Count %d: Task still running" % count) count += 1 else: break
Tasks can be killed before they complete, forcing their completion before they would naturally end.
@cocotb.test() 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,
yield statements and
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
yield within regular Python code. It can be used to create a special
type of generator function that can be iterated with
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" @cocotb.test() 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.
This style is no longer recommended and support may someday be removed.
Prior to Python 3.5, and the introduction of
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
@cocotb.coroutine 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
@cocotb.coroutine 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) @cocotb.coroutine 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 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
@cocotb.coroutine def run_for(coro, time, units): timeout = Timer(time, units='ps') # block until first trigger fires yield [timeout, coro]
# just need the test decorator @cocotb.test() def run_test(dut): yield start_clock(dut.clk) checker = check_incrementing( clk=dut.clk, valid=dut.valid, data=dut.cnt) yield run_for(checker, 1, 'us')