Simulator

The amaranth.sim module, also known as the simulator, makes it possible to evaluate a design’s functionality in a virtual environment before it is implemented in hardware.

Simulating circuits

The following examples simulate one of the two designs below: synchronous counter running in the sync clock domain, and combinational adder. They assume familiarity with the language guide.

from amaranth.lib import wiring
from amaranth.lib.wiring import In, Out

class Counter(wiring.Component):
    en: In(1, init=1)
    count: Out(4)

    def elaborate(self, platform):
        m = Module()
        with m.If(self.en):
            m.d.sync += self.count.eq(self.count + 1)
        return m

class Adder(wiring.Component):
    a: In(16)
    b: In(16)
    o: Out(17)

    def elaborate(self, platform):
        m = Module()
        m.d.comb += self.o.eq(self.a + self.b)
        return m

Running a simulation

Simulating a design always requires the three basic steps: constructing the DUT, constructing a Simulator for it, and running the simulation with the Simulator.run() or Simulator.run_until() method:

from amaranth.sim import Simulator

dut = Counter()
sim = Simulator(dut)
sim.run()

However, the code above neither stimulates the DUT’s inputs nor measures the DUT’s outputs; the Simulator.run() method also immediately returns if no stimulus is added to the simulation. To make it useful, several changes are necessary:

• The Simulator.add_clock() method adds a stimulus: a process external to the DUT that manipulates its inputs (in this case, toggles the clock of the sync domain).

• The Simulator.run_until() method runs the simulation until a specific deadline is reached.

• The Simulator.write_vcd() method captures the DUT’s inputs, state, and outputs, and writes it to a VCD file.

The following code simulates a design and capture the values of all the signals used in the design for each moment of simulation time:

dut = Counter()
sim = Simulator(dut)
sim.add_clock(1e-6) # 1 µs period, or 1 MHz
with sim.write_vcd("example1.vcd"):
    sim.run_until(1e-6 * 15) # 15 periods of the clock

The captured data is saved to a VCD file example1.vcd, which can be displayed with a waveform viewer such as Surfer or GTKWave:

The Simulator.reset() method reverts the simulation to its initial state. It can be used to speed up tests by capturing the waveforms only when the simulation is known to encounter an error:

try:
    sim.run()
except:
    sim.reset()
    with sim.write_vcd("example1_error.vcd"):
        sim.run()

Testing synchronous circuits

To verify that the DUT works as intended during a simulation, known values are provided as the inputs, and the outputs are compared with the expected results.

This is done by adding a different type of stimulus to the simulator, a testbench: an async Python function that runs concurrently with the DUT and can manipulate the signals used in the simulation. A testbench is added using the Simulator.add_testbench() method, and receives a SimulatorContext object through which it can interact with the simulator: inspect the value of signals using the ctx.get() method, change the value of signals using the ctx.set() method, or wait for an active edge of a clock domain using the ctx.tick() method.

The following example simulates a counter and verifies that it can be stopped using its en input:

dut = Counter()

async def testbench_example2(ctx):
    await ctx.tick().repeat(5)     # wait until after the 5th edge of the `sync` domain clock
    assert ctx.get(dut.count) == 5 # verify that the counter has the expected value
    ctx.set(dut.en, False)         # deassert `dut.en`, disabling the counter
    await ctx.tick().repeat(5)     # wait until after the 10th edge of clock
    assert ctx.get(dut.count) == 5 # verify that the counter has not been incrementing
    ctx.set(dut.en, True)          # assert `dut.en`, enabling the counter again

sim = Simulator(dut)
sim.add_clock(1e-6)
sim.add_testbench(testbench_example2) # add the testbench; run_until() calls the function
with sim.write_vcd("example2.vcd"):
    sim.run_until(1e-6 * 15)

Since this circuit is synchronous, and the ctx.tick() method waits until after the circuit has reacted to the clock edge, the change to the en input affects the behavior of the circuit on the next clock cycle after the change:

Testing combinational circuits

A testbench that tests a combinational circuit advances simulation time using the ctx.delay() method instead of the ctx.tick() method, since the simulation does not contain a clock in this case. The Simulator.run() method stops the simulation and returns once all testbenches finish executing.

The following example simulates an adder:

dut = Adder()

async def testbench_example3(ctx):
    await ctx.delay(1e-6)
    ctx.set(dut.a, 2)
    ctx.set(dut.b, 2)
    assert ctx.get(dut.o) == 4

    await ctx.delay(1e-6)
    ctx.set(dut.a, 1717)
    ctx.set(dut.b, 420)
    assert ctx.get(dut.o) == 2137

    await ctx.delay(2e-6)

sim = Simulator(dut)
sim.add_testbench(testbench_example3)
with sim.write_vcd("example3.vcd"):
    sim.run()

Since this circuit is entirely combinational, and the Amaranth simulator uses a zero-delay model of combinational circuits, the outputs change in the same instant as the inputs do:

Replacing circuits with code

Note

This section describes an advanced technique that is not commonly used. If you are first learning how to use the simulator, you can skip it.

During simulation, it is possible to replace an Amaranth circuit with the equivalent Python code. This can be used to improve simulation performance, or to avoid reimplementing complex Python algorithms in Amaranth if they do not need to be synthesized.

This is done by adding a process to the simulator: an async Python function that runs as an integral part of the simulation, simultaneously with the DUT. A process is added using the Simulator.add_process() method, and receives a SimulatorContext object through which it can interact with the simulator. A process is conceptually similar to a testbench but differs from it in two important ways:

• Testbenches run in a well-defined order (from first to last in the order they were added, yielding control only at await points) and cannot observe inconsistent intermediate states of a design, but processes run in an undefined order while the design is converging after a change to its inputs.

• In a process, it is not possible to inspect the value of a signal using the ctx.get() method, which guarantees that inconsistent intermediate states of a design cannot be observed by a process either.

A process communicates with the rest of the design in the same way an elaboratable would: through Signals.

Replacing synchronous circuits

Processes cannot inspect values of signals using the ctx.get() method. Instead, values of signals in a synchronous process are sampled at each active edge of the clock domain (or, for domains with asynchronous reset, at the assertion of the reset signal) using the ctx.tick() method.

The following code replaces the Counter elaboratable with the equivalent Python code in a process, and uses a testbench to verify its correct operation:

m = Module()
m.domains.sync = cd_sync = ClockDomain()
en = Signal(init=1)
count = Signal(4)

async def process_example4(ctx):
    count_value = 0 # initialize counter to 0
    async for clk_edge, rst_value, en_value in ctx.tick().sample(en):
        if rst_value: # can be asserted with or without clk_edge
            count_value = 0 # re-initialize counter
        elif clk_edge and en_value:
            count_value += 1 # advance the counter
            ctx.set(count, count_value) # publish its value to the simulation

async def testbench_example4(ctx):
    await ctx.tick().repeat(5)
    assert ctx.get(count) == 5
    ctx.set(en, False)
    await ctx.tick().repeat(5)
    assert ctx.get(count) == 5
    ctx.set(en, True)

sim = Simulator(m)
sim.add_clock(1e-6)
sim.add_process(process_example4)
sim.add_testbench(testbench_example4)
with sim.write_vcd("example4.vcd", traces=(cd_sync.clk, cd_sync.rst, en, count)):
    sim.run()

Unless it is instructed otherwise, the Simulator.write_vcd() method only captures values of signals that appear in the circuit provided to the simulator when it is created. The en and count signals do not, and are added explicitly using the traces argument so that they will appear in the VCD file.

Replacing combinational circuits

Values of signals in a combinational process are sampled anytime they change using the ctx.changed() method.

The following code replaces the Adder elaboratable with the equivalent Python code in a process, and uses a testbench to verify its correct operation:

m = Module()
a = Signal(16)
b = Signal(16)
o = Signal(17)

async def process_example5(ctx):
    async for a_value, b_value in ctx.changed(a, b):
        ctx.set(o, a_value + b_value)

async def testbench_example5(ctx):
    await ctx.delay(1e-6)
    ctx.set(a, 2)
    ctx.set(b, 2)
    assert ctx.get(o) == 4

    await ctx.delay(1e-6)
    ctx.set(a, 1717)
    ctx.set(b, 420)
    assert ctx.get(o) == 2137

    await ctx.delay(2e-6)

sim = Simulator(m)
sim.add_process(process_example5)
sim.add_testbench(testbench_example5)
with sim.write_vcd("example5.vcd", traces=[a, b, o]):
    sim.run()

Reference

class amaranth.sim.Simulator(toplevel)

Simulator for Amaranth designs.

The simulator accepts a top-level design (an elaboratable), processes that replace circuits with behavioral code, clocks that drive clock domains, and testbenches that exercise the circuits and verify that they work correctly.

The simulator lifecycle consists of four stages:

1. The simulator is created:

   sim = Simulator(design)
   

2. Processes, clocks, and testbenches are added as necessary:

   sim.add_clock(1e-6)
   sim.add_clock(1e-7, domain="fast")
   sim.add_process(process_instr_decoder)
   sim.add_testbench(testbench_cpu_execute)
   

3. The simulation is run:

   with sim.write_vcd("waveform.vcd"):
       sim.run()
   

4. (Optional) The simulator is reset:

   sim.reset()
   

After the simulator is reset, it may be reused to run the simulation again.

Note

Resetting the simulator can also be used to amortize the startup cost of repeatedly simulating a large design.

Parameters:

toplevel (Elaboratable) – Simulated design.

add_clock(period, *, phase=None, domain='sync', if_exists=False)

Add a clock to the simulation.

Adds a stimulus that toggles the clock signal of domain at a 50% duty cycle.

The driven clock signal will toggle every half-period seconds starting at phase seconds after the beginning of the simulation; if not specified, phase defaults to half-period to avoid coinciding the first active edge with the beginning of the simulation.

The clock domain to drive is selected by the domain argument, which may be a ClockDomain object or a str. If it is a string, the clock domain with that name is retrieved from the toplevel elaboratable.

Raises:

• NameError – If domain is a str, the toplevel elaboratable does not have a clock domain with that name, and if_exists is False.

• DriverConflict – If domain already has a clock driving it.

• RuntimeError – If the simulation has been advanced since its creation or last reset.

add_testbench(constructor, *, background=False)

Add a testbench to the simulation.

Adds a testbench that runs concurrently with the toplevel elaboratable and is able to manipulate its inputs, outputs, and state.

The behavior of the testbench is defined by its constructor function, which is an async function that takes a single argument, the SimulatorContext:

async def testbench(ctx):
    ...
    await ctx.tick()
    ...

sim.add_testbench(testbench)

This method does not accept coroutines. Rather, the provided constructor coroutine function is called immediately when the testbench is added to create a coroutine, as well as by the reset() method.

The testbench can be critical (the default) or background (if the background=True argument is specified). The run() method will continue advancing the simulation while any critical testbenches or processes are running, and will exit when only background testbenches or processes remain. A background testbench can temporarily become critical using the critical() context manager.

At each point in time, all of the non-waiting testbenches are executed in the order in which they were added. If two testbenches share state, or must manipulate the design in a coordinated way, they may rely on this execution order for correctness.

Raises:

RuntimeError – If the simulation has been advanced since its creation or last reset.

add_process(process)

Add a process to the simulation.

Adds a process that is evaluated as a part of the toplevel elaboratable and is able to replace circuits with Python code.

The behavior of the process is defined by its constructor function, which is an async function that takes a single argument, the SimulatorContext:

async def process(ctx):
    async for clk_edge, rst, ... in ctx.tick().sample(...):
        ...

sim.add_process(process)

This method does not accept coroutines. Rather, the provided constructor coroutine function is called immediately when the procss is added to create a coroutine, as well as by the reset() method.

Processes can be critical or background, and are always background when added. The run() method will continue advancing the simulation while any critical testbenches or processes are running, and will exit when only background testbenches or processes remain. A background process can temporarily become critical using the critical() context manager.

At each point in time, all of the non-waiting processes are executed in an arbitrary order that may be different between individual simulation runs.

Warning

If two processes share state, they must do so in a way that does not rely on a particular order of execution for correctness.

Preferably, the shared state would be stored in Signals (even if it is not intended to be a part of a circuit), with access to it synchronized using await ctx.tick().sample(...). Such state is visible in a waveform viewer, simplifying debugging.

Raises:

RuntimeError – If the simulation has been advanced since its creation or last reset.

run()

Run the simulation indefinitely.

This method advances the simulation while any critical testbenches or processes continue executing. It is equivalent to:

while self.advance():
    pass

run_until(deadline)

Run the simulation until a specific point in time.

This method advances the simulation until the simulation time reaches deadline, without regard for whether there are critical testbenches or processes executing.

advance()

Advance the simulation.

This method advances the simulation by one time step. After this method completes, all of the events scheduled for the current point in time will have taken effect, and the current point in time was advanced to the closest point in the future for which any events are scheduled (which may be the same point in time).

The non-waiting testbenches are executed in the order they were added, and the processes are executed as necessary.

Returns True if the simulation contains any critical testbenches or processes, and False otherwise.

write_vcd(vcd_file, gtkw_file=None, *, traces=())

Capture waveforms to a file.

This context manager captures waveforms for each signal and memory that is referenced from toplevel, as well as any additional signals or memories specified in traces, and saves them to vcd_file. If gtkw_file is provided, it is populated with a GTKWave save file displaying traces when opened.

Use this context manager to wrap a call to run() or run_until():

with sim.write_vcd("simulation.vcd"):
    sim.run()

The vcd_file and gtkw_file arguments accept either a file object or a filename. If a file object is provided, it is closed when exiting the context manager (once the simulation completes or encounters an error).

The traces argument accepts a trace specification, which can be one of:

• A ValueLike object, such as a Signal;

• A MemoryData object or an individual row retrieved from one;

• A tuple or list containing trace specifications;

• A dict associating str names to trace specifications;

• An interface object.

Raises:

TypeError – If a trace specification refers to a signal with a private name.

reset()

Reset the simulation.

This method reverts the simulation to its initial state:

• The value of each signal is changed to its initial value;

• The contents of each memory is changed to its initial contents;

• Each clock, testbench, and process is restarted.

class amaranth.sim.SimulatorContext(...)

Simulator context.

Simulator processes and testbenches are async Python functions that interact with the simulation using the only argument they receive: the context. Using a context, it is possible to sample or update signals and wait for events to occur in the simulation.

The context has two kinds of methods: async methods and non-async methods. Calling an async method may cause the caller to be preempted (be paused such that the simulation time can advance), while calling non-async methods never causes that.

Note

While a testbench or process is executing without calling async methods, no other testbench or process will run, with one exception: if a testbench calls set(), all processes that wait (directly or indirectly) for the updated signals to change will execute before the call returns.

get(expr)

Sample the value of an expression.

The behavior of this method depends on the type of expr:

• If it is a ValueCastable whose shape is a ShapeCastable, its numeric value is converted to a higher-level representation using from_bits() and then returned.

• If it is a Value or a ValueCastable whose shape is a Shape, the numeric value is returned as an int.

This method is only available in testbenches.

Raises:

TypeError – If the caller is a process.

set(expr, value)

Update the value of an expression.

The behavior of this method depends on the type of expr:

• If it is a ValueCastable whose shape is a ShapeCastable, value is converted to a numeric representation using const() and then assigned.

• If it is a Value or a ValueCastable whose shape is a Shape, value is assigned as-is.

This method is available in both processes and testbenches.

When used in a testbench, this method runs all processes that wait (directly or indirectly) for the signals in expr to change, and returns only after the change propagates through the simulated circuits.

critical()

Context manager that temporarily makes the caller critical.

Testbenches and processes may be background or critical, where critical ones prevent Simulator.run() from finishing. Processes are always created background, while testbenches are created critical by default, but may also be created background. This context manager makes the caller critical for the span of the with statement.

This may be useful in cases where an operation (for example, a bus transaction) takes multiple clock cycles to complete, and must be completed after starting, but the testbench or process performing it never finishes, always waiting for the next operation to arrive. In this case, the caller would elevate itself to become critical only for the duration of the operation itself using this context manager, for example:

async def testbench_bus_transaction(ctx):
    # On every cycle, check whether the bus has an active transaction...
    async for clk_edge, rst_active, bus_active_value in ctx.tick().sample(bus.active):
        if bus_active_value: # ... if it does...
            with ctx.critical(): # ... make this testbench critical...
                addr_value = ctx.get(bus.r_addr)
                ctx.set(bus.r_data, ...) # ... perform the access...
                await ctx.tick()
                ctx.set(bus.done, 1)
                await ctx.tick()
                ctx.set(bus.done, 0) # ... and complete the transaction later.
            # The `run()` method could return at this point, but not before.

tick(domain='sync', *, context=None)

Wait until an active clock edge or an asynchronous reset occurs.

This method returns a TickTrigger object that, when awaited, pauses the execution of the calling process or testbench until the active edge of the clock, or the asynchronous reset (if applicable), occurs. The returned object may be used to repeatedly wait for one of these events until a condition is satisfied or a specific number of times. See the tick trigger reference for more details.

The domain may be either a ClockDomain or a str. If it is a str, a clock domain with this name is looked up in the elaboratable context, or in toplevel if context is not provided.

Raises:

• ValueError – If domain is "comb".

• ValueError – If domain is a ClockDomain and context is provided and not None.

• ValueError – If context is an elaboratable that is not a direct or indirect submodule of toplevel.

• NameError – If domain is a str, but there is no clock domain with this name in context or toplevel.

delay(interval)

Wait until a time interval has elapsed.

This method returns a TriggerCombination object that, when awaited, pauses the execution of the calling process or testbench by interval seconds. The returned object may be used to wait for multiple events.

The value captured by this trigger is True if the delay has expired when the wait has completed, and False otherwise.

The interval may be zero, in which case the caller will be scheduled for execution immediately after all of the processes and testbenches scheduled for the current time step finish executing. In other words, if a call to Simulator.advance() schedules a process or testbench and it performs await ctx.delay(0), this process or testbench will continue execution only during the next call to Simulator.advance().

Note

Although the behavior of await ctx.delay(0) is well-defined, it may make waveforms difficult to understand and simulations hard to reason about.

Raises:

ValueError – If delay is negative.

changed(*signals)

Asynchronously wait until one of the signals change.

This method returns a TriggerCombination object that, when awaited, pauses the execution of the calling process or testbench until any of the signals change. The returned object may be used to wait for multiple events.

The values captured by this trigger are the values of signals at the time the wait has completed.

Warning

The simulation may produce glitches: transient changes to signals (e.g. from 0 to 1 and back to 0) during combinational propagation that are invisible in testbenches or waveform captures. Glitches will wake up both processes and testbenches that use this method to wait for a signal to change, and both processes and testbenches must be prepared to handle such spurious wakeups. The presence, count, and sequence in which glitches occur may also vary between simulation runs.

Testbenches that wait for a signal to change using an await statement might only observe the final value of the signal, and testbenches that wait for changes using an async for loop will crash with a BrokenTrigger exception if they encounter a glitch.

Processes will observe all of the transient values of the signal.

edge(signal, polarity)

Asynchronously wait until a low-to-high or high-to-low transition of a signal occurs.

This method returns a TriggerCombination object that, when awaited, pauses the execution of the calling process or testbench until the value of signal (a single-bit signal or a single-bit slice of a signal) changes from not polarity to polarity. The returned object may be used to wait for multiple events.

The value captured by this trigger is True if the relevant transition has occurred at the time the wait has completed, and False otherwise.

Warning

In most cases, this method should not be used to wait for a status signal to be asserted or deasserted in a testbench because it is likely to introduce a race condition. Whenever a suitable clock domain is available, use await ctx.tick().until(signal == polarity) instead.

Raises:

• TypeError – If signal is neither a single-bit Signal nor a single-bit slice of a Signal.

• TypeError – If the shape of signal is a ShapeCastable.

• ValueError – If polarity is neither 0 nor 1.

posedge(signal)

Asynchronously wait until a signal is asserted.

Equivalent to edge(signal, 1).

negedge(signal)

Asynchronously wait until a signal is deasserted.

Equivalent to edge(signal, 0).

exception amaranth.sim.BrokenTrigger

Exception raised when a trigger that is repeatedly awaited using an async for loop has a matching event occur while the body of the async for loop is still executing.

exception amaranth.sim.DomainReset

Exception raised when a tick trigger is repeatedly awaited, and its domain has been reset.

class amaranth.sim.TickTrigger(...)

A trigger that wakes up the caller when the active edge of a clock domain occurs or the domain is asynchronously reset.

A TickTrigger is an immutable object that stores a reference to a clock domain and a list of expressions to sample.

The SimulatorContext.tick() method creates a tick trigger with an empty list of sampled expressions, and the TickTrigger.sample() method creates a tick trigger based on another tick trigger that additionally samples the specified expressions.

To wait for a tick trigger to be activated once (a one-shot wait), a process or testbench calls await trigger, usually on a newly created tick trigger:

async def testbench(ctx):
    clk_hit, rst_active, a_value, b_value = await ctx.tick().sample(dut.a, dut.b)

To repeatedly wait for a tick trigger to be activated (a multi-shot wait), a process or testbench asynchronously iterates the tick trigger, usually using the async for loop:

async def testbench(ctx):
    async for clk_hit, rst_active, a_value, b_value in ctx.tick().sample(dut.a, dut.b):
        ...

Both one-shot and multi-shot waits return the same tuple (clk_hit, rst_active, *values) of return values:

1. clk_hit is True if there was an active clock edge at the moment the wait has completed, and False otherwise (that is, if the clock domain was asynchronously reset).

2. rst_active is True if the clock domain is reset (synchronously or asynchronously) at the moment the wait has completed, False otherwise.

3. All following return values correspond to the sampled expressions in the order in which they were added.

Aside from the syntax, there are two differences between one-shot and multi-shot waits:

1. A multi-shot wait continues to observe the tick trigger while the process or testbench responds to the event. If the tick trigger is activated again before the next iteration of the asynchronous iterator (such as while the body of the async for loop is executing), the next iteration raises a BrokenTrigger exception to notify the caller of the missed event.

2. A repeated one-shot wait may be less efficient than a multi-shot wait.

Note

The exact behavior of rst_active differs depending on whether domain uses synchronous or asynchronous reset; in both cases it is True if and only if the domain reset has been asserted. Reusable processes and testbenches, as well as their building blocks, should handle both cases.

sample(*exprs)

Sample expressions when this trigger is activated.

This method returns a new TickTrigger object. When awaited, this object returns, in addition to the values that would be otherwise returned by await trigger, the values of exprs (any ValueLike) at exactly the moment at which the active clock edge, or the asynchronous reset (if applicable), has occurred.

Combining tick() with sample() can be used to capture the state of a circuit after the active clock edge, but before propagation of signal values that have been updated by that clock edge:

async for clk_edge, rst_active, in_a_value, in_b_value in \
        ctx.tick().sample(in_a, in_b):
    ...

Chaining calls to sample() has the same effect as calling it once with the combined list of arguments. The code below has the same behavior as the code above:

async for clk_edge, rst_active, in_a_value, in_b_value in \
        ctx.tick().sample(in_a).sample(in_b):
    ...

Note

Chaining calls to this method is useful for defining reusable building blocks. The following (simplified for clarity) implementation of until() takes advantage of it by first appending condition to the end of the list of captured expressions, checking if it holds, and then removing it from the list of sampled values:

async def until(trigger, condition):
    async for clk_edge, rst_active, *values, done in trigger.sample(condition):
        if done:
            return values

async until(condition)

Repeat this trigger until a condition is met.

This method awaits this trigger at least once, and returns a tuple of the values that are sample()d when condition evaluates to a non-zero value. Values sampled during previous repeats are discarded.

Awaiting a trigger returns values indicating the state of the clock and reset signals, while awaiting trigger.until(...) does not:

while True:
    clk_edge, rst_active, *values, flag_value = await trigger.sample(flag) # never raises
    if flag_value:
        break
# `values` may be used after the loop finishes

values = await trigger.until(flag) # may raise `DomainReset`

Raises:

• TypeError – If the shape of condition is a ShapeCastable.

• DomainReset – If the clock domain has been synchronously or asynchronously reset during the wait.

async repeat(count)

Repeat this trigger a specific number of times.

This method awaits this trigger at least once, and returns a tuple of the values that are sample()d during the last repeat. Values sampled during previous repeats are discarded.

Awaiting a trigger returns values indicating the state of the clock and reset signals, while awaiting trigger.repeat(...) does not:

for _ in range(3):
    clk_edge, rst_active, *values = await trigger # never raises
# `values` may be used after the loop finishes

values = await trigger.repeat(3) # may raise `DomainReset`

Raises:

• ValueError – If count is less than 1.

• DomainReset – If the clock domain has been synchronously or asynchronously reset during the wait.

class amaranth.sim.TriggerCombination(...)

A list of triggers, the activation of any of which will wake up the caller.

A TriggerCombination is an immutable object that stores a list of triggers and expressions to sample. The trigger combination wakes up the caller when any of these triggers activate, and it samples all of the signals at the same moment.

The SimulatorContext.delay(), SimulatorContext.changed(), and SimulatorContext.edge() methods create a trigger combination that consists of just that one trigger, while TriggerCombination.delay(), TriggerCombination.changed(), and TriggerCombination.edge() methods create a trigger combination based on another trigger combination by extending it with an additional trigger. The TriggerCombination.sample() method creates a trigger combination based on another trigger combination that wakes up the caller in the same conditions but additionally samples the specified expressions.

To wait for a trigger combination to be activated once (a one-shot wait), a process or testbench calls await triggers, usually on a newly created trigger combination:

async def testbench(ctx):
    a_value, b_value = await ctx.changed(dut.a, dut.b)

To repeatedly wait for a trigger combination to be activated (a multi-shot wait), a process or testbench asynchronously iterates the trigger combination, usually using the async for loop:

async def testbench(ctx):
    async a_value, b_value in ctx.changed(dut.a, dut.b):
        ...

Both one-shot and multi-shot waits return the same tuple of return values, the elements of which are determined by the triggers and sampled expressions that have been added to the trigger combination, in the order in which they were added. For a detailed description of the return values, refer to SimulatorContext.delay(), SimulatorContext.changed(), SimulatorContext.edge(), and TriggerCombination.sample().

Aside from the syntax, there are two differences between one-shot and multi-shot waits:

1. A multi-shot wait continues to observe the trigger combination while the process or testbench responds to the event. If the trigger combination is activated again before the next iteration of the asynchronous iterator (such as while the body of the async for loop is executing), the next iteration raises a BrokenTrigger exception to notify the caller of the missed event.

2. A repeated one-shot wait may be less efficient than a multi-shot wait.

sample(*exprs)

Sample signals when a trigger from this combination is activated.

This method returns a new TriggerCombination object. When awaited, this object returns, in addition to the values that would be returned by await trigger, the values of exprs at exactly the moment at which the wait has completed.

Combining delay(), changed(), or edge() with sample() can be used to capture the state of a circuit at the moment of the event:

async for arst_edge, delay_expired, in_a_value, in_b_value in \
        ctx.posedge(arst).delay(1e-3).sample(in_a, in_b):
    ...

Chaining calls to sample() has the same effect as calling it once with the combined list of arguments. The code below has the same behavior as the code above:

async for arst_edge, delay_expired, in_a_value, in_b_value in \
        ctx.posedge(arst).delay(1e-3).sample(in_a).sample(in_b):
    ...

Note

Chaining calls to this method is useful for defining reusable building blocks. See the documentation for TickTrigger.sample() for a detailed example.

delay(interval)

Add a delay trigger to the list of triggers.

This method returns a new TriggerCombination object. When awaited, this object also waits for the same trigger as SimulatorContext.delay(), and returns, in addition to the values that would be returned by await trigger, the value returned by SimulatorContext.delay().

changed(*signals)

Add a signal change trigger to the list of triggers.

This method returns a new TriggerCombination object. When awaited, this object also waits for the same trigger as SimulatorContext.changed(), and returns, in addition to the values that would be returned by await trigger, the values returned by SimulatorContext.changed().

edge(signal, polarity)

Add a low-to-high or high-to-low transition trigger to the list of triggers.

This method returns a new TriggerCombination object. When awaited, this object also waits for the same trigger as SimulatorContext.edge(), and returns, in addition to the values that would be returned by await trigger, the values returned by SimulatorContext.edge().

posedge(signal)

Add a low-to-high transition trigger to the list of triggers.

Equivalent to edge(signal, 1).

negedge(signal)

Add a high-to-low transition trigger to the list of triggers.

Equivalent to edge(signal, 0).