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axilent provides python tools for describing a sequence of Axi4Lite command in python.

A python test can be written describing the sequence of commands to be sent and the expected responses. This test can then be run against both a simulation of the design and against the implemented design running on an FPGA.

axilent is structured so that tests can be run both against both interactive simulations and simulations where inputs cannot depend on outputs such as file-based testbenchs.


Currently there is handler for working with file-based testbenchs generated by slvcodec and for simulations on FPGAs run over JTAG using pyvivado. In the likely event you are using neither of these methods, then you will have to write you own handler. The handler has a single required method.

Sends a command to the FPGA, or adds the command to the list of commands that will be run in the simulation.

The handler also processes the responses from the simulation or FPGA and updates the command.future values appropriately. It calls the command.process_responses method to help process the responses.


A Command object represents a simple axi4lite read or write, or a more complex aggregrate consisting of many read and write commands.

Returns a list of AxiCommand objects. Each AxiCommand object is a simple axi4lite read or write command to a single address or a range of addresses.
command.process_responses(read_responses, write_responses, resolve_future)
Processes two lists of AxiResponse objects, one is a list of responses to write requests and one is a list of responses to read requests. The two lists must begin with the responses to this command, and these entries are removed from the lists. An (exception, result) tuple is returned by the function. If resolve_future is True then the commands future is resolved with the result and exception.

As an example consider an AxiAdder module, which uses an Axi4Lite interface. We can write to registers A and B, and when we read from register C the returned result is the sum of the contents in regsiters A and B. We can create a Command object to represent using this hardware to add two numbers.

class AddNumbersCommand(comms.CombinedCommand):
    A command that writes to the intA and intB registers
    and then reads from the intC register.
    The effect is the add the two inputs.

    def __init__(self, a, b, addresses):
        write_a_command = comms.SetUnsignedCommand(
            address=addresses['intA'], value=a,
            description='Setting A in AddNumbers',
        write_b_command = comms.SetUnsignedCommand(
            address=addresses['intB'], value=b,
            description='Setting B in AddNumbers',
        read_c_command = comms.GetUnsignedCommand(
            description='Getting C from AddNumbers',
        commands = (write_a_command, write_b_command, read_c_command)
            description='Add 2 numbers with AddNumber',

    def process_responses(self, read_responses, write_responses, resolve_future=True):
        Return the third response (from the final read command)
        Don't return any errors.
        e, result = super().process_responses(read_responses, write_responses, resolve_future=False)
        intc = result[2]
        if resolve_future:
            self.resolve_future(e, intc)
        return e, intc


Typically command objects are hidden from the testing interface by wrapping them with a Comm object. Methods on this comm object create Command objects, send them to a handler, and return the command futures.

class AxiAdderComm:
    Class to communicate with the AxiAdder module.


    def __init__(self, address_offset, handler):
        `address_offset` is any addition that is made to the address that is
        consumed during routing.
        `handler` is the object responsible for dispatching the commands.
        self.handler = handler
        self.address_offset = address_offset
        self.addresses = {
            'intA': address_offset + self.INTA_ADDRESS,
            'intB': address_offset + self.INTB_ADDRESS,
            'intC': address_offset + self.INTC_ADDRESS,

    def add_numbers(self, a, b):
        A complex complex command that write to two registers and
        then reads from another.
        Sets 'a' and 'b' then reads 'c' (should be a+b)
        command = AddNumbersCommand(a, b, self.addresses)
        return command.future


A possible way to write a test to is define a class with a prepare method that defines the requests to send to the module, and a check method that analyzes the responses.

The prepare method uses a handler to generate the requests and creates a number of futures to hold the results of processing the responses.

The responses are then processed by a handler-dependent method and then the check method can be run to check the contents of the resolved futures.

class AxiAdderTest(object):

    def __init__(self):
        self.expected_intcs = []
        self.intc_futures = []

    def prepare(self, handler):
        Sends a number of 'add_numbers' commands.
        comm = AxiAdderComm(address_offset=0, handler=handler)
        n_data = 20
        max_int = pow(2, 16)-1
        logger.debug('preparing data')
        for i in range(n_data):
            inta = random.randint(0, max_int)
            intb = random.randint(0, max_int)
            self.expected_intcs.append(inta + intb)
            future = comm.add_numbers(inta, intb)
        # Flush the communication for simulations.
        # Ignored in FPGA.

    def check(self):
        Check that the output of the commands matches the expected values.
        output_intcs = [f.result() for f in self.intc_futures]
        assert output_intcs == self.expected_intcs

Repeatability of Simulations

Although the simulations are repeatable the FPGA-based tests are currently not repeatable because of the changing number of clock-cycles between when requests are received. I would like to fix this by allowing the ability of specify on which clock cycle at AXI request should be sent (they would be gathered in a delayed in a buffer on the FPGA until the correct clock cycle). TODO: Add delaying of requests to allow repeatability.

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