3.2.2.2 Example

from random import randrange
from myhdl import *

ACTIVE_LOW, INACTIVE_HIGH = 0, 1

def Inc(count, enable, clock, reset, n):

    """ Incrementer with enable.
    
    count -- output
    enable -- control input, increment when 1
    clock -- clock input
    reset -- asynchronous reset input
    n -- counter max value

    """

    @always(clock.posedge, reset.negedge)
    def incLogic():
        if reset == ACTIVE_LOW:
            count.next = 0
        else:
            if enable:
                count.next = (count + 1) % n

    return incLogic

For the test bench, we will use an independent clock generator, stimulus generator, and monitor. After applying enough stimulus patterns, we can raise the StopSimulation exception to stop the simulation run. The test bench for a small incrementer and a small number of patterns is a follows:

def testbench():
    count, enable, clock, reset = [Signal(intbv(0)) for i in range(4)]

    inc_1 = Inc(count, enable, clock, reset, n=4)

    HALF_PERIOD = delay(10)

    @always(HALF_PERIOD)
    def clockGen():
        clock.next = not clock

    @instance
    def stimulus():
        reset.next = ACTIVE_LOW
        yield clock.negedge
        reset.next = INACTIVE_HIGH
        for i in range(12):
            enable.next = min(1, randrange(3))
            yield clock.negedge
        raise StopSimulation

    @instance
    def monitor():
        print "enable  count"
        yield reset.posedge
        while 1:
            yield clock.posedge
            yield delay(1)
            print "   %s      %s" % (enable, count)

    return clockGen, stimulus, inc_1, monitor


tb = testbench()

def main():
    Simulation(tb).run()

% python inc.py
enable  count
   0      0
   1      1
   0      1
   1      2
   1      3
   1      0
   0      0
   1      1
   0      1
   0      1
   0      1
   1      2
StopSimulation