myhdl/example/cookbook/johnson/jc2.py

from myhdl import *

ACTIVE = 0
DirType = enum('RIGHT', 'LEFT')

def jc2(goLeft, goRight, stop, clk, q):
    
    """ A bi-directional 4-bit Johnson counter with stop control.

    I/O pins:
    --------
    clk      : input free-running slow clock 
    goLeft   : input signal to shift left (active-low switch)
    goRight   : input signal to shift right (active-low switch)
    stop     : input signal to stop counting (active-low switch)
    q        : 4-bit counter output (active-low LEDs; q[0] is right-most)

    Operation:
    ---------
    The counter is triggered on the rising edge of the clock (clk). 
    A low pulse on the goLeft input will cause the counter to start 
    shifting left from its current state. A low pulse on the goRight
    input will cause the counter to start shifting right from its 
    current state. A low pulse on the stop input will cause the 
    counter to hold its current state until goLeft or goRight is pulsed.

    After power-up, the counter is stopped with all outputs low (LEDs lit).

    """

    dir = Signal(DirType.LEFT)
    run = Signal(False)

    @always(clk.posedge)
    def logic():
        # direction
        if goRight == ACTIVE:
            dir.next = DirType.RIGHT
            run.next = True
        elif goLeft == ACTIVE:
            dir.next = DirType.LEFT
            run.next = True
        # stop
        if stop == ACTIVE:
            run.next = False
        # counter action
        if run:
            if dir == DirType.LEFT:
                q.next[4:1] = q[3:]
                q.next[0] = not q[3]
            else:
                q.next[3:] = q[4:1]
                q.next[3] = not q[0]

    return logic