myhdl/example/manual/fsm.py

from myhdl import *

ACTIVE_LOW = 0
FRAME_SIZE = 8
t_State = enum('SEARCH', 'CONFIRM', 'SYNC')

def FramerCtrl(SOF, state, syncFlag, clk, reset_n):
    
    """ Framing control FSM.

    SOF -- start-of-frame output bit
    state -- FramerState output
    syncFlag -- sync pattern found indication input
    clk -- clock input
    reset_n -- active low reset
    
    """
    
    index = Signal(0) # position in frame

    @always(clk.posedge, reset_n.negedge)
    def FSM():
        if reset_n == ACTIVE_LOW:
            SOF.next = 0
            index.next = 0
            state.next = t_State.SEARCH

        else:
            index.next = (index + 1) % FRAME_SIZE
            SOF.next = 0

            if state == t_State.SEARCH:
                index.next = 1
                if syncFlag:
                    state.next = t_State.CONFIRM

            elif state == t_State.CONFIRM:
                if index == 0:
                    if syncFlag:
                        state.next = t_State.SYNC
                    else:
                        state.next = t_State.SEARCH

            elif state == t_State.SYNC:
                if index == 0:
                    if not syncFlag:
                        state.next = t_State.SEARCH
                SOF.next = (index == FRAME_SIZE-1)

            else:
                raise ValueError("Undefined state")

    return FSM


def testbench():

    SOF = Signal(bool(0))
    syncFlag = Signal(bool(0))
    clk = Signal(bool(0))
    reset_n = Signal(bool(1))
    state = Signal(t_State.SEARCH)
            
    framectrl = FramerCtrl(SOF, state, syncFlag, clk, reset_n)

    @always(delay(10))
    def clkgen():
        clk.next = not clk

    @instance
    def stimulus():
        for i in range(3):
            yield clk.posedge
        for n in (12, 8, 8, 4):
            syncFlag.next = 1
            yield clk.posedge
            syncFlag.next = 0
            for i in range(n-1):
                yield clk.posedge
        raise StopSimulation
        
    return framectrl, clkgen, stimulus


def main():
    tb_fsm = traceSignals(testbench)
    sim = Simulation(tb_fsm)
    sim.run()

if __name__ == '__main__':
    main()