ram_window.py

class RamWindow(Elaboratable):
    """
    Note - the maximum (0xFF..) address isn't usable while shifting samples in!
    I don't plan to do anything about it.
    
    Latency (input to output) - (2 + addr) cycles 

    Latency (addr to output) - 1 cycle (!)
    """

    def __init__(self, width, addr_width):
        self.width = width
        self.addr_width = addr_width

        self.shift_in = Signal(reset=1)
        self.addr = Signal(self.addr_width)

        self.input = Signal(width)
        self.output = Signal(width)

    def elaborate(self, platform):
        depth = 2**self.addr_width

        cursor = Signal(self.addr_width, reset=0)

        mem = Memory(width=self.width,
                depth=depth,
                init=[0]*depth)

        m = Module()

        m.submodules.rdport = rdport = mem.read_port()
        m.submodules.wrport = wrport = mem.write_port()

        m.d.comb += [
                wrport.en.eq(self.shift_in),
                wrport.data.eq(self.input),
                wrport.addr.eq(cursor+1),

                rdport.addr.eq(cursor-self.addr),
                self.output.eq(rdport.data),
        ]

        with m.If(self.shift_in):
            m.d.sync += cursor.eq(cursor + 1)

        return m