parameter     //atmBytes            = 53,
                ValidCounterInit    = 9'd423, // [{atmBytes * 8) - 1]
                ValidCounterWidth   = 9,
                LockUpCause         = 5,
                LockDownCause       = 5;

parameter       DemuxHunt           = 2'b01,
                DemuxPreSync        = 2'd10,
                DemuxSync           = 2'd11;

always @ (posedge clk)
begin
    if (~rst) begin
        crc_       <=  8'd0;
        atm        <= 40'd0; end
    else if (demuxFlowClk) begin
//     crc_[7:0] <= {crc_[7:6], crc_[5]^atm[39], crc_[4]^atm[39], ...
//     crc_[3:2], crc_[1]^crc_[7], crc_[0]^crc_[7]^atm[39], crc_[7]^demuxFlow};
        crc_[0]    <=    crc_[7]^demuxFlow;
        crc_[1]    <=    crc_[0]^crc_[7]^atm[39];
        crc_[2]    <=    crc_[1]^crc_[7];
        crc_[3]    <=    crc_[2];
        crc_[4]    <=    crc_[3];
        crc_[5]    <=    crc_[4]^atm[39];
        crc_[6]    <=    crc_[5]^atm[39];
        crc_[7]    <=    crc_[6];
        
        atm[39:0] <= {atm[39:0], demuxFlow};
    end
end

assign  crcValid    = ~|crc_ & (flowCtrlReg0 == 8'h73);

always @ (posedge clk)
begin
    if (~rst) begin validCounter    <= ValidCounterInit;
                    demuxStatus     <= 0;
                    upCounter       <= 0;
                    downCounter     <= 0;    end

    else if (demuxFlowClk) begin
        if (|validCounter & (~crcValid | demuxSync)) validCounter <= validCounter - 1;
        else validCounter <= ValidCounterInit;


        if (demuxHunt) begin            // HUNT
            if (crcValid & demuxCycleEvent) demuxStatus <= DemuxPreSync;
                upCounter   <= 0;
                downCounter <= 0;
         end
        
        else if (demuxPreSync) begin    // PRESYNC
            if (crcValid & demuxCycleEvent) begin
                    if (~lockUp)    upCounter   <= upCounter + 1;
                    else            demuxStatus <= DemuxSync; end
            else if (crcValid ^ demuxCycleEvent)
                                    demuxStatus <= DemuxHunt;
         end
        
        else if (demuxSync) begin       // SYNC
            if (crcValid ^ demuxCycleEvent) begin
                    if (~lockDown)  downCounter <= downCounter + 1;
                    else            demuxStatus <= DemuxHunt; end
            else if (crcValid & demuxCycleEvent)
                                    downCounter <= 0;

         end
        
        else    demuxStatus <= DemuxHunt;
    end
end

assign demuxCycleEvent  = (~|validCounter);
assign lockUp           = (upCounter   == LockUpCause);
assign lockDown         = (downCounter == LockDownCause);
assign demuxHunt        = (demuxStatus == DemuxHunt);
assign demuxPreSync     = (demuxStatus == DemuxPreSync);
assign demuxSync        = (demuxStatus == DemuxSync);

assign demuxLock        = crcValid & demuxSync;

wire    [7:0]    flowCtrlReg0   = atm[39:32];
wire    [7:0]    flowCtrlReg1   = atm[31:24];
wire    [7:0]    flowCtrlReg2   = atm[23:16];
wire    [7:0]    flowCtrlReg3   = atm[15:8];
wire    [7:0]    crcDataReg     = atm[7:0];