source: trunk/MultiChannelUSB/usb_fifo.v@ 44

module usb_fifo
        (
                input   wire                    usb_clk,
                inout   wire    [7:0]   usb_data,
                input   wire                    usb_full, usb_empty,
                output  wire                    usb_wrreq, usb_rdreq, usb_rden, usb_pktend,
                output  wire    [1:0]   usb_addr,
                
                input   wire                    clk, aclr,
                input   wire                    tx_wrreq, rx_rdreq,
                input   wire    [7:0]   tx_data,
                output  wire                    tx_full, rx_empty,
                output  wire    [7:0]   rx_q
        );

        wire                    int_rx_full, int_tx_empty;
        wire                    rx_ready, tx_ready;
        wire                    int_rdreq, int_wrreq, int_pktend;
        reg                             is_rx_addr_ok;
        reg             [8:0]   byte_counter;
        reg             [4:0]   idle_counter;

        wire    [7:0]   int_rx_data = usb_data;
        wire    [7:0]   int_tx_q;

        fifo32x8 fifo_tx_unit (
                .aclr(aclr),
                .data(tx_data),
                .rdclk(usb_clk),
                .rdreq(int_wrreq),
                .wrclk(clk),
                .wrreq(tx_wrreq),
                .q(int_tx_q),
                .rdempty(int_tx_empty),
                .wrfull(tx_full));

        fifo32x8 fifo_rx_unit (
                .aclr(aclr),
                .data(int_rx_data),
                .rdclk(clk),
                .rdreq(rx_rdreq),
                .wrclk(usb_clk),
                .wrreq(int_rdreq),
                .q(rx_q),
                .rdempty(rx_empty),
                .wrfull(int_rx_full));
        
        assign  rx_ready = (~usb_empty) & (~int_rx_full) & (~int_pktend);
        assign  tx_ready = (~rx_ready) & (~usb_full) & (~int_tx_empty) & (~int_pktend);

        assign  int_rdreq = (rx_ready) & (is_rx_addr_ok);
        assign  int_wrreq = (tx_ready) & (~is_rx_addr_ok);
        
        assign  int_pktend = (&idle_counter);

        always @ (posedge usb_clk)
        begin
                // respect 1 clock delay between fifo selection
                // and data transfer operations
                is_rx_addr_ok <= rx_ready;

                // assert pktend if buffer contains unsent data
                // and fifo_tx_unit stays empty for more than 30 clocks
                if (int_pktend)
                begin
                        byte_counter <= 9'd0;
                        idle_counter <= 5'd0;
                end
                else if (int_wrreq)
                begin
                        byte_counter <= byte_counter + 9'd1;
                        idle_counter <= 5'd0;
                end
                else if ((|byte_counter) & (int_tx_empty) & (~rx_ready))
                begin
                        byte_counter <= byte_counter;
                        idle_counter <= idle_counter + 5'd1;
                end

        end

        assign  usb_pktend = int_pktend;
        assign  usb_rdreq = int_rdreq;
        assign  usb_wrreq = int_wrreq;
        assign  usb_rden = int_rdreq;
        assign  usb_addr = {1'b1, ~rx_ready};
        assign  usb_data = int_wrreq ? int_tx_q : 8'bz;

endmodule