source: sandbox/MultiChannelUSB/spi_fifo.v@ 123

module spi_fifo
        (               
                input           wire                            clock, reset,

                input           wire                            bus_ssel, bus_wren,
                input           wire    [15:0]  bus_mosi,

                output  wire                            bus_busy,

                output  wire                            spi_sel,
                output  wire                            spi_sdo,
                output  wire                            spi_clk
        );

        wire                            int_rdempty, int_wrfull;
        wire    [15:0]  int_q;

        reg                             int_bus_busy;
        reg                             int_rdreq, int_wrreq;
        reg                             int_clken, int_sdo, int_sel;
        reg     [15:0]  int_bus_mosi;
        reg     [15:0]  int_data;
        reg     [2:0]           clk_cntr;
        reg     [3:0]           bit_cntr;
        reg     [1:0]           state;

        scfifo #(
                .add_ram_output_register("OFF"),
                .intended_device_family("Cyclone III"),
                .lpm_numwords(16),
                .lpm_showahead("ON"),
                .lpm_type("scfifo"),
                .lpm_width(16),
                .lpm_widthu(4),
                .overflow_checking("ON"),
                .underflow_checking("ON"),
                .use_eab("OFF")) fifo_tx (
                .rdreq((~int_rdempty) & (int_rdreq) & (&clk_cntr)),
                .aclr(1'b0),
                .clock(clock),
                .wrreq(int_wrreq),
                .data(int_bus_mosi),
                .empty(int_rdempty),
                .q(int_q),
                .full(int_wrfull),
                .almost_empty(),
                .almost_full(),
                .sclr(),
                .usedw());
        
        always @ (posedge clock)
        begin
                int_bus_busy <= int_wrfull;

                if (bus_ssel)
                begin
                        if (~int_wrfull & bus_wren)
                        begin
                                int_bus_mosi <= bus_mosi;
                                int_wrreq <= 1'b1;
                        end
                end
                
                if (~int_wrfull & int_wrreq)
                begin
                        int_wrreq <= 1'b0;
                end

        end

        always @ (posedge clock)
        begin
                clk_cntr <= clk_cntr + 3'd1;
                if (&clk_cntr)
                begin
                        case (state)
                                0:
                                begin
                                        int_sdo <= 1'b0;
                                        int_sel <= 1'b1;
                                        int_clken <= 1'b0;
                                        int_rdreq <= 1'b1;
                                        state <= 2'd1;
                                end
        
                                1: 
                                begin
                                        if (~int_rdempty)
                                        begin
                                                int_rdreq <= 1'b0;
                                                int_data <= int_q;
                                                bit_cntr <= 4'd0;
                                                state <= 2'd2;
                                        end
                                end
        

                                2:
                                begin // data
                                        int_clken <= 1'b1;
                                        int_sel <= 1'b0;
                                        int_sdo <= int_data[11];
                                        int_data <= {int_data[10:0], 1'b0};
                                        bit_cntr <= bit_cntr + 4'd1;
                                        if (bit_cntr == 4'd11)
                                        begin
                                                state <= 2'd3;
                                        end
                                end
        
                                3:
                                begin
                                        int_sdo <= 1'b0;
                                        int_clken <= 1'b0;
                                        state <= 2'd0;
                                end
        
                        endcase
                end
        end

        // output logic
        assign  bus_busy = int_bus_busy;
        assign  spi_clk = (int_clken ? clk_cntr[2] : 1'b1);     
        assign  spi_sdo = int_sdo;      
        assign  spi_sel = int_sel;      

endmodule