source: sandbox/MultiChannelUSB/i2c_fifo.v@ 190

module i2c_fifo
        (               
                input   wire                    clock, reset,

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

                output  wire                    bus_busy,

                inout   wire                    i2c_sda,
                inout   wire                    i2c_scl
        );

        wire                    int_rdempty, int_wrfull, i2c_clk, start, stop;
        wire    [15:0]  int_q;

        reg                             int_bus_busy;
        reg                             int_rdreq, int_wrreq, int_clken, int_sdo, int_scl, int_ack;
        reg             [15:0]  int_bus_mosi;
        reg             [15:0]  int_data;
        reg             [9:0]   counter;
        reg             [4:0]   state;

        assign i2c_sda = int_sdo ? 1'bz : 1'b0;
        assign i2c_scl = int_scl | (int_clken ? counter[9] : 1'b0);     

        assign start = int_data[8];
        assign stop = int_data[9];

        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) & (&counter)),
                .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
                counter <= counter + 10'd1;
                if (&counter)
                begin
                        case (state)
                                0:
                                begin
                                        int_ack <= 1'b0;
                                        int_sdo <= 1'b1;
                                        int_scl <= 1'b1;
                                        int_rdreq <= 1'b1;
                                        state <= 5'd1;
                                end
        
                                1: 
                                begin
                                        if (~int_rdempty)
                                        begin
                                                int_data <= int_q;
                                                int_rdreq <= 1'b0;
                                                state <= 5'd2;
                                        end
                                end
        
                                2: 
                                begin
                                        if (start)
                                        begin
                                                int_sdo <= 1'b1;
                                                int_scl <= 1'b1;
                                                state <= 5'd3;
                                        end
                                        else
                                        begin
                                                state <= 5'd5;
                                        end
                                end
                        
                                3:
                                begin // start
                                        int_sdo <= 1'b0;
                                        state <= 5'd4;
                                end
        
                                4:
                                begin
                                        int_scl <= 1'b0;
                                        state <= 5'd5;
                                end
                        
                                5:
                                begin // data
                                        int_clken <= 1'b1;
                                        int_sdo <= int_data[7];
                                        state <= 5'd6;
                                end
        
                                6:
                                begin
                                        int_sdo <= int_data[6];
                                        state <= 5'd7;
                                end
        
                                7:
                                begin
                                        int_sdo <= int_data[5];
                                        state <= 5'd8;
                                end
        
                                8:
                                begin
                                        int_sdo <= int_data[4];
                                        state <= 5'd9;
                                end
        
                                9:
                                begin
                                        int_sdo <= int_data[3];
                                        state <= 5'd10;
                                end
        
                                10:
                                begin
                                        int_sdo <= int_data[2];
                                        state <= 5'd11;
                                end
        
                                11:
                                begin
                                        int_sdo <= int_data[1];
                                        state <= 5'd12;
                                end
        
                                12:
                                begin
                                        int_sdo <= int_data[0];
                                        state <= 5'd13;
                                end
                                
                                13:
                                begin // ack
                                        int_sdo <= 1'b1;
                                        int_rdreq <= 1'b1;
                                        state <= 5'd14;
                                end
        
                                14:
                                begin 
                                        int_ack <= i2c_sda;
                                        int_rdreq <= 1'b0;
                                        if (stop | int_rdempty)
                                        begin
                                                int_clken <= 1'b0;
                                                int_sdo <= 1'b0;
                                                int_scl <= 1'b0;
                                                state <= 5'd15;
                                        end
                                        else if (~int_rdempty)
                                        begin
                                                int_data <= int_q;
                                                int_sdo <= int_q[7];
                                                state <= 5'd6;
                                        end
                                end
        
                                15:
                                begin // stop
                                        int_scl <= 1'b1;
                                        state <= 5'd16;
                                end
        
                                16:
                                begin
                                        int_sdo <= 1'b1;
                                        state <= 5'd0;
                                end
        
                        endcase
                end
        end

        // output logic
        assign  bus_busy = int_bus_busy;

endmodule