/* show4let:  by karttu, Jan 1 2005.
              Show four letters specified as (inverted)
              7-segment bit masks given in MASKS[0:3] in the
              four 7-segment LED display of Spartan-3 board.

   For some reason input [6:0] LET [3:0]
   does not work!
 */

module show4let(input CLK,
                input [6:0] LET0,
                input [6:0] LET1,
                input [6:0] LET2,
                input [6:0] LET3,
                output [7:0] SEG_OUT,
                output [3:0] DIGIT_OUT);

parameter msb = 16;
reg [msb:0] delay_counter = 0;

wire [1:0] d = delay_counter[msb:msb-1];

assign SEG_OUT[7] = 1'b1; // Don't light the dp.
assign SEG_OUT[6:0] = (0 == d ? LET0 : 1 == d ? LET1 : 2 == d ? LET2 : LET3);

assign DIGIT_OUT = (0 == d) ? 4'b1110 : (1 == d) ? 4'b1101 : (2 == d) ? 4'b1011 : 4'b0111;
// = (4'b1110 << d); // Works in principle, not in practice.

always @(posedge CLK)
  begin
    delay_counter <= delay_counter+1; // Wraps around, eventually.
  end

endmodule