yosys/tests/asicworld/code_hdl_models_lfsr_updown.v

`define WIDTH 8
module lfsr_updown (
clk       ,   // Clock input
reset     ,   // Reset input
enable    ,   // Enable input
up_down   ,   // Up Down input
count     ,   // Count output
overflow      // Overflow output
);

 input clk;
 input reset;
 input enable;
 input up_down;

 output [`WIDTH-1 : 0] count;
 output overflow;

 reg [`WIDTH-1 : 0] count;

 assign overflow = (up_down) ? (count == {{`WIDTH-1{1'b0}}, 1'b1}) :
                               (count == {1'b1, {`WIDTH-1{1'b0}}}) ;

 always @(posedge clk)
 if (reset)
    count <= {`WIDTH{1'b0}};
 else if (enable) begin
    if (up_down) begin
      count <= {~(^(count & `WIDTH'b01100011)),count[`WIDTH-1:1]};
    end else begin
      count <= {count[`WIDTH-2:0],~(^(count &  `WIDTH'b10110001))};
    end
 end

endmodule