Generic Gray Code Counter

[omara007]

Added after 3 minutes:

I don't want to have the glitches resulted from the binary counting

if you use combination logic, you must have glitches, even the simplest combination circuit with only 2 ANDs, by some inputs.

Counters are not COMBINATIONAL circuits

 

[echo47]

I whipped up this synchronous Gray counter without using any intermediate counter or tedious 'case' statement. The 'bin' value is combinatorial. The number of bits is easily adjusted.

module top (clk, gray);
  parameter             bits = 8;
  input                 clk;
  reg        [bits-1:0] bin = 0;
  output reg [bits-1:0] gray = 0;

  integer i;
  always @ (gray)
    for (i = 0; i < bits; i = i + 1)
      bin[i] <= ^(gray >> i);

  always @ (posedge clk)
    gray <= ((bin + 1'd1) >> 1) ^ (bin + 1'd1);  // to count down, change both + to -
endmodule