Question

I am writing Verilog code to convert 8 bit pixels into half tone pixels using Floyd-Stiengburg algorithm. Below is the code to convert 6 pixels (8 bit) to half tone pixels. I have complied this code successfully on Modelsim, but on simulation, the output (half tone pixel value) is XXXXXX (undefined).

  module half_tone(pixel,htpv);
  input [0:47]pixel;
  output reg [1:6]htpv;
  reg [8:1]error[1:6];
  reg [8:1]pixel_1;
  reg [9:0]cpv,cpv_round,e_av;
  parameter threshold =128;
  integer i=1;
  initial
   begin
   error[0]=8'b00;
   for(i=0;i<6;i=i+1)
   begin
   e_av=(2*error[i])>>4;
   cpv=pixel[(i*8)+:8]+e_av;
   cpv_round=(cpv<threshold)?0:255;
   htpv[i]=(cpv_round==0)?0:1;
   error[i]=cpv-cpv_round;
   #10;
   end
 end
 endmodule

I am unable to sort out why the output is XXXXXX(undefined).

Answer 1

When I compiled your code with the Incisive simulator, I got this warning message:

   error[0]=8'b00;
       |
ncelab: *W,BNDMEM (./half_tone.v,11|7): Memory index out of declared bounds [4.2.2(IEEE)].

You need to initialize error properly. Maybe error[1]=8'b00;?