Why Won't My Verilog Code Synthesize in Xilinx?

[freezer]

I am trying to build a BCD driver for my BASYS2 board that i can supply a 16bit data stream and will display the number on the four 7 segment LED.

In this module, i have just hard coded the numbers to display. It does not currently have the 16bit input. It scans across each of the four 7 segment led and will display 1 2 3 4.

The module tests fine in the test bench but when i go to synthesize it with the zilinx software, i get an Unsupported Control Statement error.

I have read through my verilog book and everything seems to be in order.

I have annotated the code where xilinx point to the issue.

Here is the verilog code:

module scanner(AN, BCD, segments, scan, clk);
  
  input wire clk;
  
  output reg [1:0] scan;
  output reg [3:0] AN;
  output reg [3:0] BCD;
  output reg [6:0] segments;
  
  initial scan = 2'b00;
  
  always
    begin
      @(posedge clk)   //<<<<<<<Error points here
        scan = scan + 1'b1;
  
           case(scan)
//Turns on or off each of the 4 7 seg LEDs  
// AN0 1 on,  0 off
// AN1 1 off, 0 on
// AN2 1 off, 0 on
// AN3 1 on,  0 off
                         //LED     3210
              2'b00 : AN <= 4'b0111;   //AN0 ON
              2'b01 : AN <= 4'b0100;   //AN1 ON
              2'b10 : AN <= 4'b0010;   //AN2 ON
              2'b11 : AN <= 4'b1110;   //AN3 ON  
          endcase
          
          case(AN)
              //Temp to give BCD input some values
              2'b00 : BCD <= 4'b0100;   
              2'b01 : BCD <= 4'b0011;  
              2'b10 : BCD <= 4'b0010; 
              2'b11 : BCD <= 4'b0001;   
          endcase
          
          case(BCD)
              //Due to the physical FPGA 7 Seg LED, the segments is inverted
              //to drive the correct segments on the display.
              //                                abcdefg
              4'b0000 : segments = 7'b0000001; //zero
              4'b0001 : segments = 7'b1001111; //one
              4'b0010 : segments = 7'b0010010; //two
              4'b0011 : segments = 7'b0000110; //three
              4'b0100 : segments = 7'b1001100; //four
              4'b0101 : segments = 7'b0100100; //five
              4'b0110 : segments = 7'b0100000; //six
              4'b0111 : segments = 7'b0001111; //seven
              4'b1000 : segments = 7'b0000000; //eight
              4'b1001 : segments = 7'b0001100; //nine
              4'b1010 : segments = 7'b0001000; //a
              4'b1011 : segments = 7'b1100000; //b
              4'b1100 : segments = 7'b0110001; //c
              4'b1101 : segments = 7'b1000010; //d
              4'b1110 : segments = 7'b0110000; //e
              4'b1111 : segments = 7'b0111000; //f
              default : segments = 7'b0110000; //e
            
            
          endcase
  end  
endmodule

 

[freezer]

Found out that the @(posedge clk) needed to be before the begin...

 

[meBigGuy]

A good habit when writing synthesizable verilog is to use the standard flop template and to include a reset term.
The reset term is most important for simulation (since everything starts as X and X+1 = X). The flops all start at zero in Xilinx unless you set them to 1 during the reset, but that's not true for simulation.

Also, you should use nonblocking assignment (<=) for synchronous elements (flip-flops) or you will have huge simulation problems in the future and will be throwing around #1's to get your sim to work.

//async reset flop
always @ (posedge clk or posedge reset) begin
   if(reset == 1) begin
       scan <= 0;
   else begin
       scan <= scan+1;
   end
end

//sync reset flop
always @ (posedge clk) begin
   if(reset == 1) begin
       scan <= 0;
   else begin
       scan <= scan+1;
   end
end