VGA

By xantus on Friday 5 February 2010 20:22
Category: Verilog examples, Views: 1.736

The verilog provided at 4.3_psp_lcd can also be used to drive a VGA display. Only the timing and resolution in h_sync, v_sync and in_picture and the pixel clock in clk should be changed to the correct values as explained in vga_timing.

Example for 640x480

in_picture module

Change the values of the front and backporch in in_picture by changing

C: in_picture

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assign h_pixel = h_addr - 10'h30;
assign v_line = v_addr - 9'h10;
assign in_picture = (h_addr > 10'h30 && h_addr < 10'h213 && v_addr > 9'h10 && v_addr < 9'h120)?1'b1:1'b0;

C: in_picture

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assign h_pixel = h_addr - 10'h90;
assign v_line = v_addr - 9'h23;
assign in_picture = (h_addr > 10'h90 && h_addr < 10'h310 && v_addr > 9'h23 && v_addr < 9'h1E2)?1'b1:1'b0;

clk module

Change the pixel clock in clk by changing

C: clk

  always @ (posedge clk_50MHz)
    begin
      if (count == 3'h5)
        count <= 0;
      else
        count <= count + 3'h1;
    end
 
  assign clk_9MHz = count[2];

C: clk

  always @ (posedge clk_50MHz)
    count <= ~count;
 
  assign clk_9MHz = count[0];

v_sync module

In v_sync module, change the sync pulse duration by changing the following:

C: v_sync

      if (count == 286)
        count <= 9'h0;
 
      if (count < 12)
        sync <= 1'h0;
      else
        sync <= 1'h1;

C: v_sync

      if (count == 525) 
        count <= 9'h0;
 
      if (count < 2)
        sync <= 1'h0;
      else
        sync <= 1'h1;

h_sync module

In h_sync module, change the sync pulse duration by changing the following:

C: h_sync

      if (count == 525) //525
        count <= 10'h0;
 
      if (count < 45) //45
        sync <= 1'h0;
      else
        sync <= 1'h1;

C: h_sync

      if (count == 799)
        count <= 10'h0;
 
      if (count < 96)
        sync <= 1'h0;
      else
        sync <= 1'h1;

Signal widths

Also, the width of h_addr, v_addr, h_pixel, v_line and address should be changed and the depth of the display memory

	old	new
h_addr	10	11
v_addr	9	10
h_pixel	10	11
v_line	9	10
address	10	11
memory depth	2048	4096

Result

This will result in a display memory of 128×32 character with 30×80 visible character at a resolution of 680×480 (60 Hz refresh rate). It will use up 64 kB (70%) of the available ram bits. It is recommended to reduce the font table to 64 characters and (if needed) to increase to size of the characters to 16×32 instead of 8×16. This is done by shifting the h_addr and v_addr lines one to the left and increasing the shift register (in the color module) every other clock cycle.