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module `VARIANT`SVGA_TIMING_GENERATION
#(parameter CHARACTER_DECODE_DELAY=4,
  parameter H_ACTIVE=640,        
  parameter H_FRONT_PORCH=16,        
  parameter H_SYNCH=96,        
  parameter H_BACK_PORCH=48,        
  parameter H_TOTAL=800,        
  parameter V_ACTIVE=480,        
  parameter V_FRONT_PORCH=11,        
  parameter V_SYNCH=2,        
  parameter V_BACK_PORCH=31,        
  parameter V_TOTAL=524        
)
(

input                clk,          // pixel clock
input                reset,        // reset
output reg           h_synch,      // horizontal synch for VGA connector
output reg           v_synch,      // vertical synch for VGA connector
output reg           blank,        // composite blanking
output reg [10:0]    pixel_count,  // counts the pixels in a line
output reg [9:0]     line_count,   // counts the display lines
output reg [2:0]     subchar_pixel,// pixel position within the character
output reg [2:0]     subchar_line, // identifies the line number within a character block
output reg [6:0]     char_column,  // character number on the current line
output reg [6:0]     char_line     // line number on the screen
 );
   


 
reg                    h_blank;                         // horizontal blanking
reg                    v_blank;                         // vertical blanking



reg [9:0]           char_column_count;     // a counter used to define the character column number
reg [9:0]           char_line_count;          // a counter used to define the character line number
reg                     reset_char_line;           // flag to reset the character line during VBI
reg                    reset_char_column;     // flag to reset the character column during HBI



// CREATE THE HORIZONTAL LINE PIXEL COUNTER
always @ (posedge clk) begin
     if (reset)
          // on reset set pixel counter to 0
          pixel_count <= 11'd0;
     
     else if (pixel_count == (H_TOTAL - 1))
          // last pixel in the line, so reset pixel counter
          pixel_count <= 11'd0;
     
     else
          pixel_count <= pixel_count + 1;
end

// CREATE THE HORIZONTAL SYNCH PULSE
always @ (posedge clk ) begin
     if (reset)
          // on reset remove h_synch
          h_synch <= 1'b0;
     
     else if (pixel_count == (H_ACTIVE + H_FRONT_PORCH - 1))
          // start of h_synch
          h_synch <= 1'b1;
     
     else if (pixel_count == (H_TOTAL - H_BACK_PORCH - 1))
          // end of h_synch
          h_synch <= 1'b0;
end

// CREATE THE VERTICAL FRAME LINE COUNTER
always @ (posedge clk ) begin
     if (reset)
          // on reset set line counter to 0
          line_count <= 10'd0;
     
     else if ((line_count == (V_TOTAL - 1)) & (pixel_count == (H_TOTAL - 1)))
          // last pixel in last line of frame, so reset line counter
          line_count <= 10'd0;
     
     else if ((pixel_count == (H_TOTAL - 1)))
          // last pixel but not last line, so increment line counter
          line_count <= line_count + 1;
end

// CREATE THE VERTICAL SYNCH PULSE
always @ (posedge clk ) begin
     if (reset)
          // on reset remove v_synch
          v_synch <= 1'b0;

     else if ((line_count == (V_ACTIVE + V_FRONT_PORCH - 1) &
             (pixel_count == H_TOTAL - 1))) 
          // start of v_synch
          v_synch <= 1'b1;
     
     else if ((line_count == (V_TOTAL - V_BACK_PORCH - 1)) &
             (pixel_count == (H_TOTAL - 1)))
          // end of v_synch
          v_synch <= 1'b0;
end


// CREATE THE HORIZONTAL BLANKING SIGNAL
// the "-2" is used instead of "-1" because of the extra register delay
// for the composite blanking signal 
always @ (posedge clk ) begin
     if (reset)
          // on reset remove the h_blank
          h_blank <= 1'b0;

     else if (pixel_count == (H_ACTIVE -2)) 
          // start of HBI
          h_blank <= 1'b1;
     
     else if (pixel_count == (H_TOTAL -2))
          // end of HBI
          h_blank <= 1'b0;
end


// CREATE THE VERTICAL BLANKING SIGNAL
// the "-2" is used instead of "-1"  in the horizontal factor because of the extra
// register delay for the composite blanking signal 
always @ (posedge clk ) begin
     if (reset)
          // on reset remove v_blank
          v_blank <= 1'b0;

     else if ((line_count == (V_ACTIVE - 1) &
             (pixel_count ==  H_TOTAL - 2))) 
          // start of VBI
          v_blank <= 1'b1;
     
     else if ((line_count == (V_TOTAL - 1)) &
             (pixel_count == (H_TOTAL - 2)))
          // end of VBI
          v_blank <= 1'b0;
end


// CREATE THE COMPOSITE BANKING SIGNAL
always @ (posedge clk ) begin
     if (reset)
          // on reset remove blank
          blank <= 1'b0;

     // blank during HBI or VBI
     else if (h_blank || v_blank)
          blank <= 1'b1;
          
     else
          // active video do not blank
          blank <= 1'b0;
end


/* 
   CREATE THE CHARACTER COUNTER.
   CHARACTERS ARE DEFINED WITHIN AN 8 x 8 PIXEL BLOCK.

     A 640  x 480 video mode will display 80  characters on 60 lines.
     A 800  x 600 video mode will display 100 characters on 75 lines.
     A 1024 x 768 video mode will display 128 characters on 96 lines.

     "subchar_line" identifies the row in the 8 x 8 block.
     "subchar_pixel" identifies the column in the 8 x 8 block.
*/

// CREATE THE VERTICAL FRAME LINE COUNTER
always @ (posedge clk ) begin
     if (reset)
           // on reset set line counter to 0
          subchar_line <= 3'b000;

     else if  ((line_count == (V_TOTAL - 1)) & (pixel_count == (H_TOTAL - 1) - CHARACTER_DECODE_DELAY))
          // reset line counter
          subchar_line <= 3'b000;

     else if (pixel_count == (H_TOTAL - 1) - CHARACTER_DECODE_DELAY)
          // increment line counter
          subchar_line <= line_count[2:0] + 3'b001;
end

// subchar_pixel defines the pixel within the character line
always @ (posedge clk ) begin
     if (reset)
          // reset to 5 so that the first character data can be latched
          subchar_pixel <= 3'b101;
     
     else if (pixel_count == ((H_TOTAL - 1) - CHARACTER_DECODE_DELAY))
          // reset to 5 so that the first character data can be latched
          subchar_pixel <= 3'b101;
     
     else
          subchar_pixel <= subchar_pixel + 1;
end


wire [9:0] char_column_count_iter = char_column_count + 1;

always @ (posedge clk ) begin
     if (reset) begin
          char_column_count <= 10'd0;
          char_column <= 7'd0;
     end
     
     else if (reset_char_column) begin
          // reset the char column count during the HBI
          char_column_count <= 10'd0;
          char_column <= 7'd0;
     end
     
     else begin
          char_column_count <= char_column_count_iter;
          char_column <= char_column_count_iter[9:3];
     end
end

wire [9:0] char_line_count_iter = char_line_count + 1;

always @ (posedge clk ) begin
     if (reset) begin
          char_line_count <= 10'd0;
          char_line <= 7'd0;
     end
     
     else if (reset_char_line) begin
          // reset the char line count during the VBI
          char_line_count <= 10'd0;
          char_line <= 7'd0;
     end
     
     else if (pixel_count == ((H_TOTAL - 1) - CHARACTER_DECODE_DELAY)) begin
          // last pixel but not last line, so increment line counter
          char_line_count <= char_line_count_iter;
          char_line <= char_line_count_iter[9:3];
     end
end

// CREATE THE CONTROL SIGNALS FOR THE CHARACTER ADDRESS COUNTERS
/* 
     The HOLD and RESET signals are advanced from the beginning and end
     of HBI and VBI to compensate for the internal character generation
     pipeline.
*/
always @ (posedge clk ) begin
     if (reset)
          reset_char_column <= 1'b0;

     else if (pixel_count == ((H_ACTIVE - 2) - CHARACTER_DECODE_DELAY))
          // start of HBI
          reset_char_column <= 1'b1;
     
     else if (pixel_count == ((H_TOTAL - 1) - CHARACTER_DECODE_DELAY))
           // end of HBI                         
          reset_char_column <= 1'b0;
end

always @ (posedge clk ) begin
     if (reset)
          reset_char_line <= 1'b0;

     else if ((line_count == (V_ACTIVE - 1)) &
             (pixel_count == ((H_ACTIVE - 1) - CHARACTER_DECODE_DELAY)))
          // start of VBI
          reset_char_line <= 1'b1;
     
     else if ((line_count == (V_TOTAL - 1)) &
             (pixel_count == ((H_TOTAL - 1) - CHARACTER_DECODE_DELAY)))
          // end of VBI                         
          reset_char_line <= 1'b0;
end
endmodule //SVGA_TIMING_GENERATION