/*
SuperVGA timing from NEC monitor manual
Horizontal :
                ______________                 _____________
               |              |               |
_______________|  VIDEO       |_______________|  VIDEO (next line)

___________   _____________________   ______________________
           |_|                     |_|
            B C <------D-----><-E->
            <----------A---------->


Vertical :
                ______________                 _____________
               |              |               |           
_______________|  VIDEO       |_______________|  VIDEO (next frame)

___________   _____________________   ______________________
           |_|                     |_|
            P Q <------R-----><-S->
            <----------O---------->

For VESA 800*600 @ 60Hz:
Fh (kHz) :37.88
A  (us)  :26.4
B  (us)  :3.2
C  (us)  :2.2
D  (us)  :20.0
E  (us)  :1.0

Fv (Hz)  :60.32
O  (ms)  :16.579
P  (ms)  :0.106
Q  (ms)  :0.607
R  (ms)  :15.84
S  (ms)  :0.026


Horizonal timing information:

Mode name          Pixel      sync      back  active  front whole line
                   clock      pulse     porch  time   porch  period
                   (MHz)   (us)  (pix)  (pix)  (pix)  (pix)  (pix)
 
VGA 800x600 60Hz   40       3.2   128     85    806    37    1056

Vertical timing information:
Mode name          Lines   line    sync      back        active     front     whole frame
                   Total   width   pulse     porch        time      porch        period
                           (us)   (us)(lin) (us)(lin)  (us)  (lin)  (us)(lin)  (us)  (lin)

VGA 800x600 60Hz    628    26.40  106   4   554   21   15945  604         -1*  16579  628

*/


module vga_vl(resetn,
              clock,
              orient,
              hsync,
              vsync,
              pixel,
              blank
              );
 input        resetn,clock,orient;
 output       hsync,vsync,blank;
 output [2:0] pixel;
 wire         hsync;
 reg          vsync;
 reg    [2:0] pixel;
 
 //Horizontal timing constants
 parameter H_PIXELS      = 'd806,//'d640,
           H_FRONTPORCH  = 'd37,//16,
           H_SYNCTIME    = 'd128,//'d96,
           H_BACKPORCH   = 'd85,//48,
           H_SYNCSTART   = 'd843,// H_PIXELS + H_FRONTPORCH,
           H_SYNCEND     = 'd971,// H_SYNCSTART + H_SYNCTIME,
           H_PERIOD      = 'd1056,//H_SYNCEND + H_BACKPORCH,
 //Vertical timing constants
           V_LINES       = 'd604,//48,         
           V_FRONTPORCH  = -1,//'d10,
           V_SYNCTIME    = 'd4,//2,
           V_BACKPORCH   = 'd21,//33,
           V_SYNCSTART   = 'd603, //V_LINES + V_FRONTPORCH,
           V_SYNCEND     = 'd607,// V_SYNCSTART + V_SYNCTIME,
           V_PERIOD      = 'd628; //V_SYNCEND + V_BACKPORCH;
    
 reg [10:0] hcnt,vcnt;
 reg enable,hsyncint; 
 
 //Horizontal counter of pixels
 always @ (posedge clock or negedge resetn)
 if(!resetn)
     hcnt <= 0;
 else if(hcnt<H_PERIOD)
     hcnt <= hcnt + 1;  
 else 
     hcnt <= 0;  
 
 //Internal horizontal synchronization pulse generation (negative polarity)
 always @ (posedge clock or negedge resetn)
 if(!resetn)
     hsyncint<= 1;
 else if(hcnt >= H_SYNCSTART && hcnt < H_SYNCEND)
     hsyncint <= 0; 
 else
     hsyncint<= 1; 
 
 //Horizontal synchronization output
 assign hsync = hsyncint;   
 
 //Vertical counter of lines
 always @ (posedge hsyncint or negedge resetn)
 if(!resetn)
    vcnt<=0;
 else if(vcnt < V_PERIOD) 
    vcnt <= vcnt + 1;
 else
    vcnt<=0;
    
//Vertical synchronization pulse generation (negative polarity)
always @ (posedge hsyncint or negedge resetn)
if(!resetn)
    vsync <=1;
else if(vcnt >= V_SYNCSTART && vcnt < V_SYNCEND)
    vsync <=0;
else 
    vsync <=1;
    
//Enabling of color outputs 
always @ (posedge clock or negedge resetn)
if(!resetn)
    enable<=0;
else if(hcnt >= H_PIXELS || vcnt >= V_LINES)
    enable<=0;
else
    enable<=1;
    
//Output image generation (horizontal or vertical color stripes)
always @ ( enable or orient or hcnt or vcnt)    
if(enable==0)
   pixel = 0;
else 
if(orient)
 begin
 if(vcnt<75)
   pixel='h1;
 else
   if(vcnt<150)
     pixel='h2;
   else
     if(vcnt<225)
       pixel='h3;
     else
       if(vcnt<300)
         pixel='h4;
       else
        if(vcnt<375)
         pixel='h5;
        else
         if(vcnt<450)
           pixel='h6;
         else
           if(vcnt<525)
              pixel='h7;
         else
              pixel='h0;
  end
else
 begin
 if(hcnt<100)
   pixel='h1;
 else
   if(hcnt<200)
     pixel='h2;
   else
     if(hcnt<300)
       pixel='h3;
     else
       if(hcnt<400)
         pixel='h4;
       else
        if(hcnt<500)
         pixel='h5;
        else
         if(hcnt<600)
           pixel='h6;
         else
           if(hcnt<700)
              pixel='h7;
         else
              pixel='h0;
  end
	     


assign blank = enable;

endmodule