//video gen
//D.5 is sync:1000 ohm + diode to 75 ohm resistor
//D.6 is video:330 ohm + diode to 75 ohm resistor
                    
#include <Mega163.h>   
   
#define lineTime 507 //cycles = 63.5 * 8
#define begin {
#define end   }
#define ScreenTop 30
#define ScreenBot 230

char syncON, syncOFF, v1, v2, v3, v4, v5, v6, v7, v8; 
int i,LineCount;
//#pragma regalloc- 
//register char temp @16; 
//#pragma regalloc+  
int j; 
char screen[800]; 

flash char bitmap[8]={
	0b00000000,
	0b00010000,
	0b00010000,
	0b00010000,
	0b00010000,
	0b00010000,
	0b00010000,
	0b00111000};

interrupt [TIM1_COMPA] void t1_cmpA(void)  
begin 
  //start the Horizontal sync pulse    
  PORTD = syncON;     
  //count timer 0 at 1/usec
  TCNT0=0;
  //update the curent scanline number
  LineCount ++ ;   
  //begin inverted synch after line 247
  if (LineCount==248)
  begin 
    syncON = 0b00100000;
    syncOFF = 0;
  end
  //back to regular sync after line 250
  if (LineCount==251)	
  begin
    syncON = 0;
    syncOFF = 0b00100000;
  end  
  //start new frame after line 262
  if (LineCount==263) 
  begin
     LineCount = 1;
  end 
  //end sync pulse
  PORTD = syncOFF;  
end  

void main(void)
begin 
  //init timer 1 to generate sync
  OCR1A = lineTime; 	//One NTSC line
  TCCR1B = 9; 		//full speed; clear-on-match
  TCCR1A = 0x00;	//turn off pwm and oc lines
  TIMSK = 0x10;		//enable interrupt
  
  //init ports
  DDRD = 0xf0;		//video out and switches
  //D.5 is sync:1000 ohm + diode to 75 ohm resistor
  //D.6 is video:330 ohm + diode to 75 ohm resistor
   
  //init timer 0 to 1/uSec  
  TCCR0 = 2;
  
  //initialize synch constants 
  LineCount = 1;
  syncON = 0b00000000;
  syncOFF = 0b00100000;  
  
  //initialize screen   
  
  for (i=0;i<799;i=i+16) 
  begin
  	for(j=0;j<8;j=j+1)
  	begin
  	   	screen[i+j]=0b10101010; 
   	end 
  	for(j=0;j<8;j=j+1)
   	begin
  	  	screen[i+j+8]=0b01010101; 
   	end
  end
  //top line
  for (i=0;i<8;i++) 
  begin
     //screen[i+8]=0b11111111; 
     //screen[i+32]=0b11111111;
     //screen[i+256]=0b11111111;
  end
  
  MCUCR = 0b01000000;
  #asm ("sei");
  
  while(1)
  begin
  
    //precompute pixel index for next line
    if (LineCount<ScreenBot && LineCount>=ScreenTop) 
    begin 
       //left-shift 3 would be individual lines
       // <<2 means line-double the pixels
       i=(LineCount-ScreenTop)<<2 & 0xfff8; 
       
   end
   
   //stall here until next line starts
    //sleep enable; mode=idle  
    //use sleep to make entry into sync ISR uniform time  
       
    #asm ("sleep"); 
     
    //Put code here to execute once/line
    //During the active portion of a line;
    //--TCNT1 goes from about 130 to about 480 
    //--Usable lines 1 to about 240
    
    if (LineCount<ScreenBot && LineCount>ScreenTop) 
    begin
      //load the pixels into registers
      v1 = screen[i]; 
       v2 = screen[i+1];
       v3 = screen[i+2];
       v4 = screen[i+3]; 
      v5 = screen[i+4]; 
      v6 = screen[i+5];
      v7 = screen[i+6];
      v8 = screen[i+7];
        
      //now blast them out to the screen
        PORTD.6=v1 & 0b10000000; 
        PORTD.6=v1 & 0b01000000;
        PORTD.6=v1 & 0b00100000; 
        PORTD.6=v1 & 0b00010000;
        PORTD.6=v1 & 0b00001000; 
        PORTD.6=v1 & 0b00000100;
        PORTD.6=v1 & 0b00000010; 
        PORTD.6=v1 & 0b00000001;
        
        PORTD.6=v2 & 0b10000000; 
        PORTD.6=v2 & 0b01000000;
        PORTD.6=v2 & 0b00100000; 
        PORTD.6=v2 & 0b00010000;
        PORTD.6=v2 & 0b00001000; 
        PORTD.6=v2 & 0b00000100;
        PORTD.6=v2 & 0b00000010; 
        PORTD.6=v2 & 0b00000001; 
        
        PORTD.6=v3 & 0b10000000; 
        PORTD.6=v3 & 0b01000000;
        PORTD.6=v3 & 0b00100000; 
        PORTD.6=v3 & 0b00010000;
        PORTD.6=v3 & 0b00001000; 
        PORTD.6=v3 & 0b00000100;
        PORTD.6=v3 & 0b00000010; 
        PORTD.6=v3 & 0b00000001; 
            
        PORTD.6=v4 & 0b10000000; 
        PORTD.6=v4 & 0b01000000;
        PORTD.6=v4 & 0b00100000; 
        PORTD.6=v4 & 0b00010000;
        PORTD.6=v4 & 0b00001000; 
        PORTD.6=v4 & 0b00000100;
        PORTD.6=v4 & 0b00000010; 
        PORTD.6=v4 & 0b00000001; 
             
        PORTD.6=v5 & 0b10000000; 
        PORTD.6=v5 & 0b01000000;
        PORTD.6=v5 & 0b00100000; 
        PORTD.6=v5 & 0b00010000;
        PORTD.6=v5 & 0b00001000; 
        PORTD.6=v5 & 0b00000100;
        PORTD.6=v5 & 0b00000010; 
        PORTD.6=v5 & 0b00000001;
             
        PORTD.6=v6 & 0b10000000; 
        PORTD.6=v6 & 0b01000000;
        PORTD.6=v6 & 0b00100000; 
        PORTD.6=v6 & 0b00010000;
        PORTD.6=v6 & 0b00001000; 
        PORTD.6=v6 & 0b00000100;
        PORTD.6=v6 & 0b00000010; 
        PORTD.6=v6 & 0b00000001; 
            
        PORTD.6=v7 & 0b10000000; 
        PORTD.6=v7 & 0b01000000;
        PORTD.6=v7 & 0b00100000; 
        PORTD.6=v7 & 0b00010000;
        PORTD.6=v7 & 0b00001000; 
        PORTD.6=v7 & 0b00000100;
        PORTD.6=v7 & 0b00000010; 
        PORTD.6=v7 & 0b00000001;
             
        PORTD.6=v8 & 0b10000000; 
        PORTD.6=v8 & 0b01000000;
        PORTD.6=v8 & 0b00100000; 
        PORTD.6=v8 & 0b00010000;
        PORTD.6=v8 & 0b00001000; 
        PORTD.6=v8 & 0b00000100;
        PORTD.6=v8 & 0b00000010; 
        PORTD.6=v8 & 0b00000001;  
      
        PORTD.6=0 ; 
    end
    //following code executes during the vertical blanking
    //Code here can be as long as  63 uSec/line
    //20 lines x 63.5 uSec/line x 8 cycles/uSec
   // if (LineCount>170 && LineCount<178)
   // begin
    //	i=LineCount-170;
   // end
    
    if (LineCount==243)
    begin
    end //if (LineCount==243)  
    
  end  //while
end  //main