Mini AV Test Box

The Software
           The timing needs of VGA, specifically 800x600 resolution are such that we must program in assembly. Keeping constant and correct timing for the signals output is critical if the monitor is going to receive and display the signal. Since assmembly is crude and not friendly on the eyes, I'm just going to cover same of the main points of the code.

Variables Used

;Delay Counters For Delay Functions And
;For Red, Green and Blue Color Routines

delay_cnt		equ		0x3F
counter1		equ		0x2F
counter2		equ		0x1F
counter3		equ		0x0F

           Many times delays of 1, 10 or 20 micro or miliseconds become necessary and the delay_cnt variable makes it so that we can have a variable run a for loop the necessary number of times to create the delay we need. The other integers counter1, counter2 and counter3 are used to display 200 lines of red, 200 lines of blue and 200 lines of green. You can play around with these variables in the code and change them to 100/100/400 or whatever combination you like, as long as it adds up to 600 lines.

;200 Horizontal Lines
HSync1
   HSYNC_short_X 0
   decfsz counter1,1 ;1.8uS
   goto HSync1 ;2.2uS
   nop

;200 Horizontal Lines
HSync2
   HSYNC_short_X 1
   decfsz counter2,1 ;1.8uS
   goto HSync2 ;2.2uS
   nop
;200 Horizontal Lines
HSync3
   HSYNC_short_X 2
   decfsz counter3,1 ;1.8uS
   goto HSync3 ;2.2uS
   nop

           This is the section of the code where 3 seperate for loops display 200 lines each of Red, Green or Blue, denoted by the 0,1 or 2 next to the HSYNC_short_X macro call. All the rest of the code is just keeping the correct vertical syncing, but this section is where the color output action is happening.