CRT Display Technology z Cathode Ray Tube y

CRT Display Technology z Cathode Ray Tube y Electron gun fires electrons at the screen y Electric field steers the electron (X field and Y field) y Screen phosphor emits light when electron hits z Electron beams scans the screen, left-to-right, top-to-bottom y As the beam moves, we can set the brightness of each color y We turn on each pixel for the right about of time z We set the horizontal and vertical scan rate using SYNC signals y HSYNC - horizontal scan frequency y VSYNC - vertical scan frequency z VGA - Video Graphics Adapter y Ancient PC CRT interface standard y But still used, just like the 8086 ISA VGA Interface 1

VGA Timing VGA Interface 2

One Frame z Vertical Synch tells the monitor when to go back to the top z The Blanking Interval turns off the video at the top and bottom of the screen VGA Interface 3

One Line z Each frame is divided into many lines y Horizontal synch tells the monitor to go back to the start of the next line z Each line is divided into pixels y No timing signal: just change the value from one pixel to the next VGA Interface 4

VGA Interface z This is what the image really looks like y Horizontal retrace during HSYNC y Vertical retrace during VSYNC y “Front porch” y “Back porch” VGA Interface 5

Timing with a 12 MHz clock (XS 40 clock) z Line: 31. 77 usec x 12 MHz = 381 clocks/line y we will display 256 pixels/line y one pixel/clock y rest is front porch, back porch and HSYNC z Frame: 16. 784 msec x 12 MHz = 201, 406 clocks/frame (!) y 201, 406 / 381 clocks/line = 528 lines y we can display 480 lines y rest is front porch, back porch and VSYNC VGA Interface 6

"VGA industry standard" 640 x 480 pixel mode z Clock frequency 25. 175 MHz z Line frequency 31469 Hz z Field frequency 59. 94 Hz z One line y y y y 8 pixels front porch 96 pixels horizontal sync 40 pixels back porch 8 pixels left border 640 pixels video 8 pixels right border --800 pixels total per line One field 2 lines front porch 2 lines vertical sync 25 lines back porch 8 lines top border 480 lines video 8 lines bottom border --525 lines total per field VGA Interface 7

VGA Interface z Row and column counts VGA Interface 8

VGA Interface z Simple counting will work! y HCNT y VCNT VGA Interface 9

VGA Interface z When HCNT is (0, 255) and VCNT is (0, 479) y Display the pixel at (x, y) = (VCNT, HCNT) z When HCNT > 255 y 313 <= HCNT <= 359 - HSYNC z When VCNT > 480 y 494 <= VCNT <= 495 - VSYNC VGA Interface 10

VGA Interface z Where does the pixel value come from? z The image to be displayed is stored in memory y “frame buffer” y XS 40 SRAM is 32 Kx 8 xmax image size = x 6 bits/pixel = 2 bits each of RGB (modest color) z Frame buffer model y User fills memory with image x. User can change the image by just writing to memory y Monitor reads memory whenever it needs a pixel y Memory is shared, but user doesn’t need to know what monitor is doing xmodularization, information hiding y VGA interface only needs to worry about reading the memory VGA Interface 11