VGA Interface1 CRT Display Technology zCathode Ray Tube yElectron gun fires electrons at the screen yElectric field steers the electron (X field and Y field) yScreen phosphor emits light when electron hits zElectron beams scans the screen, left-to-right, top-to-bottom yAs the beam moves, we can set the brightness of each color yWe turn on each pixel for the right about of time zWe set the horizontal and vertical scan rate using SYNC signals yHSYNC - horizontal scan frequency yVSYNC - vertical scan frequency zVGA - Video Graphics Adapter yAncient PC CRT interface standard yBut still used, just like the 8086 ISA

VGA Interface2 VGA Timing

VGA Interface3 One Frame zVertical Synch tells the monitor when to go back to the top zThe Blanking Interval turns off the video at the top and bottom of the screen

VGA Interface4 One Line zEach frame is divided into many lines yHorizontal synch tells the monitor to go back to the start of the next line zEach line is divided into pixels yNo timing signal: just change the value from one pixel to the next

VGA Interface5 zThis is what the image really looks like yHorizontal retrace during HSYNC yVertical retrace during VSYNC y“Front porch” y“Back porch”

VGA Interface6 Timing with a 12 MHz clock (XS40 clock) zLine: usec x 12 MHz = 381 clocks/line ywe will display 256 pixels/line yone pixel/clock yrest is front porch, back porch and HSYNC zFrame: msec x 12 MHz = 201,406 clocks/frame (!) y201,406 / 381 clocks/line = 528 lines ywe can display 480 lines yrest is front porch, back porch and VSYNC

VGA Interface7 zRow and column counts

VGA Interface8 zSimple counting will work! yHCNT yVCNT

VGA Interface9 zWhen HCNT is (0, 255) and VCNT is (0, 479) yDisplay the pixel at (x, y) = (VCNT, HCNT) zWhen HCNT > 255 zWhen VCNT > 480

VGA Interface10 VGA Interface zWhere does the pixel value come from? zThe image to be displayed is stored in memory y“frame buffer” yXS40 SRAM is 32Kx8 xmax image size = x6 bits/pixel = 2 bits each of RGB (modest color) zFrame buffer model yUser fills memory with image xUser can change the image by just writing to memory yMonitor reads memory whenever it needs a pixel yMemory is shared, but user doesn’t need to know what monitor is doing xmodularization, information hiding yVGA interface only needs to worry about reading the memory