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

VGA Timing VGA Interface

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

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

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

Timing with a 12 MHz clock (XS40 clock) Line: 31.77 usec x 12 MHz = 381 clocks/line we will display 256 pixels/line one pixel/clock rest is front porch, back porch and HSYNC Frame: 16.784 msec x 12 MHz = 201,406 clocks/frame (!) 201,406 / 381 clocks/line = 528 lines we can display 480 lines rest is front porch, back porch and VSYNC VGA Interface

"VGA industry standard" 640x480 pixel mode Clock frequency 25.175 MHz Line frequency 31469 Hz Field frequency 59.94 Hz One line 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

VGA Interface Row and column counts VGA Interface

VGA Interface Simple counting will work! HCNT VCNT VGA Interface

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

VGA Interface Where does the pixel value come from? The image to be displayed is stored in memory “frame buffer” XS40 SRAM is 32Kx8 max image size = 6 bits/pixel = 2 bits each of RGB (modest color) Frame buffer model User fills memory with image User can change the image by just writing to memory Monitor reads memory whenever it needs a pixel Memory is shared, but user doesn’t need to know what monitor is doing modularization, information hiding VGA interface only needs to worry about reading the memory VGA Interface