1. 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

2. VGA Timing
VGA Interface

3. 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

4. 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

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

6. Timing with a 12 MHz clock (XS40 clock)
Line: 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: 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

7. "VGA industry standard" 640x480 pixel mode
Clock frequency MHz
Line frequency Hz
Field frequency 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

8. VGA Interface
Row and column counts
VGA Interface

9. VGA Interface
Simple counting will work!
HCNT
VCNT
VGA Interface

10. 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 <= HSYNC
When VCNT > 480
494 <= VCNT <= VSYNC
VGA Interface

11. 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