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					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                       1
VGA Timing




             VGA Interface   2
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                          3
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                             4
VGA Interface

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


                                                         HSYNC

                                               Visible
                                               Image




                                              VSYNC
                              VGA Interface               5
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                6
VGA Interface

 Row and column counts
                   0   x                   x+255   ?   381
               0


               y




                                                        HSYNC

                                 Visible
                                 Image



           y+479



              ?
             527
                                VSYNC

                           VGA Interface                        7
VGA Interface

 Simple counting will work!                           HCNT
    HCNT                       381 0                          255   ?   ?

    VCNT              527
                        0




                                                                             HSYNC

                                                     Visible
                VCNT                                 Image



                       479


                            ?
                            ?
                                                    VSYNC


                                    VGA Interface                            8
VGA Interface

 When HCNT is (0, 255) and VCNT is (0, 479)
    Display the pixel at (x, y) = (VCNT, HCNT)
 When HCNT > 255




 When VCNT > 480




                               VGA Interface      9
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                          10

				
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posted:8/12/2012
language:English
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