Digilent DIO2 - Manual (Page 13)

Digilab DIO2 Reference Manual Digilent, Inc.
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The VGA controller circuit
must generate the HS and VS
timings signals and coordinate
the delivery of video data based
on the pixel clock. The pixel
clock defines the time available
to display 1 pixel of
information. The VS signal
defines the “refresh” frequency
of the display, or the frequency
at which all information on the
display is redrawn. The
minimum refresh frequency is
a function of the display’s
phosphor and electron beam
intensity, with practical refresh
frequencies falling in the 60Hz
to 120Hz range. The number of
lines to be displayed at a given
refresh frequency defines the
horizontal “retrace” frequency.
For a 640-pixel by 480-row
display using a 25MHz pixel
clock and 60 +/-1Hz refresh,
the following signal timings can be derived. Timings for sync pulse width and front and back porch
intervals (porch intervals are the pre- and post-sync pulse times during which information cannot be
displayed) are based on observations taken from VGA displays.
VGA Signal Timing
T
S
T
disp
T
pw
T
fp
T
bp
T
S
T
disp
T
pw
T
fp
T
bp
Sync pulse time
Display time
VS pulse width
VS front porch
VS back porch
16.7ms
15.36ms
64 us
320 us
928 us
416,800
384,000
1,600
8,000
23,200
521
480
2
10
29
Symbol Parameter
Time Clocks Lines
Vertical Sync
32 us
25.6 us
3.84 us
640 ns
1.92 us
800
640
96
16
48
Clocks
Horizontal Sync
Time
A VGA controller circuit decodes the output of a horizontal-sync counter driven by the pixel clock to
generate HS signal timings. This counter can be used to locate any pixel location on a given row.
Likewise, the output of a vertical-sync counter that increments with each HS pulse can be used to
generate VS signal timings, and this counter can be used to locate any given row. These two
continually running counters can be used to form an address into video RAM. No time relationship
Current
through
horizontal
defletion
coil
Stable current ramp - information
displayed during this time
Retrace - no
information
displayed
during this
time
Total horizontal time
Horizontal display time
Horizontal sync signal
sets retrace frequency
retrace time
time
HS
"back porch" "front porch"
VGA display
surface
640 pixels are displayed each time
the beam travels across the screen
pixel 0,639 pixel 0,0
pixel 479,0 pixel 479,639