60
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Spartan-3A/3AN FPGA Starter Kit Board User Guide
UG334 (v1.1) June 19, 2008
Chapter 6:
VGA Display Port
R
flows through the deflection coils, and it ensures that pixel or video data is applied to the
electron guns at the correct time.
Video data typically comes from a video refresh memory with one or more bytes assigned
to each pixel location. The Spartan-3A/3AN Starter Kit board uses 12 bits per pixel,
producing one of the 4,096 possible colors. The controller indexes into the video data buffer
as the beams move across the display. The controller then retrieves and applies video data
to the display at precisely the time the electron beam is moving across a given pixel.
As shown in
Figure 6-2
, the VGA controller generates the horizontal sync (HS) and vertical
sync (VS) timing signals and coordinates the delivery of video data on each pixel clock. The
pixel clock defines the time available to display one 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 in the 60 Hz to 120 Hz
range. The number of horizontal lines displayed at a given refresh frequency defines the
horizontal
retrace
frequency.
VGA Signal Timing
The signal timings in
Table 6-2
are derived for a 640-pixel by 480-row display using a
25 MHz pixel clock and 60 Hz ± 1 refresh.
Figure 6-3
shows the relation between each of
the timing symbols. The timing for the sync pulse width (T
PW
) and front and back porch
intervals (T
FP
and T
BP
) is based on observations from various VGA displays. The front and
back porch intervals are the pre- and post-sync pulse times. Information cannot be
displayed during these times.
Generally, a counter clocked by the pixel clock controls the horizontal timing. Decoded
counter values generate the HS signal. This counter tracks the current pixel display
location on a given row.
Table 6-2:
640x480 Mode VGA Timing
Symbol
Parameter
Vertical Sync
Horizontal Sync
Time
Clocks
Lines
Time
Clocks
T
S
Sync pulse time
16.7 ms
416,800
521
32 µs
800
T
DISP
Display time
15.36 ms
384,000
480
25.6 µs
640
T
PW
Pulse width
64 µs
1,600
2
3.84 µs
96
T
FP
Front porch
320 µs
8,000
10
640 ns
16
T
BP
Back porch
928 µs
23,200
29
1.92 µs
48
Figure 6-3:
VGA Control Timing
T
fp
T
disp
T
S
T
pw
T
bp
UG230_c6_03_021706
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