Xilinx Spartan-3 User Manual page 23

Signal Timing for a 60Hz, 640x480 VGA Display
same signal timings as CRT displays. Consequently, the following discussion pertains to
both CRTs and LCD displays.
Within a CRT display, current waveforms pass through the coils to produce magnetic fields
that deflect electron beams to transverse the display surface in a "raster" pattern,
horizontally from left to right and vertically from top to bottom. As shown in
information is only displayed when the beam is moving in the "forward" direction—left to
right and top to bottom—and not during the time the beam returns back to the left or top
edge of the display. Much of the potential display time is therefore lost in "blanking"
periods when the beam is reset and stabilized to begin a new horizontal or vertical display
pass.
Current
through the
horizontal
deflection
coil
time
"back porch"
HS
The size of the beams, the frequency at which the beam traces across the display, and the
frequency at which the electron beam is modulated determine the display resolution.
Spartan-3 Starter Kit Board User Guide
UG130 (v1.1) May 13, 2005
pixel 0,0
640 pixels are displayed each
time the beam traverses the screen
VGA Display
pixel 479,0
Stable current ramp: Information is
displayed during this time
Total horizontal time
Horizontal display time
Horizontal sync signal
sets the retrace frequency
Figure 5-2: CRT Display Timing Example
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pixel 0,639
pixel 479,639
retrace time
Figure 5-2,
Retrace: No
information
is displayed
during
this time
"back porch"
"front porch"
UG130_c5_02_051305
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