Digilent 410-044-10P-KIT - Manual (Page 23)

Spartan-3 Starter Kit Board User Guide www.xilinx.com 23
UG130 (v1.1) May 13, 2005 1-800-255-7778
Signal Timing for a 60Hz, 640x480 VGA Display R
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
Figure 5-2 ,
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.
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.
Figure 5-2: CRT Display Timing Example
Current
through the
horizontal
deflection
coil
Stable current ramp: Information is
displayed during this time
Retrace: No
information
is displayed
during
this time
Total horizontal time
Horizontal display time
Horizontal sync signal
sets the retrace frequency
retrace time
time
HS
"front porch"
"back porch"
VGA Display
640 pixels are displayed each
time the beam traverses the screen
pixel 0,639 pixel 0,0
pixel 479,0 pixel 479,639
"back porch"
UG130_c5_02_051305