Spartan-3A/3AN FPGA Starter Kit Board User Guide
www.xilinx.com
59
UG334 (v1.1) June 19, 2008
Signal Timing for a 60 Hz, 640x480 VGA Display
R
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 6-2
, 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 display resolution defines 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.
Modern VGA displays support multiple display resolutions, and the VGA controller
dictates the resolution by producing timing signals to control the raster patterns. The
controller produces TTL-level synchronizing pulses that set the frequency at which current
Figure 6-2:
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
"back porch"
"front 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
"front porch"
UG230_c6_02_021706
