Nexys 3™ FPGA Board Reference Manual
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
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video color signals that proceed in equal increments between 0V (fully off) and 0.7V (fully on). Using this circuit,
256 different colors can be displayed, one for each unique 8-bit pattern. A video controller circuit must be created
in the FPGA to drive the sync and color signals with the correct timing in order to produce a working display
system.
8.1 VGA System Timing
VGA signal timings are specified, published,
copyrighted and sold by the VESA organization
(www.vesa.org). The following VGA system timing
information is provided as an example of how a VGA
monitor might be driven in 640 by 480 mode. For
more precise information, or for information on other
VGA frequencies, refer to documentation available at
the VESA website.
CRT-based VGA displays use amplitude-modulated
moving electron beams (or cathode rays) to display
information on a phosphor-coated screen. LCD
displays use an array of switches that can impose a
voltage across a small amount of liquid crystal, thereby
changing light permittivity through the crystal on a
pixel-by-pixel basis. Although the following description is limited to CRT displays, LCD displays have evolved to use
the same signal timings as CRT displays (so the "signals" discussion below pertains to both CRTs and LCDs). Color
CRT displays use three electron beams (one for red, one for blue, and one for green) to energize the phosphor that
coats the inner side of the display end of a cathode ray tube (see illustration). Electron beams emanate from
"electron guns" which are finely-pointed heated cathodes placed in close proximity to a positively charged annular
plate called a "grid". The electrostatic force imposed by the grid pulls rays of energized electrons from the
cathodes, and those rays are fed by the current that flows into the cathodes. These particle rays are initially
accelerated towards the grid, but they soon fall under the influence of the much larger electrostatic force that
results from the entire phosphor-coated display surface of the CRT being charged to 20kV (or more). The rays are
focused to a fine beam as they pass through the center of the grids, and then they accelerate to impact on the
phosphor-coated display surface. The phosphor surface glows brightly at the impact point, and it continues to glow
for several hundred microseconds after the beam is removed. The larger the current fed into the cathode, the
brighter the phosphor will glow.
Between the grid and the display surface, the beam passes through the neck of the CRT where two coils of wire
produce orthogonal electromagnetic fields. Because cathode rays are composed of charged particles (electrons),
they can be deflected by these magnetic fields. Current waveforms are passed through the coils to produce
magnetic fields that interact with the cathode rays and cause them to transverse the display surface in a "raster"
pattern, horizontally from left to right and vertically from top to bottom. As the cathode ray moves over the
surface of the display, the current sent to the electron guns can be increased or decreased to change the
brightness of the display at the cathode ray impact point.
Anode (entire screen)
High voltage
supply (>20kV)
Deflection coils
Grid
Electron guns
(Red, Blue, Green)
gun
control
grid
control
deflection
control
R,G,B signals
(to guns)
Cathode ray tube
Cathode ray
VGA
cable
