Digilent DIO5 - Manual (Page 11)

DIO5 Reference Manual Digilent, Inc.
www.digilentinc.com Page 11
Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.
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 higher VGA frequencies, refer
to document available at the VESA website (or
experiment!).
VGA system timing
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 permitivity 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).
CRT displays use 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 drawing
below). Electron beams emanate from
“electron guns”, which are a 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 away rays of energized electrons as current
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 the phosphor 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.
Cathode ray tube display system
Anode (entire screen)
High voltage supply (>20kV) Control board
Deflection coils
Grid
Electron guns
(Red, Blue, Green)
gun
control grid
control
deflection
control
R,G,B signals (to guns)
Sync signals
(to deflection control)
Cathode ray tube
Cathode ray
VGA cable