Digilent DIO2 - Manual (Page 12)

Digilab DIO2 Reference Manual Digilent, Inc.
www.digilentinc.com page 12 of 19
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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 illuminate 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 bent by these magnetic fields. Current waveforms are passed
through the coils to produce magnetic fields that cause the electron beams to transverse the display
surface in a “raster” pattern, horizontally from left to right and vertically from top to bottom.
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 is reset 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 can be traced
across the display, and
the frequency at which
the electron beam can be
modulated determine the
display resolution.
Modern VGA displays
can accommodate
different resolutions, and
a VGA controller circuit
dictates the resolution by
producing timing signals
to control the raster
patterns. The controller
must produce TTL-level synchronizing pulses to set the frequency at which current flows through the
deflection coils, and it must ensure that pixel (or video) data is applied to the electron guns at the
correct time. Video data typically comes from a video refresh memory, with one or more bytes
assigned to each pixel location (the DIO2 board uses 8-bits per pixel). The controller must index into
video memory as the beams move across the display, and retrieve and apply video data to the display
at precisely the time the electron beam is moving across a given pixel.
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