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Nexys2 Reference Manual

Digilent

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Copyright Digilent, Inc.

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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.

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
synchronizing pulses at 3.3V (or 5V) to
set the frequency at which current
flows through the deflection coils, and
it must ensure that video data is
applied to the electron guns at the
correct time. Raster video displays
define a number of “rows” that
corresponds to the number of
horizontal passes the cathode makes
over the display area, and a number of
“columns” that corresponds to an area
on each row that is assigned to one
“picture element” or pixel. Typical
displays use from 240 to 1200 rows
and from 320 to 1600 columns. The
overall size of a display and the
number of rows and columns
determines the size of each pixel.

Video data typically comes from a
video refresh memory, with one or