Digilent DIO4 - Manual (Page 7)

DIO4 Reference Manual Digilent, Inc.
www.digilentinc.com Page 7
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 more bytes assigned to
each pixel location (the DIO4 board uses 3-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.
A VGA controller circuit must generate the HS
and VS timings signals and coordinate the
delivery of video data based on the pixel clock.
The pixel clock defines the time available to
display 1 pixel of information. The VS signal
defines the “refresh” frequency of the display, or
Current
through
horizontal
defletion
coil
Stable current ramp - information
displayed during this time
Retrace - no
information
displayed
during this
time
Total horizontal time
Horizontal display time
Horizontal sync signal
sets retrace frequency
retrace
time time
HS
"back porch" "front porch"
VGA display
surface
640 pixels are displayed each
time the beam travels across
the screen
pixel 0,639 pixel 0,0
pixel 479,0 pixel 479,639