Digilent 410-279P-KIT - Manual (Page 19)

ZYBO™ FPGA Board Reference Manual
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Page 19 of 26
pattern, horizontally from left to right and vertically from top to bottom, as shown in Fig. 10. 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.
Current
waveform
through
horizontal
defletion
coil
Stable current ramp - information
is 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"
Display Surface
640 pixels per row are displayed
during forward beam trace
pixel 0,639 pixel 0,0
pixel 479,0 pixel 479,639
Figure 10. VGA horizontal synchronization.
Video data typically comes from a video refresh memory; with one or more bytes assigned to each pixel location
(the ZYBO uses 16 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.