Digilent NXVGA User Manual

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Revision: November 9, 2006 215 E Main Suite D | Pullman, WA 99163

Overview

The Digilent Nexys VGA Module provides a 12bit VGA
interface for use with the Digilent Nexys board. The 12
bit interface allows up to 4096 colors displayed on a
standard VGA Monitor.

The Nexys VGA Module interfaces with the Nexys board
via the 16 pin header at J8 and connects to a VGA
monitor using a standard 15 pin VGA cable.

Functional Description

VGA Port

The five standard VGA signals Red, Green, Blue,
Horizontal Sync (HS), and Vertical Sync (VS) are routed
directly from the FPGA to the VGA connector. There are
four signals routed from the FPGA for each of the
standard VGA color signals resulting in a video system
that can produce 4,096 colors. Each of these signals
has a series resistor that when combined in the circuit,
form a divider with the 75-ohm termination resistance of
the VGA display. These simple circuits ensure that the
video signals cannot exceed the VGA-specified
maximum voltage, and result in color signals that are
either fully on (.7V), fully off (0V) or somewhere in
between.

l

Spartan 3

FPGA

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P16

512

1K

P15

T7

2K

R5

3K

N15

512

1K

J16

K16

2K

K15

3K

L15

512

1K

M16
M15

2K

N16

3K

200

P16
P16

200

Nexys VGA Module

Block Diagram

RED

GRN

BLU

HS
VS

®

VGA signal timings are specified, published, copyrighted and sold by the VESA organization
(www.vesa.org). The following VGA system timing information is provided as an example of how a

Copyright Digilent, Inc. All rights reserved 12 pages Doc: 502-107

Basys Reference Manual

Digilent

www.digilentinc.com

VGA monitor might be driven in 640 by 480 mode. For more precise information, or for information on
higher VGA frequencies, refer to documentation available at the VESA website.

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 permittivity 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). Color CRT displays use three 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 illustration). Electron beams emanate from “electron guns”, which are
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 rays of energized electrons 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.

Anode (entire screen)

Cathode ray tube

Deflection coils

Grid

Cathode ray

Cathode Ray Tube

Display System

Electron guns
(Red, Blue, Green)

R,G,B signals (to guns)

VGA cable

High voltage

supply (>20kV)

deflection

control

grid

control

gun

control

Sync signals
(to deflection control)

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.

Copyright Digilent, Inc. Page 2/4 Doc: 502-107