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Nexys2 Reference Manual

VGA Port

The Nexys2 board uses 10 FPGA signals to

create a VGA port with 8-bit color and the two

standard sync signals (HS – Horizontal Sync,

and VS – Vertical Sync). The color signals use

resistor-divider circuits that work in conjunction

with the 75-ohm termination resistance of the

VGA display to create eight signal levels on the

red and green VGA signals, and four on blue

(the human eye is less sensitive to blue levels).

This circuit, shown in figure 13, produces video

color signals that proceed in equal increments

between 0V (fully off) and 0.7V (fully on). Using

this circuit, 256 different colors can be

displayed, one for each unique 8-bit pattern. A

video controller circuit must be created in the

FPGA to drive the sync and color signals with

the correct timing in order to produce a working

display system.

VGA System Timing

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 VGA monitor might be driven in 640 by 480 mode. For more precise information, or for

information on other VGA frequencies, refer to documentation available at the VESA website.

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

Figure 16: VGA pin definitions and Nexys2 circuit

Anode (entire screen)

Cathode ray tube

Deflection coils

Cathode ray

High voltage

deflection

supply (>20kV)

control

Figure 17: CRT deflection system

Page 10/17

Digilent

www.digilentinc.com

Electron guns

Grid

(Red, Blue, Green)

R,G,B signals

(to guns)

grid

gun

control

Doc: 502-134

VGA

cable