Spartan 6 – Digilent 410-182P-KIT User Manual

Page 15

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Nexys3 Reference Manual

Doc: 502-182

page 15 of 22

VGA Port

The Nexys3 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
di

splays 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 bea

ms 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

1 XS YS XY YY P

X0 X1 X2 X3 X4 X5 X6 X7

Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7 P

Idle state

Start bit

Mouse status byte

X direction byte

Y direction byte

Stop bit

Start bit

Stop bit

Idle state

Stop bit

Start bit

Mouse Data Format

HD-DB15

510

100

Pin 1: Red
Pin 2: Grn
Pin 3: Blue
Pin 13: HS
Pin 14: VS

Pin 5: GND
Pin 6: Red GND
Pin 7: Grn GND
Pin 8: Blu GND
Pin 10: Sync GND

RED0

RED1

RED2

510

GRN0

GRN1

GRN2

510

BLU1

BLU2

RED

GRN

BLU

Spartan 6

HSYNC

VSYNC

Figure 16: VGA pin definitions and Nexys2 circuit

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