16 Color VGA
by
Tetris Team
KhernYang Twang
Feng Gui
Taozhi Peng

Introduction

In this application note, we are discussing how to create more colors for the VGA. To simplify the explanation, we are going to use 16 colors only. Currently, people had been using 8 colors to display their programs and it does look ugly at times. Hopefully this application note can help others to create a more colorful and vivid program to display onto the monitor.

Design

Before discussing the techniques of creating 16 colors, we have to understand how the VGA monitor works. The monitor has 5 input pins. They are horizontal sync, vertical sync, red, green, and blue. We are only concern with the red, green, and blue pins. When logic '1' is applied to one of the pins in VHDL, it refers to applying +5V to the pin. And logic '0' refers to 0V. The table below shows the 8 color that can be displayed on the VGA.

Color	Red Pin	Green Pin	Blue Pin
Black	0	0	0
Blue	0	0	1
Green	0	1	0
Cyan	0	1	1
Red	1	0	0
Violet	1	0	1
Yellow	1	1	0
White	1	1	1

We noticed that the color changes when a different voltage is applied to the pins. For example, the intensity for the red color varies depending on the voltage applied to the red pin. The higher the voltage, the brighter the color will be. This discovery enables us to create a wider variety color palette.

Now we procede with the explaination of creating another 8 colors. The extra 8 colors are created by substituting 0V as logic '0' to +2.5V. MaxPlus2 does not support any variation of voltage to the output pins, therefore we have to use Pulse Width Modulation. Pulse Width Modulation is alternating a signal at regular interval to create the effects of variating voltage. The +2.5V is created by alternating a signal at 50% duty cycle. A good example of 50% duty cycle signal is the clock signal. The graph for generating +2.5V is shown below:

The table below shows all 16 colors.

( Clock gives a 50% duty cycle that modulate a +2.5V)

Color Red Green Blue

Black 0 0 0

Blue 0 0 1

Green 0 1 0

Cyan 0 1 1

Red 1 0 0

Violet 1 0 1

Yellow 1 1 0

White 1 1 1

Grey Clock Clock Clock

Intense Blue Clock Clock 1

Intense Green Clock 1 Clock

Intense Cyan Clock 1 1

Intense Red 1 Clock Clock

Intense Violet 1 Clock 1

Intense Yellow 1 1 Clock

Intense White 1 1 1

We have created another set of color palette. The colors that use +2.5V as logic '0' are called intense colors. They are slightly brighter than the colors that use 0V for logic '0'. Although the differences are not much, it is noticeable. The only color that is similar is white and intense white.

A wider set of color can be created however due to the limitation of the clock, we can only display 16 colors in 640*480 resolution at 60fps (frame per second). If we reduce the resolution or the fps, more clock cycle can be used to refresh each pixel. Therefore we can modulate more colors signal at different interval to create different level of voltage.

The limitation is caused by generating a signal at regular interval. The reference signal to use generate the regulating signal is the clock. Any signal faster than the clock is not possible on the UP1 board unless a faster external clock crystal is connected to it.

Test Program

We have developed a VHDL code that cycles through the 16 set of colors. The equipments needed are a Altera UP1 board and a VGA monitor. To cycle the colors, we need a push button to increment to each color. The pin assignment is given below

Description	Signal Name	Pin Number
Horizontal Sync	Horiz_sync	240
Vertical Sync	Vert_sync	239
Red	Red	236
Green	Green	237
Blue	Blue	238
Push Button	button	28
Clock	Clock	91

Contact Us

If you have any enquires, please contact Khern at ktwang@ee.ualberta.ca .

Color	Red	Green	Blue
Black	0	0	0
Blue	0	0	1
Green	0	1	0
Cyan	0	1	1
Red	1	0	0
Violet	1	0	1
Yellow	1	1	0
White	1	1	1
Grey	Clock	Clock	Clock
Intense Blue	Clock	Clock	1
Intense Green	Clock	1	Clock
Intense Cyan	Clock	1	1
Intense Red	1	Clock	Clock
Intense Violet	1	Clock	1
Intense Yellow	1	1	Clock
Intense White	1	1	1