Gameboy VGA adapter

Project: Gameboy VGA adapter
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Description	Adding VGA output to a gameboy classic
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Introduction

The gameboy classic is a handheld video game device which invokes a feeling of nostalgia into the hearts of many gamers (and non-gamers!).

In total nearly 120 million of these devices have been sold around the world and have delivered uncountable hours of fun (and frustration.. "DAMN YOU MARIO!" heh anyone? :P).

Nowadays the gameboy has been replaced by far more advanced (read: battery-slurping) handheld devices and as such, many people have one lying around collecting dust.

As some of you might remember, the biggest problem with the gameboy classic was the unlit screen which caused any form of sunlight to render the screen into a useless plastic mirror.

REJOICE! For I have decided to add VGA output to the gameboy so you can play on 21" screens! (TAKE THAT SUN!) No more visibility problems! (Yes you can add LEDs to the back of the screen, stop being so boring :P).

Before we get down and dirty, you'll need to be sure that you understand the terms used on this page; VGA terminology.

The gameboy

Gameboy specs:

• Resolution: 160 x 144
• Refresh rate: 59.7Hz
• Horizontal Sync: 9.2KHz
• Bits per pixel: 2 (Black, Dark grey, Light grey or White)
• Pixel Clock: 4MHz
  

As you can see we are presented with a refresh rate of 59.7Hz which nearly matches the standard VGA refresh rate. (Most likely close enough, only testing will tell)

The resolution of 160x144 however poses a problem, no such VGA resolution exists and as such we will be forced to pick a different resolution and do some pixel juggling.

The solution? We will use double buffering and a different resolution.

Calculations

Double buffer

To determine the size of our buffers we simply take the resolution of the gameboy screen and multiply the amount of horizontal pixels with the vertical pixels (160x144 = 23040).

The 23040 bits translate roughly to 3 kilobyte.

Since we are dealing with 2 bits per pixel, we need 2 buffers per frame, which adds up to 4 buffers in total.

Resolution

Multiplying horizontal pixels by a float is only a matter of altering the pixel clock.

Multiplying vertical pixels by a float however is difficult as this has to be done with a digital solution.

As such we prefer to use a resolution whose vertical pixels are a multiple of 144. (Any integer)

There so happens to be a resolution which adheres to this: 768x576

VESA signal 768x576 source

General timing

Screen refresh rate	60 Hz
Vertical refresh	35.819672131148 kHz
Pixel freq.	34.96 MHz

Horizontal timing (line)
Polarity of horizontal sync pulse is negative.

Scanline part	Pixels	Time [µs]
Visible area	768	21.967963386728
Front porch	24	0.68649885583524
Sync pulse	80	2.2883295194508
Back porch	104	2.974828375286
Whole line	976	27.9176201373

Vertical timing (frame)
Polarity of vertical sync pulse is positive.

Frame part	Lines	Time [ms]
Visible area	576	16.080549199085
Front porch	1	0.0279176201373
Sync pulse	3	0.083752860411899
Back porch	17	0.4745995423341
Whole frame	597	16.666819221968

To 'convert' 160 pixels to 768 we simply adjust the pixel clock: 34.96 MHz / 768 pixels = x MHz / 160 pixels x = 160 pixels * (34.96 MHz / 768 pixels) = 7.283 MHz So if we use a pixel clock of 7.283 MHz, the 160 pixels will stretch themselves over a width of 768 pixels.