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Homemade VGA Adapter using ATmega644




Introduction

Motivation

The goal of our project is to create a VGA video adapter. This “homemade video card” should be able to connect to any monitor that subscribes to VGA standards with a standard connector and display the desired material reliably. The challenges involved here stem from adapting a general use microprocessor that we are familiar with to a specific task that it may (or may not) be suited to. The project required the researching and understanding the VGA standard of how a picture is displayed on a computer screen, identifying the shortcomings or advantages of the MEGA644 processor in accomplishing this, development and fabrication of the necessary hardware to interface with the screen, and in converting images to a format that can be stored in memory and displayed on the microprocessor.

Homemade VGA Adapter

We divided our goals for this project into a progression of three different tasks, each building off of the previous one.

  • First, we wanted to display color bars on the screen, by means of direct output from the microcontroller to an analog circuit that transformed pin outs to VGA output.
  • Next, we wanted to display color bars or a static image to the screen, by means of triggering RGB outputs from static random-access memory (SRAM).
  • Finally, we wanted to render an animation or video to the screen, by means of triggering outputs from SRAM but also writing to SRAM live data simultaneously.

Our ultimate goal, originally, was to stream a live CCD camera to VGA output using our device. However, upon delivery of the CCD camera and studying its output, we observed very quickly that its transmitted signals were not suitable to be converted to VGA in the scope of the remaining time in our 5-week project. This will be described at greater length after a brief background about the standards and parameters relating to VGA.

While this is a “solved problem” by industry standards, it poses a number of interesting challenges to the inquiring student.

  • The clock speed of the processor versus the needed clock speed of the VGA standard (overclocking).
  • The onboard memory the MEGA644 has versus the needed memory (external memory).
  • The exact timing needed for the VGA output standard (cycle accuracy).
  • Fabrication of appropriate hardware to address shortcomings of the processor for the above tasks or in simply building hardware filters/interfaces for the VGA standard.

Research

Video Graphics Array (VGA) is a video standard devised by IBM for their PS/2 computers in 1987. The widespread adoption has since made this the baseline for all displays and is still the baseline for operation today. The standard specifies a set of screen resolutions, operating modes, and minimum hardware requirements.

There are five signals in the VGA connection that we are most interested in: two for timing conditioning and three for colorization.

For conditioning, the vertical sync pulse is a digital active low signal whose negative edge triggers the monitor moving focus to the topmost line, leftmost pixel of the screen to display RGB; the horizontal sync pulse is a digital active low signal whose negative edge triggers the monitor focus to the leftmost pixel of the next line down from where current focus is. When not in the presence of sync pulses, the monitor moves focus to the pixel to the right of existing focus, one pixel per clock cycle on a 25.175 MHz clock.

Homemade VGA Adapter diagram

The other three signals with which we are concerned are for Red, Blue, and Green, which are each analog signals sent to the monitor. Thus, since we store values (like the colors in an image) as digital elements in the MCU, part of the hardware for such a device would require a Digital-to-Analog conversion.

Given these five signals, we can divide each line into four distinct sections. During the first section (Vertical and Horizontal Syncs), the necessary syncs are driven low and RGB must be set to digital low, as well, for the monitor to observe the syncs correctly. To skip ahead to the third section (RGB), this is when the syncs are kept high and the RGB signals are of variable output depending on screen colorization. The two other sections (the second: Back Porch, and the fourth: Front Porch) are spare cycles that keep the syncs high and RGB low, with the direction (Back, Front) referring to the location of the porch relative to nearest sync section of relative magnitude.

 

For more detail: Homemade VGA Adapter using ATmega644

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