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A Simple VGA Interface

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Introduction 

The VGA interface is common to most modern computer displays and is based on a pixel map, color planes, and horizontal and vertical sync signals. A VGA monitor has three color signals (Red, Green, and Blue) that set one of these colors on or off on the screen. The intensity of each of those colors sets the final color seen on the display. For example, if the Red was fully on, but the Blue and Green off, then the color would be seen as a strong red. Each analog intensity is defined by a 2-bit digital word for each color (e.g., red0 and red1) that are connected to a simple digital-to-analog converter to obtain the correct output signal. 

The resolution of the screen can vary from 480 × 320 up to much larger screens, but a standard default size is 640 × 480 pixels. This is 480 lines of 640 pixels in each line, so the aspect ratio is 640/480, leading to the classic landscape layout of a conventional monitor screen. 

The VGA image is controlled by two signals—horizontal sync and vertical sync. The horizontal sync marks the start and finish of a line of pixels with a negative pulse in each case. The actual image data is sent in a 25.17 µs window in a 31.77 µs space between the sync pulses. (The time that image data is not sent is where the image is defined as a blank space and the image is dark.) The vertical sync is similar to the horizontal sync except that in this case the negative pulse marks the start and finish of each frame as a whole and the time for the frame (image as a whole) takes place in a 15.25 ms window in the space between pulses, which is 16.784 ms. 

There are some constraints about the spacing of the data between pulses which will be considered later in this chapter, but it is clear that the key to a correct VGA output is the accurate definition of timing and data by the VHDL.

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