Hệ thống nhúng - Chương 7

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Nội dung Text: Hệ thống nhúng - Chương 7

Embedded Systems Hệ thống nhúng Thạc sĩ Lê Mạnh Hải 1
Lesson 8 :THE DARK SCREEN Motivation: • Generating the composite video signal • Using the Output Compare modules • Memory allocation • Image serialization • Building the video module • Testing the video generator • Measuring performance • The dark screen • A Test Pattern 2
Flight plan • In this lesson we will consider techniques to interface to a TV screen or, for that matter, to any display that can accept a standard composite video signal. • It will be a good excuse to use new features of several peripheral modules of the PIC24 and review new programming techniques. • Our first project objective will be to get a nice dark screen (a well-synchronized video frame), but we will soon fill it up with several entertaining graphical applications. 3
Preflight checklist • MPLAB® IDE, • MPLAB C30 compiler and the • MPLAB SIM simulator, • Explorer16 demonstration board and the • MPLAB ICD2 In Circuit Debugger 4
The flight • There are many different formats and standards today in use in the world of video, but perhaps the oldest and most common one is the so-called “composite” video format. • This is what was originally used by the very first TV sets to appear in the consumer market, and today it represents the minimum common denominator of every video display, whether a modern high-definition flat-screen TV of the latest generation, a DVD player, or a VHS tape recorder. 5
Video image • All video devices are based on the same basic concept: the image is “painted” one line at a time, starting from the top left corner of the screen and moving horizontally to the right edge, then quickly jumping back to the left edge at a lower position and painting a second line, and so on and on, in a zig-zag motion, until the entire screen has been scanned. • Then the process repeats and the entire image is refreshed fast enough for our eyes to be tricked into believing that the entire image is present at the same time, and if there is motion, it is fluid and continuous. 6
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Basic features • the horizontal synchronization pulse, used by the display to identify the beginning of each line. • the so-called back porch, that creates a dark frame around the image. • the actual line luminosity signal; the higher the voltage, the more luminous the point. • the so-called front porch, producing the right edge of the image. 9
• The color information is transmitted separately, modulated on a high frequency subcarrier. • The three main standards differ significantly in the way they encode the color information but, for our purposes, it will be easy to ignore the problem altogether to obtain a simple black-and-white display output. 10
• In our project we will avoid “interlacing” as well, sacrifi cing half of the image resolution in favor of a more stable and readable display output. In other words, we will transmit frames of 262 lines (for NTSC) at the double rate of 60 frames per second. Readers that have easier access to PAL or SECAM TV sets/monitors will fi nd it relatively easy to modify the project for 312-line resolution with a refresh rate of 50 frames per second. 11
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Generating the composite video signal 13
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