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Problem Statement Currently, there are no commercially available 3D FFT displays in existence. Just about every home stereo system has a 2D frequency vs. amplitude display and every computer can produce one graphically. My project is to create a true 3D FFT display of audio frequencies on an 8X8X8 3D array of LEDs. The general idea for my project is to display an eight point 8-bit resolution FFT on a vertical 8X8 plane of LEDs. Then this 8x8 plane of LEDs, which is part of a cube 8 LEDs deep, will be translated through the cube to produce a moving 3D FFT display. So, there are 8 3 = 512 LEDs. At some rate that is viewable to human eyes (discovered by trial and error), the single current 8x8 FFT plane regress back through the 8 planes till it is lost from the scope of the cube. This occurs continuously so that a volumetric real- time 8 frame FFT is created. The time between each frame is variable, but must be slow enough to perceive and yet fast enough to appear to change in real-time. Method In order to accomplish this goal, three things must occur. First the hardware of the display must support the task which is to allow communication with the EVMOMAPL137 development board to accept FFT data and be able to light up 512 LEDs correctly at a data rate that appears seamless to human vision. Second, the software on both the display and the EVMOMAPL137 must be compatible so that the FFT produced by the DSP can be meaningfully represented on the display. Lastly, a basic understanding of digital signal processing is necessary. More specifically, knowledge of how to use the results of the FFT is needed. Hardware The hardware chosen for the task of displaying the FFT is centered on ATMEL’s ATMEGA32 microprocessor. The ATMEGA32 is an 8 bit 16MHz microprocessor with 32 input/output (i/o) ports. Switching 512 LEDs would normally require 512 ports, however using multiplexing, the task can be accomplished with a lot fewer. By connecting the cathodes of each layer together, each layer can have ground applied individually. Combining that with having each column’s anodes tied together allows individual addressing of every LED individually with only 64 + 8 microprocessor ports (64 anodes + 8 cathodes). The appearance of several or even all of the LEDs being on is based on persistence of vision. With a very high refresh rate, the switching is invisible to human eyes. Now, the method of converting 32 (i/o) ports into 64+8 = 72 is based on multiplexing. First, 3 ports are designated for addressing. Using a 74HC138 (3 to 8) de- multiplexer chip, those 3 pins are converted into 8 which address 8 separate 74HC574 8-
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bit latch ICs. These latch ICs all share the same 8 bit input data bus which is supplied by directly by 8 ports on the ATMEGA32. At any instant all the latches contain the same data on their inputs; however their outputs are controlled by an output enable pin which is fed by the 3 to 8 de-multiplexer.
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