DSP Applications and Student Projects - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 362

FIGURE 10.12. Line driver used with external LEDs to display the spectrum in project
graphic_FFT .

EMIF-LEDs
A total of 32 LEDs connect through four line drivers (74LS244). Current limiting
resistors of 300 ohms are connected between each LED and ground. The line drivers
allow for the needed current to light up the LEDs. The current drawn by the LED
is limited to 10 mA so that the line drivers are not overloaded. Figure 10.12 shows
one of the line drivers. Pin 20 is connected to 5 V and pin 10 to ground. Pins 1 and

19 are also connected to ground to enable the output of the line driver. Each line
driver supports eight inputs and eight outputs. The pins labeled with “Y” are output
pins. Each of the output pins (on a line driver) is connected to pins 33-40, which
correspond to data pins 31-24 on the EMIF bus. The arrangement is the same with
the other three line drivers connecting to pins 43-50 (data pins 23-16), pins 53-60
(data pins 15-8), and pins 63-70 (data pins 7-0), respectively. Pin 79 on the EMIF
bus is used for universal ground. See also the schematics of connectors J3 and J4
shown in the file c6713_dsk_schem.pdf , included with CCS. Table 10.3 shows the
EMIF signals.
Note : Pin 75 on J3 (not J4), the 80-pin connector for the external peripheral inter-
face, is to be connected to ground since it is an enable pin for the EMIF interface
and enables the output voltages on these pins.

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Implementation
The real-time radix-2 FFT program example in Chapter 6 is slightly modified to
check the amplitude of a specific frequency and determine whether or not it is above
a set threshold value of 20000. If so, the value of that specific frequency is sent to
the EMIF output port to light the appropriate LED(s). From Figure 10.11, when a
value of the FFT magnitude is larger than the set threshold, the variable out is
output. This output corresponds to a bit that is shifted by the value of the index i
that is the corresponding frequency location in the FFT array. This bit shift moves
a binary 1 to the appropriate bit location corresponding to the specific LED to be
lit. This process is repeated for every value in the magnitude FFT array. If multiple
values in the FFT array are larger than the set threshold of 20000, then the appro-
priate bit-shifted value is accumulated. This process lights up all the LEDs that have