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C fft routine

A bit-reversing and FFT routine have also been provided in lab4fft.c , listed in Appendix C . Again, make sure you understand how the bit reversal is takingplace. In lab4main.c , the line defining C_FFT must not be commented for use of the C FFT routine. The sine tables (twiddle factors) are located in sinetables.h . This fft requires its inputs in two buffers, the real buffer real and the imaginary buffer imag , and the output is placed in the same buffers.The length of the FFT, N , and logN are defined in lab4.h , which is also listed in Appendix C . When experimenting with the C FFT make sure you modify these length values instead of the onesin the assembly code and lab4main.c !

Creating the window

As mentioned, you will be using the FFT to compute the spectrum of a windowed input. For your implementation youwill need to create a 1024-point Hamming window. First, create a Hamming window in matlab using the function hamming . For the assembly FFT, use save_coef to save the window to a file that can then be included in your code with the .copy directive. For the C FFT, use the matlab function write_intvector_headerfile with name set to 'window' and elemperline set to 8 to create the header file that is included in lab4main.c .

Displaying the spectrum

Once the DFT has been computed, you must calculate the squared magnitude of the spectrum for display.

X k 2 X k 2 X k 2
You may find the assembly instructions squr and squra useful in implementing .

Because the squared magnitude is always nonnegative, you can replace one of the magnitude values with a -1.0 as a triggerpulse for display on the oscilloscope. This is easily performed by replacing the DC term ( k 0 ) with a -1.0 when copying the magnitude values to the output buffer. Thetrigger pulse is necessary for the oscilloscope to lock to a specific point in the spectrum and keep the spectrum fixed on the scope.

Intrinsics

If you are planning on writing some of the code in C, then you may be forced to use intrinsics. Intrinsic instructionsprovide a way to use assembly instructions directly in C. An example of an intrinsic instruction is bit_rev_data[0]=_smpyr(bit_rev_data[0],window[0]) which performs the assembly signed multiply round instruction. You may also find the _lsmpy instruction useful. For more information on intrinsics, see page 6-22 of the TI-C54x Optimizing C/C++ Compiler User's Guide .

The following lines of code were borrowed from the C FFT to serve as an example of arithmetic operations in C. Savethis code in a file called mathex.c and compile this file. Look at the resulting assembly file and investigate the differences betweeneach block. Be sure to reference the compiler user's guide to find out what the state of the FRCT and OVM bits are. Does each blockwork properly for all possible values?

void main(void) { int s1, s2; int t1, t2; int i1, i2; int n1 = 16383, n2 = 16382, n3 = 16381, n4 = 16380; /* Code for standard 32-bit hardware, */ /* with x,y limited to 16 bits */ s1 = (n1*n2 + n3*n4) >> 15; s2 = (n1 + n2) >> 1; /* Code for TI TMSC5000 series */ t1 = ((long int)(n1*n2) + (long int)(n3*n4)) >> 15; t2 = ((long int)n1 + (long int)n2) >> 1; /* Intrinsic code for TMS320C54X series */ i1 = _sadd(_smpy(n1,n2), _smpy(n3,n4)); i2 = _sshl(_sadd(n1, n2),-1); }

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Source:  OpenStax, Ece 320 spring 2004. OpenStax CNX. Aug 24, 2004 Download for free at http://cnx.org/content/col10225/1.12
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