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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 . 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
lab4.c .
The window has already been created for you.
Once the DFT has been computed, you must calculate the squared magnitude of the spectrum for display.
sqrm and
sqam 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 ( ) 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.
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 3-29 of the
TI-C55x
DSP Programmer's Guide .
The following lines of code were borrowed from the C FFT to serve as an example of arithmetic operations in C. Save this code in a file called mathex.c and create a new project by going to
Project->New... . In the following window, enter mathex as the name for the project and save it in its own folder on the W: drive. Verify that the Project Type is Executable (.out) and that the target is TMS320C55XX before clicking Finish.
int s1, s2;
int t1, t2;
int i1, i2;
int n1 = 16383, n2 = 16382, n3 = 16381, n4 = 16380;
void main(void)
{
/* 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 TMS320C55X series */
t1 = ((long int)(n1*n2) + (long int)(n3*n4)) >> 15;
t2 = ((long int)n1 + (long int)n2) >> 1;
/* Intrinsic code for TMS320C55X series */
i1 = _sadd(_smpy(n1,n2), _smpy(n3,n4));
i2 = _sshl(_sadd(n1, n2),-1);
while(1);
}
Add the mathex.c file to the project by left-clicking on the mathex.pjt file in the left-hand window and selecting
Add Files to Project.. . By default, the generated assembly code is not saved. To save the generated assembly for later comparison, go to
Project->Build Options . Under the Compiler tab, click on the Assembly category and make sure
Keep Generated .asm Files is selected.
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