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Now, we explain how to use the FFT routine provided by TI
for the C54x. The FFT routine
fft.asm located
in
v:\ece420\54x\dsplib\ computes an in-place,
complex FFT. The length of the FFT is defined as a label
K_FFT_SIZE and the algorithm assumes that the
input starts at data memory location
_fft_data .
To have your code assemble for an
-point FFT, you will have to include the following
label definitions in your assembly code.
N .set 1024
K_FFT_SIZE .set N ; size of FFT
K_LOGN .set 10 ; number of stages (log_2(N))
In addition to defining these constants, you will have to
include twiddle-factor tables for the FFT. These tables(
twiddle1 and
twiddle2 ) are available in the shared
directory
v:\ece420\54x\dsplib\ . Note that the
tables are each
points long representing values from 0 to just shy
of 180 degrees and must be accessed using a
circular pointer . To include these
tables at the proper location in memory with the appropriatelabels referenced by the FFT, use the following
.sect ".data"
.align 1024
sine .copy "v:\ece420\54x\dsplib\twiddle1"
.align 1024
cosine .copy "v:\ece420\54x\dsplib\twiddle2"
The FFT provided requires that the input be in bit-reversed
order, with alternating real and imaginarycomponents. Bit-reversed addressing is a convenient way to
order input
into a FFT so that the output
is in sequential order (
| Input Order | Binary Representation | Bit-Reversed Representation | Output Order |
|---|---|---|---|
| 0 | 000 | 000 | 0 |
| 1 | 001 | 100 | 4 |
| 2 | 010 | 010 | 2 |
| 3 | 011 | 110 | 6 |
| 4 | 100 | 001 | 1 |
| 5 | 101 | 101 | 5 |
| 6 | 110 | 011 | 3 |
| 7 | 111 | 111 | 7 |
The following routine performs the bit-reversed reordering
of the input data. The routine assumes that the input isstored in data memory starting at the location labeled
_bit_rev_data , which must be aligned to the
least power of two greater than the input buffer length, andconsists of alternating real and imaginary parts. Because
our input data is going to be purely real in this lab, youwill have to make sure that you set the imaginary parts to
zero by zeroing out every other memory location.
1 bit_rev:
2 STM #_bit_rev_data,AR3 ; AR3 -> original input
3 STM #_fft_data,AR7 ; AR7 -> data processing buffer
4 MVMM AR7,AR2 ; AR2 -> bit-reversed data
5 STM #K_FFT_SIZE-1,BRC
6 RPTBD bit_rev_end-1
7 STM #K_FFT_SIZE,AR0 ; AR0 = 1/2 size of circ buffer
8 MVDD *AR3+,*AR2+
9 MVDD *AR3-,*AR2+
10 MAR *AR3+0B
11 bit_rev_end:
12 NOP
13 RET
As mentioned, in the above code
_bit_rev_data is a label indicating the start of the input data and
_fft_data is a label indicating the start of a
circular buffer where the bit-reversed data will bewritten. Note that although
AR7 is not used by
the bit-reversed routine directly, it is used extensively inthe FFT routine to keep track of the start of the FFT data
space.
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