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In this exercise, you will program in the DSP's assembly language to create FIR filters. Begin by studying theassembly code for the basic FIR filter filter.asm .
1 .copy "core.asm" ; Copy in core file
2 ; This initializes DSP and jumps to "main"
3
4 FIR_len .set 8 ; This is an 8-tap filter.
5
6 .sect ".data" ; Flag following as data declarations
7
8 .align 16 ; Align to a multiple of 16
9 coef ; assign label "coeff"
10 .copy "coef.asm" ; Copy in coefficients
11
12 .align 16
13 firstate
14 .space 16*8 ; Allocate 8 words of storage for
15 ; filter state.
16
17 .sect ".text" ; Flag the following as program code
18 main
19 ; Initialize various pointers
20 stm #FIR_len,BK ; initialize circular buffer length
21 stm #coef,AR2 ; initialize coefficient pointer
22 stm #firstate,AR3 ; initialize state pointer
23 stm #1,AR0 ; initialize AR0 for pointer increment
24
25 loop
26 ; Wait for a new block of 64 samples to come in
27 WAITDATA
28
29 ; BlockLen = the number of samples that come from WAITDATA (64)
30 stm #BlockLen-1, BRC ; Put repeat count into repeat counter
31 rptb endblock-1 ; Repeat between here and 'endblock'
32
33 ld *AR6,16, A ; Receive ch1 into A accumulator
34 mar *+AR6(2) ; Rcv data is in every other channel
35 ld *AR6,16, B ; Receive ch2 into B accumulator
36 mar *+AR6(2) ; Rcv data is in every other channel
37
38 ld A,B ; Transfer A into B for safekeeping
39
40 ; The following code executes a single FIR filter.
41
42 sth A,*AR3+% ; store current input into state buffer
43 rptz A,(FIR_len-1) ; clear A and repeat
44 mac *AR2+0%,*AR3+0%,A ; multiply coef. by state & accumulate
45
46 rnd A ; Round off value in 'A' to 16 bits
47
48 ; end of FIR filter code. Output is in the high part of 'A.'
49
50 sth A, *AR7+ ; Store filter output (from A) into ch1
51 sth B, *AR7+ ; Store saved input (from B) into ch2
52
53 sth B, *AR7+ ; Store saved input to ch3...ch6 also
54 sth B, *AR7+ ; ch4
55 sth B, *AR7+ ; ch5
56 sth B, *AR7+ ; ch6
57
58 endblock:
59 b loop
filter.asm applies an FIR filter to the signal
from input channel 1 and sends the resulting output to outputchannel 1. It also sends the original signal to output
channel 2.
First, create a work directory on your network drive for the
files in this exercise, and copy
filter.asm and
core.asm into it. Then use MATLAB to generate two 20-tap FIR
filters. The first filter should pass signals from 4 kHz to 8kHz; the second filter should pass from 8 kHz to 12 kHz. For
both filters, allow a 1 kHz transition band on each edge ofthe filter passband. To create these filters, first convert
these band edges to digital frequencies based on the 44.1 kHzsample rate of the system, then use the MATLAB command
remez to generate this filter; you can type
help remez for more information. Use the
save_coef command to save each of these filters
into different files. (Make sure you reverse the vectors of
filter coefficients before you save them.) Also save yourfilters as a MATLAB matrix, since you will need them later to
generate test vectors. This can be done using the MATLAB
save command. Once this is done, use the
freqz command to plot the frequency response of
each filter.
For now, you will implement only the filter with a 4 kHz to 8
kHz passband. Edit
filter.asm to use the
coefficients for this filter by making several changes.
First, the length of the FIR filter for this exercise is 20,
not 8. Therefore, you need to change
FIR_len to
20.
FIR_len is set using the
.set directive, which assigns a number to a symbolic name. You will
need to change this to
FIR_len .set 20 .
Second, you will need to ensure that the
.copy directive brings in the correct coefficients. Change the
filename to point to the file that contains the coefficientsfor your first filter.
Third, you will need to modify the
.align and
.space directives appropriately. The TI
TMS320C54x DSP requires that circular buffers, which are usedfor the FIR filter coefficient and state buffers, be aligned
so that they begin at an address that is a multiple of a powerof two greater than the length of the buffer. Since you are
using a 20-tap filter (which uses 20-element state andcoefficient buffers), the next greater power of two is 32.
Therefore, you will need to align both the state andcoefficient buffers to an address that is a multiple of 32.
(16-element buffers would also require alignment to a multipleof 32.) This is done with the
.align command. In
addition, memory must be reserved for the state buffer. Thisis done using the
.space directive, which takes
as its input the number of
bits of space
to allocate. Therefore, to allocate 20 words of storage, usethe directive
.space 16*20 as shown below:
1 .align 32 % Align to a multiple of 32
2 coef .copy "filter1.asm" % Copy FIR filter coefficients
3
4 .align 32 % Align to a multiple of 32
5 state .space 16*20 % Allocate 20 words of data space
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