As a final exercise, you will find the output of the DSP
for an input specified by a test vector. Then you will
compare that output with the output of a MATLAB simulation
of the same filter processing the same input; if the DSP
implementation is correct, the two outputs should be
almost identical. To do this, you will generate a
waveform in MATLAB and save it as a test vector. You will
then run your DSP filter using the test vector as input
and import the results back into MATLAB for comparison
with a MATLAB simulation of the filter.
The first step in using test vectors is to generate an
appropriate input signal. One way to do this is to use
the MATLAB function sweep (available as sweep.m) to generate a sinusoid that
sweeps across a range of frequencies. The MATLAB function
save_test_vector (available as save_test_vector.m can
then save the sinusoidal sweep to a file you will later
include in the DSP code.
Generate a sinusoidal sweep and save it to a DSP
test-vector file using the following MATLAB commands:
>> t=sweep(0.1*pi,0.9*pi,0.25,500); % Generate a frequency sweep
>> save_test_vector('testvect.asm',t); % Save the test vector
Next, use the MATLAB conv command to generate
a simulated response by filtering the sweep with the
filter
h
h
you generated using gen_filt above. Note that
this operation will yield a vector of length 507 (which is
n+m−1
n
m
1
, where
n
n is the length of the filter and
m
m is the length of the input). You should keep
only the first 500 elements of the resulting vector.
>> out=conv(h,t) % Filter t with FIR filter h
>> out=out(1:500) % Keep first 500 elements of out
Now, modify the file filter.asm to use the
alternative "test vector" core file, vectcore.asm. Rather than
accepting input from the A/D converters and sending output
to the D/A, this core file takes its input from, and saves
its output to, memory on the DSP. The test vector is
stored in a block of memory on the DSP evaluation board
that will not interfere with your program code or data.
The
memory block that holds the test vector is large enough to
hold a vector up to 4,000 elements long. The test vector
stores data for both channels of input and from all six
channels of output.
To run your program with test vectors, you will need to
modify filter.asm. The assembly source
is simply a text file and can be edited using the editor of your
preference, including WordPad, Emacs, and VI. Replace the first
line of the file with two lines. Instead of:
use:
.copy "testvect.asm"
.copy "vectcore.asm"
Note that, as usual, the whitespace in front of the
.copy directive is required.
These changes will copy in the test vector you created and
use the alternative core file. After modifying your code,
assemble it, then load and run the file using Code
Composer as before. After a few seconds, halt the DSP
(using the Halt command under the
Debug menu) and verify that the DSP has
halted at a branch statement that branches to itself. In
the disassembly window, the following line should be
highlighted: 0000:611F F073 B 611fh.
Next, save the test output file and load it back into
MATLAB. This can be done by first saving 3,000 memory
elements (six channels times 500 samples) starting with
location 0x8000 in program memory. Do this
by choosing File->Data->Save... in Code
Composer Studio, then entering the filename
output.dat and pressing Enter.
Next, enter 0x8000 in the Address field of
the dialog box that pops up, 3000 in the
Length field, and choose Program from the
drop-down menu next to Page. Always make sure
that you use the correct length (six times the length of
the test vector) when you save your results.
Last, use the read_vector (available as read_vector.m) function to read
the saved result into MATLAB. Do this using the following
MATLAB command:
>> [ch1, ch2] = read_vector('output.dat');
Now, the MATLAB vector ch1 corresponds to the
filtered version of the test signal you generated. The
MATLAB vector ch2 should be nearly identical
to the test vector you generated, as it was passed from
the DSP system's input to its output unchanged.
Because of quantization error introduced in
saving the test vector for the 16-bit memory of the DSP,
the vector ch2 will not be identical to the
MATLAB generated test vector.
After loading the output of the filter into MATLAB,
compare the expected output (calculated as
out above) and the output of the filter (in
ch1 from above). This can be done graphically
by simply plotting the two curves on the same axes; for
example:
>> plot(out,'r'); % Plot the expected curve in red
>> hold on % Plot the next plot on top of this one
>> plot(ch1,'g'); % Plot the expected curve in green
>> hold off
You should also ensure that the difference between the two
outputs is near zero. This can be done by plotting the
difference between the two vectors:
>> plot(out-ch1); % Plot error signal
You will observe that the two sequences are not exactly
the same; this is due to the fact that the DSP computes
its response to 16 bits precision, while MATLAB uses
64-bit floating point numbers for its arithmetic.
Note that to compare two vectors in this way, the two
vectors must be exactly the same length, which is ensured
after using the MATLAB command out=out(1:500)
above.
