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The parameter passing convention between assembly and C is
simple for single input, single output assembly functions.From a C program, the input to an assembly program is in the
low part of accumulator
A
with the output
returned in the same place. In this example, the function
filter
takes the right input sample from
A
and returns a single output in
A
(note the left shift by 16 to put the result in the low part
of
A
). When more than one parameter is passed
to an assembly function, the parameters are passed on thestack (see the core file description for more information).
We suggest that you avoid passing or returning more than oneparameter. Instead, use global memory addresses to pass in
or return more than one parameter. Another alternative isto pass a pointer to the start of a buffer intended for
passing and returning parameters.
When entering and leaving an assembly function, the
ENTER_ASM
and
LEAVE_ASM
macros
ensure that certain registers are saved and restored. Sincethe C program may use any and all registers, the state of a
register cannot be expected to remain the same between callsto assembly function(s).
Therefore, any
information that needs to be preserved across calls to anassembly function must be saved to memory! In
this example,
stateptr
keeps track of the
location of the current sample in the circular buffer
firstate
. Why don't we need to keep track of
the location of the coefficient pointer (
AR2
in this example) after every sample?
A working program can be produced by compiling the C code
and linking assembly modules and the core module. Thecompiler translates C code to a relocatable assembly form.
The linker assigns physical addresses on the DSP to therelocatable data and code segments, resolves
.global
references and links runtime libraries.
The procedure for compiling C code and linking assembly
modules has been automated for you in the batch file
v:\ece320\54x\dsptools\C_ASM.bat
. Copy the
files
lab3main.c
, and
lab3filt.asm
from the
v:\ece320\54x\dspclib\
directory into
your own directory on the
W:
drive. Using
Matlab, write the coefficients you created in the prelabinto a
coef1.asm
file. Then, type
c_asm
lab3main lab3filt
to produce a
lab3main.out
file to be loaded onto the DSP.
Load the output file onto the DSP as usual and check that isthe FIR filter you designed.
Modify the
lab3filt.asm
assembly module to
implement a cascade of filters FIR1 and FIR2. Note thatboth
_filter
and
_init_filter
will
need to be modified. Compile and link the new assemblymodule and confirm it has the frequency response which you
expect from cascading FIR1 and FIR2.
Once you have the cascaded system working, implement the
multirate system composed of the three FIR filters bymodifying the assembly modules in
lab3filt.asm
.
In order to implement the sample rate converters, you willneed to use a counter or a loop.
The upsampling
block and downsampling block are not implemented as seperatesections of code. Your counter or loop will
determine when the decimated rate processing is to occur aswell as when to insert zeros into FIR3 to implement the
zero-filling up-sampler.
Some instructions that may be useful for implementing your
multirate structure are the
addm
(add to
memory) and
bc
(branch conditional)
instructions. You may also find the
banz
(branch on auxiliary register not zero) instruction useful,
depending on how you implement your code.
As the
counter is state information that needs to be preservedbetween calls to
filter
, the counter must be
saved in memory.
In order to experiment with multirate effects in yoursystem, make the downsampling factor ( ) a constant which can be changed easily in your code. Is there a critical( ) associated with this system above which aliasing occurs?
It will be useful both for debugging and for experimentation
to show the output of your system at various points in theblock diagram. By modifying the C code in
lab3main.c
and the assembly modules in
lab3filt.asm
, send the following sequences to
the DSP output
For the quiz, you should be prepared to change the decimation rate upon request, and explain the effects ofchanging the decimation rate on the system's output.
As usual, your grade will be split up into three sections:
One of the main benefits of multirate systems is efficiency. Because of downsampling, the output of FIR1 is used only oneof times. Make your assembly module more efficient by using this fact.
Similarly, at the input of FIR3, of every samples is zero. So, for a fixed downsampling factor , it is possible to make use of this fact to create different filters (each a subset of the coefficients of FIR3) to be used at the time instances. This technique is referred to as polyphase filtering and can be found in most modern DSPtextbooks. These filters are more efficient as the sum of the lengths of the filters is equal to the length of FIR3.Apply this fact for .
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