Analog Devices ee-88 Application Notes

Engineer to Engineer Note EE-88

Technical Notes on using Analog Devices’ DSP components and development tools

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Initializing Variables in C
using the 21xx C compiler

Last modified 9/2/98

Overview

This Engineer’s Note will address the
procedure of initializing variables in C using the
21xx C compiler. Example FFT code is provided at
the end of this EE note.

For many applications using the 21xx
family of DSPs, data is gathered by using one of the
many IO interfaces on the DSP. However, there
may be an occasion where an engineer may wish to
initialize a variable with a data array in the
internal memory of the DSP. This is not difficult to
do using assembly code. However, this procedure is
a little trickier for the engineer who is
programming a 21xx DSPs using C code.

To initialize variables in C for the 21xx
family of DSPs, it is necessary to define arrays in
an external assembly module. Additionally, these
arrays should be declared as “.global” in the
assembly code. Then, these arrays may be called in
your C code, by declaring them as “extern.” Then,
to initialize these variables, use a “.init” statement
to initialize the arrays in the assembly module. For
more details on the “.init” statement, please read
page 3-33 of the Assembler Tools and Simulator
Manual and read the example code in this EE note.

real_input and imag_input, are initialized with a
data file that contains 16 bit hex values in 1.15 data
format. They are stored in the files named real.hex
and imag.hex. The format of these files is
demonstrated below:

Real.hex Imag.hex
0000 0000

080a 0000
100b 0000

. .
. . 252 more values
. .

d872 0000

The arrays imag_output and real_output
are the results of the fft function, which then are
fed into the inverse fft function. The outputs of the
inverse fft function, rout and iout, are nearly
identical to the inputs, real_input and imag_input.
Discrepancies are due to the limited resolution of
the 21xx DSP family.

This code was compiled using VDSP,
version 6.0, with the following command lines:

g21 cifft.c -a adsp2181.ach -runhdr 2181_hdr.dsp -g ­save-temps -c

asm21 var_init.dsp -c -cp -2181
ld21 cifft var_init 2181_hdr -a adsp2181 -e cifft -gcc -lib -x

-g

The example code on the next page performs
a fft and an inverse fft upon two data arrays that
are stored in data memory. These arrays, called

a

References:

ADSP-2100 Family User’s Manual, Third Edition
ADSP-2100 Family C Runtime Library Manual,

Second Edition
ADSP-2100 Family C Tools Manual, Second Edition

EE-88 Page 2

Technical Notes on using Analog Devices’ DSP components and development tools from the DSP division

Phone: (800) ANALOG-D, FAX: [phone], FTP: ftp.analog.com, EMAIL: [e-mail]

Listing fft_ifft.c

/* The code performs a fft and inverse fft upon an array of 256 data points which are stored in the variable s real_input and
imag_input. These arrays are declared and initialized in the file array_init.dsp. The output of this program, rout and iout, is
nearly identical to the inputs, real_input and imag_input. Discrepancies are due to the limited resolution of the 21xx DSP family.
In this test case, a regular sine wave input was used for real_input and imag_input. */

#include <ffts.h> /*include C callable fft functions */
#define N 256 /*number of data points, can be configured */

/*valid values for N can be found in the C runtime */
/*library manual under fft or ifft. */

extern int real_input[N], imag_input[N]; /* these variables are declared in array_init.dsp */

/* and describe the real and imaginary components */
/* of the input signal, a sine wave */

int imag_output[N], real_output[N]; /* these values are the result of the fft */

/* which are in turn passed to the inverse fft */

int rout[N], iout[N]; /* results of the inverse fft */

main()
{
int i;
fft256(real_input, imag_input, real_output, imag_output); /* call fft program */
ifft256(real_output, imag_output, rout, iout); /* call inverse fft program */
asm("idle;"); /* after done, idle */
}

Listing, array_init.dsp

.module array_init;

.var/dm real_input_[256]; /* declares array of 256 points */
.global real_input_; /* makes this array accessable by fft_ifft.c */
.INIT real_input_: <real.hex>; /* initializes the array with real part of a sine wave */

.var/dm imag_input_[256]; /* declares array of 256 points */
.global imag_input_; /* makes this array accessable by fft_ifft.c */
.INIT imag_input_: <imag.hex>; /* initializes the array with imaginary part of a sine wave */

.endmod;

EE-88 Page 3

Technical Notes on using Analog Devices’ DSP components and development tools from the DSP division

Phone: (800) ANALOG-D, FAX: [phone], FTP: ftp.analog.com, EMAIL: [e-mail]

Real input and output from fft_ifft.c

Imaginary input and output from fft_ifft.c

EE-88 Page 4

Technical Notes on using Analog Devices’ DSP components and development tools from the DSP division

Phone: (800) ANALOG-D, FAX: [phone], FTP: ftp.analog.com, EMAIL: [e-mail]