DSP Applications and Student Projects - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 351

// DTMF_Bios_FFT.c Core program using radix-4 FFT and onboard LEDs
. . . //see radix-4 example in Chapter 6
short input_buffer[N] = {0}; //to store input samples...same as x
float output_buffer[7] = {0}; //to store magnitude of FFT
short buffer_count, i, J;
short nFlag; //indicator to begin FFT

short nRow, nColumn;
double delta;
float tempvalue;
interrupt void c_int11()
{
input_buffer[buffer_count] = input_sample();
output_sample((short)input_buffer[buffer_count++]);
if(buffer_count >= N) //if accum more than N points->begin FFT
{
buffer_count = 0; //reset buffer_count

nFlag = 0; //flag to signal completion

for(i = 0; i < N; i++)
{

x[2*i] = (float)input_buffer[i]; //real part of input

x[2*i+1] = 0; //imaginary part of input

}

}
}
void main(void)
{
nFlag = 1;
buffer_count = 0;
. . . //generate twiddle constants, then index for digit reversal
comm_intr();
while(1) //infinite loop
{
while(nFlag); //wait for ISR to finish buffer accum samples
nFlag = 1;
//call radix-4 FFT, then digit reverse function
output_buffer[0]=(float) sqrt(x[2*22]*x[2*22]+x[2*22+1]*x[2*22+1]);
. . . //for weigths 25,28,31,39,43
output_buffer[6]=(float) sqrt(x[2*47]*x[2*47]+x[2*47+1]*x[2*47+1]);
tempvalue = 0; //choose largest row frequency

nRow = 0;
for(j = 0; j < 4; j++)
{
if(tempvalue < output_buffer[j])
{

if(output_buffer[j] > 0.5e4)

{

nRow = j + 1;

tempvalue = output_buffer[j];

}

}
} //end of for loop
tempvalue = 0; //choose largest column frequency
nColumn = 0;
for(j = 4; j < 7; j++)
. . . //as with the rows
FIGURE 10.4. Core C program using FFT to detect DTMF tones ( dtmf_bios_FFT.c ).