-13.53     0.00     1.09    13.97     4.73    23.37     3.00    16.20   -14.50     0.41   -44.25   -10.98   -94.92   -12.72  -266.36    -3.09  1837.12     0.00  -266.36     3.09   -94.92    12.72   -44.25    10.98   -14.50    -0.41     3.00   -16.20     4.73   -23.37     1.09   -13.97
13.53    14.01    23.84    16.48    14.51    45.59    95.77   266.38  1837.12   266.38    95.77    45.59    14.51    16.48    23.84    14.01
MAX value at magnitudes[8] : +1837.12

#include "mbed.h"
#include <ctype.h>
#include "arm_math.h"
#include "arm_const_structs.h"

Serial pc(USBTX, USBRX);

AnalogIn   left(PTC2);
AnalogIn   right(PTB3);

int SAMPLE_RATE_HZ = 40000;             // Sample rate of the audio in hertz.
const int FFT_SIZE = 16;                // Size of the FFT.

const static arm_cfft_instance_f32 *S;
Ticker samplingTimer;
float samples[FFT_SIZE*2];
float magnitudes[FFT_SIZE];
int sampleCounter = 0;

void samplingCallback()
{
    // Read from the ADC and store the sample data
    samples[sampleCounter] = (1023 * left) - 511.0;
    // Complex FFT functions require a coefficient for the imaginary part of the input.
    // Since we only have real data, set this coefficient to zero.
    samples[sampleCounter+1] = 0.0;
    // Update sample buffer position and stop after the buffer is filled
    sampleCounter += 2;
    if (sampleCounter >= FFT_SIZE*2) {
        samplingTimer.detach();
    }
}

void samplingBegin()
{
    // Reset sample buffer position and start callback at necessary rate.
    sampleCounter = 0;
    samplingTimer.attach_us(&samplingCallback, 1000000/SAMPLE_RATE_HZ);
}

bool samplingIsDone()
{
    return sampleCounter >= FFT_SIZE*2;
}

int main()
{
    // Set up serial port.
    pc.baud (38400);

    // Init arm_ccft_32
    switch (FFT_SIZE)
    {
    case 16:
        S = & arm_cfft_sR_f32_len16;
        break;
    case 32:
        S = & arm_cfft_sR_f32_len32;
        break;
    case 64:
        S = & arm_cfft_sR_f32_len64;
        break;
    case 128:
        S = & arm_cfft_sR_f32_len128;
        break;
    case 256:
        S = & arm_cfft_sR_f32_len256;
        break;
    case 512:
        S = & arm_cfft_sR_f32_len512;
        break;
    case 1024:
        S = & arm_cfft_sR_f32_len1024;
        break;
    case 2048:
        S = & arm_cfft_sR_f32_len2048;
        break;
    case 4096:
        S = & arm_cfft_sR_f32_len4096;
        break;
    }
    float maxValue = 0.0f;
    unsigned int testIndex = 0;

    // Begin sampling audio
    samplingBegin();

    while(1)
    {
        // Calculate FFT if a full sample is available.
        if (samplingIsDone())
        {
            // Run FFT on sample data.
            arm_cfft_f32(S, samples, 0, 1);
            for(int i = 0;i < FFT_SIZE*2;++i)
                printf("% 8.2f ",samples[i]);
            printf("\r\n");
            // Calculate magnitude of complex numbers output by the FFT.
            arm_cmplx_mag_f32(samples, magnitudes, FFT_SIZE);
            for(int i = 0;i < FFT_SIZE;++i)
                printf("% 8.2f ",magnitudes[i]);
            printf("\r\n");
            arm_max_f32(magnitudes, FFT_SIZE, &maxValue, &testIndex);
            printf("MAX value at magnitudes[%d] : %+ 8.2f\r\n", testIndex, maxValue);
            // Wait for user confirmation to restart audio sampling.
            pc.getc();
            samplingBegin();
        }
    }
}