File content as of revision 0:05e2c9ca68e2:

#include "mbed.h"
/* Include arm_math.h mathematic functions */
#include "arm_math.h"
/* Include mbed-dsp libraries */
#include "arm_common_tables.h"
#include "arm_const_structs.h"
#include "math_helper.h"

/* FFT settings */
#define SAMPLES                 512             /* 256 real party and 256 imaginary parts */
#define FFT_SIZE                SAMPLES / 2     /* FFT size is always the same size as we have samples, so 256 in our case */

/* Global variables */
float32_t Input[SAMPLES];
float32_t Output[FFT_SIZE];
bool      trig=0;
/* MBED class APIs */
DigitalOut myled(LED1);
AnalogIn   myADC(A1);
AnalogOut  myDAC(D13);
Serial     pc(USBTX, USBRX);
Ticker     timer;

void sample(){
    trig=1;
    }

int main() {

    //arm_cfft_instance_f32 S;   // ARM CFFT module
    float maxValue;            // Max FFT value is stored here
    uint32_t maxIndex;         // Index in Output array where max value is
    bool once=0;
    pc.baud(115200);
    pc.printf("Starting FFT\r\n");
    while(1) {
            timer.attach_us(&sample,20); //20us 50KHz sampling rate
            for (int i = 0; i < SAMPLES; i += 2) {
                while (trig==0){}
                trig=0;
                Input[i] = myADC.read() - 0.5f; //Real part NB removing DC offset
                Input[i + 1] = 0;               //Imaginary Part set to zero
                }
            timer.detach();
        // Init the Complex FFT module, intFlag = 0, doBitReverse = 1
        //NB using predefined arm_cfft_sR_f32_lenXXX, in this case XXX is 256
        arm_cfft_f32(&arm_cfft_sR_f32_len256, Input, 0, 1);

        // Complex Magniture Module put results into Output(Half size of the Input)
        arm_cmplx_mag_f32(Input, Output, FFT_SIZE);
        
        //Calculates maxValue and returns corresponding value
        arm_max_f32(Output, FFT_SIZE, &maxValue, &maxIndex);

        if (once==0){
            pc.printf("Maximum is %f\r\n",maxValue);
            once = 1;
            }
       
        //maxValue /= 100.0f;
        
        myDAC=1.0f;     //SYNC Pulse to DAC Output
        wait_us(20);    //Used on Oscilliscope set trigger level to the highest
        myDAC=0.0f;     //point on this pulse (all FFT data will be scaled
                        //90% of Max Value
        
        for (int i=0; i<FFT_SIZE/2 ; i++){
            myDAC=(Output[i]/maxValue)*0.9f; // Scale to Max Value and scale to 90%
            wait_us(10); //Each pulse of 10us is 50KHz/256 = 195Hz resolution
            }
        myDAC=0.0f;
        }
}