CMSIS_rfft.c

// ... application includes here

// CMSIS Math includes
#include "arm_math.h"
#include "arm_const_structs.h"

// using a 1024 point signal
#define FFT_SAMPLES 1024
#define FFT_SAMPLES_HALF (FFT_SAMPLES / 2)

// see gist
#include "FFTsignal.h"
static float32_t complexFFT[FFT_SAMPLES], realFFT[FFT_SAMPLES_HALF],
		imagFFT[FFT_SAMPLES_HALF], angleFFT[FFT_SAMPLES_HALF],
		powerFFT[FFT_SAMPLES_HALF];

uint32_t fftSize = FFT_SAMPLES;
uint32_t ifftFlag = 0;
arm_rfft_fast_instance_f32 S;
uint32_t maxIndex = 0;
arm_status status;
float32_t maxValue;
int i;

void main() {
  status = ARM_MATH_SUCCESS;
  status = arm_rfft_fast_init_f32(&S, fftSize);
  // input is real, output is interleaved real and complex
  arm_rfft_fast_f32(&S, inputSignal, complexFFT, ifftFlag);
	
	// first entry is all real DC offset
	float32_t DCoffset = complexFFT[0];
  
  // de-interleave real and complex values
  for (i = 0; i < (FFT_SAMPLES / 2) - 1; i++) {
    realFFT[i] = complexFFT[i * 2];
    imagFFT[i] = complexFFT[(i * 2) + 1];
  }
  
  // find angle of FFT
  for (i = 0; i < FFT_SAMPLES / 2; i++) {
    angleFFT[i] = atan2f(imagFFT[i], realFFT[i]);
  }
	// compute power
	arm_cmplx_mag_squared_f32(complexFFT, powerFFT, FFT_SAMPLES_HALF);
	arm_max_f32(&powerFFT[1], FFT_SAMPLES_HALF - 1, &maxValue, &maxIndex);
	// correct index
	maxIndex += 1;
}