transform_ex1_fft_fixed_q15.c

This example demonstrates how to use the msp_fft_fixed_q15 API to transform a real input data array to the frequency domain using the Q15 fast fourier transform (FFT) with fixed scaling. The real FFT function is identical to running a complex FFT with imaginary components set to zero. The result of the real FFT is a complex vector with half the samples but identical byte length.

//******************************************************************************
// Real FFT with fixed scaling.
//
// Brent Peterson, Jeremy Friesenhahn
// Texas Instruments Inc.
// April 2016
//******************************************************************************
#include "msp430.h"

#include <math.h>
#include <stdint.h>
#include <stdbool.h>

#include "DSPLib.h"

/* Input signal parameters */
#define FS                  8192
#define SAMPLES             256
#define SIGNAL_FREQUENCY1   200
#define SIGNAL_AMPLITUDE1   0.6
#define SIGNAL_FREQUENCY2   2100
#define SIGNAL_AMPLITUDE2   0.15

/* Hamming window parameters */
#define HAMMING_ALPHA       0.53836
#define HAMMING_BETA        0.46164

/* Constants */
#define PI                  3.1415926536

/* Generated Hamming window function */
DSPLIB_DATA(window,4)
_q15 window[SAMPLES];

/* Input signal and FFT result */
DSPLIB_DATA(input,MSP_ALIGN_FFT_Q15(SAMPLES))
_q15 input[SAMPLES];

/* Temporary data array for processing */
DSPLIB_DATA(temp,4)
_q15 temp[3*SAMPLES/2];

/* Benchmark cycle counts */
volatile uint32_t cycleCount;

/* Function prototypes */
extern void initSignal(void);
extern void initHamming(void);

void main(void)
{
    msp_status status;
    msp_mpy_q15_params mpyParams;
    msp_fft_q15_params fftParams;

    /* Disable WDT */
    WDTCTL = WDTPW + WDTHOLD;

#ifdef __MSP430_HAS_PMM__
    /* Disable GPIO power-on default high-impedance mode for FRAM devices */
    PM5CTL0 &= ~LOCKLPM5;
#endif
    
    /* Initialize input signal and Hamming window */
    initSignal();
    initHamming();

    /* Multiply input signal by generated Hamming window */
    mpyParams.length = SAMPLES;
    status = msp_mpy_q15(&mpyParams, input, window, input);
    msp_checkStatus(status);

    /* Initialize the fft parameter structure. */
    fftParams.length = SAMPLES;
    fftParams.bitReverse = true;
    fftParams.twiddleTable = MAP_msp_cmplx_twiddle_table_2048_q15;

    /* Perform real FFT with fixed scaling */
    msp_benchmarkStart(MSP_BENCHMARK_BASE, 16);
    status = MAP_msp_fft_fixed_q15(&fftParams, input);
    cycleCount = msp_benchmarkStop(MSP_BENCHMARK_BASE);
    msp_checkStatus(status);
    
    /* End of program. */
    __no_operation();
}

void initSignal(void)
{
    msp_status status;
    msp_add_q15_params addParams;
    msp_sinusoid_q15_params sinParams;

    /* Generate Q15 input signal 1 */
    sinParams.length = SAMPLES;
    sinParams.amplitude = _Q15(SIGNAL_AMPLITUDE1);
    sinParams.cosOmega = _Q15(cosf(2*PI*SIGNAL_FREQUENCY1/FS));
    sinParams.sinOmega = _Q15(sinf(2*PI*SIGNAL_FREQUENCY1/FS));
    status = msp_sinusoid_q15(&sinParams, input);
    msp_checkStatus(status);

    /* Generate Q15 input signal 2 to temporary array */
    sinParams.length = SAMPLES;
    sinParams.amplitude = _Q15(SIGNAL_AMPLITUDE2);
    sinParams.cosOmega = _Q15(cosf(2*PI*SIGNAL_FREQUENCY2/FS));
    sinParams.sinOmega = _Q15(sinf(2*PI*SIGNAL_FREQUENCY2/FS));
    status = msp_sinusoid_q15(&sinParams, temp);
    msp_checkStatus(status);

    /* Add input signals */
    addParams.length = SAMPLES;
    status = msp_add_q15(&addParams, input, temp, input);
    msp_checkStatus(status);
}

void initHamming(void)
{
    msp_status status;
    msp_sub_q15_params subParams;
    msp_copy_q15_params copyParams;
    msp_fill_q15_params fillParams;
    msp_sinusoid_q15_params sinParams;

    /* Generate sinusoid for cosine function */
    sinParams.length = 3*SAMPLES/2;
    sinParams.amplitude = _Q15(HAMMING_BETA);
    sinParams.cosOmega = _Q15(cosf(2*PI/(SAMPLES-1)));
    sinParams.sinOmega = _Q15(sinf(2*PI/(SAMPLES-1)));
    status = msp_sinusoid_q15(&sinParams, temp);
    msp_checkStatus(status);

    /* Shift sinusoid by pi/2 to create cosine function */
    copyParams.length = SAMPLES;
    status = msp_copy_q15(&copyParams, &temp[SAMPLES/4], &temp[0]);
    msp_checkStatus(status);

    /* Fill temporary array with alpha constant */
    fillParams.length = SAMPLES;
    fillParams.value = _Q15(HAMMING_ALPHA);
    status = msp_fill_q15(&fillParams, window);
    msp_checkStatus(status);

    /* Subtract generated cosine from alpha constant to generate final window */
    subParams.length = SAMPLES;
    status = msp_sub_q15(&subParams, window, temp, window);
    msp_checkStatus(status);
}