arm_cfft_sR_q31_len4096 undeclared - c

Question

I am doing some FFT calculations on a STM32F407 and I want to compare the different FFT functions that is available in the CMSIS DSP library. When I am using the f32 CFFT functions it works as one would expect but when I try to use the q31/q15 functions I get an error saying "arm_cfft_sR_q31_len4096" or arm_cfft_sR_q15_len4096 undeclared when I call their respective cfft functions. I have included arm_const_structs.h where it should be defined but apparently it isn't? It works with arm_cfft_sR_f32_len4096 for the f32 version of the functions so what might be the problem?
Here is how the f32 version of my fft calculations look:
#include "arm_const_structs.h"
float32_t fft_data[FFT_SIZE * 2];
uint16_t util_calculate_fft_value(uint16_t *buffer, uint32_t len, uint32_t fft_freq, uint32_t fft_freq2)
{
uint16_t i;
float32_t maxValue; // Max FFT value is stored here
uint32_t maxIndex; // Index in Output array where max value is
tmStartMeasurement(&time); // Record clock cycles
// Ensure in buffer is not longer than FFT buffer
if (len > FFT_SIZE)
len = FFT_SIZE;
// Convert buffer uint16 to fft input float32
for (i = 0; i < len ; i++)
{
fft_data[i*2] = (float32_t)buffer[i] / 2048.0 - 1.0; // Real part
fft_data[i*2 + 1] = 0; // Imaginary part
}
// Process the data through the CFFT module intFlag = 0, doBitReverse = 1
arm_cfft_f32(&arm_cfft_sR_f32_len4096, fft_data, 0, 1);
// Process the data through the Complex Magniture Module for calculating the magnitude at each bin
arm_cmplx_mag_f32(fft_data, fft_data, FFT_SIZE / 2);
// Find maxValue as max in fft_data
arm_max_f32(fft_data, FFT_SIZE, &maxValue, &maxIndex);
if (fft_freq == 0)
{ // Find maxValue as max in fft data
arm_max_f32(fft_data, FFT_SIZE, &maxValue, &maxIndex);
}
else
{ // Grab maxValue from fft data at freq position
arm_max_f32(&fft_data[fft_freq * FFT_SIZE / ADC_SAMP_SPEED - 1], 3, &maxValue, &maxIndex);
if (fft_freq2 != 0)
{
// Grab maxValue from fft data at freq2 position
float32_t maxValue2; // Max FFT value is stored here
uint32_t maxIndex2; // Index in Output array where max value is
arm_max_f32(&fft_data[fft_freq * FFT_SIZE / ADC_SAMP_SPEED - 1], 3, &maxValue2, &maxIndex2);
maxValue = (maxValue + maxValue2) / 2.0;
}
}
tmStopMeasurement(&time); // Get number of clock cycles
// Convert output back to uint16 for plotting
for (i = 0; i < len / 2; i++)
{
buffer[i] = (uint16_t)(fft_data[i] * 10.0);
}
// Zero the rest of the buffer
for (i = len / 2; i < len; i++)
{
buffer[i] = 0;
}
LOG_INFO("FFT number of cycles: %i\n", time.worst);
return ((uint16_t)(maxValue * 10.0));
}

Answer 1

I found a copy of arm_const_structs.h online. It includes the following line:
extern const arm_cfft_instance_q31 arm_cfft_sR_q31_len4096;
The extern keyword means that this line is a declaration of arm_cfft_SR_q31_len4096, not a definition. The variable must also be defined somewhere else in your code.
I found the definition in arm_const_structs.c.
const arm_cfft_instance_q31 arm_cfft_sR_q31_len4096 = {
4096, twiddleCoef_4096_q31, armBitRevIndexTable_fixed_4096, ARMBITREVINDEXTABLE_FIXED_4096_TABLE_LENGTH
};
Make sure you have included arm_const_structs.c in your project so that it compiles and links with your program.