ARM Cortex-M3 (STM32F103) Project - 16 Point FFT Audio Spectrum Analyzer

This project is the application of FFT algorithm from my previous post. I made an audio spectrum analyzer project using ARM Cortex-M3 (STM32F013C8) and LED matrix 8x8. I used 16 point FFT. 16 point FFT will produce 16 point frequency spectrum. I only display the frequency spectrum from index 1 to 8 on LED matrix 8x8. Index 0 of frequency spectrum (DC signal) is not displayed. Index 9 to 15 of frequency spectrum (symmetrical frequency) is also not displayed.

My audio sampling rate is 35.15kHz, but the frequency sampling of FFT sample is 35.15kHz/2=17.5kHz. So, the maximum frequency domain signals that can be displayed is up to 8.75kHz (Nyquist's theorem) and the resolution is 8.75kHz/8≈1kHz.

I have tested the spectrum analyzer using signal generator. This is the result:

The spectrum on the left side of LED matrix is about 1.1kHz and on the right side of LED matrix is about 8.8kHz. This is code for interrupt routine and main loop:

#include "stm32f10x.h"
#include "stm32f10x_rcc.h"
#include "stm32f10x_gpio.h"
#include "stm32f10x_adc.h"
#include "stm32f10x_tim.h"
#include <math.h>

// 16 point FFT
#define N     16
#define PI    3.141

#define RCC_GPIO_ROW RCC_APB2Periph_GPIOA
#define RCC_GPIO_COL RCC_APB2Periph_GPIOB
#define GPIO_ROW GPIOA
#define GPIO_COL GPIOB
#define GPIO_PIN_ROW_0 GPIO_Pin_4
#define GPIO_PIN_ROW_1 GPIO_Pin_5
#define GPIO_PIN_ROW_2 GPIO_Pin_6
#define GPIO_PIN_ROW_3 GPIO_Pin_7
#define GPIO_PIN_ROW_4 GPIO_Pin_8
#define GPIO_PIN_ROW_5 GPIO_Pin_9
#define GPIO_PIN_ROW_6 GPIO_Pin_10
#define GPIO_PIN_ROW_7 GPIO_Pin_11
#define GPIO_PIN_COL_0 GPIO_Pin_8
#define GPIO_PIN_COL_1 GPIO_Pin_9
#define GPIO_PIN_COL_2 GPIO_Pin_10
#define GPIO_PIN_COL_3 GPIO_Pin_11
#define GPIO_PIN_COL_4 GPIO_Pin_12
#define GPIO_PIN_COL_5 GPIO_Pin_13
#define GPIO_PIN_COL_6 GPIO_Pin_14
#define GPIO_PIN_COL_7 GPIO_Pin_15

volatile uint16_t adc_value = 0;
volatile uint8_t n_count = 0;
volatile uint8_t n_done = 0;
int REX[N];
int IMX[N];
uint16_t MAG[N];
uint8_t led_buf[8];

void init_adc(void);
void init_timer(void);
void init_pwm(void);
void init_led_matrix(void);
uint16_t read_adc(void);
void write_pwm(uint16_t val);
void led_matrix_update(void);
void fft(void);
void mag_to_buf(void);

void TIM3_IRQHandler()
{
    static uint8_t s = 0;
    static uint8_t l = 0;
 
    // TIM3 interrupt at 35.15kHz
    if (TIM_GetITStatus(TIM3, TIM_IT_Update))
    {
        // Read ADC value (10-bit PWM)
        adc_value = read_adc() >> 2;
        // Write to PWM (audio loopback)
        write_pwm(adc_value);

        // Sampling N point FFT at 17.5kHz
        s++;
        if (s >= 2)
        {
            if (n_done == 0)
            {
                REX[n_count++] = adc_value;

                if (n_count >= N)
                {
                    n_done = 1;
                    n_count = 0;   
                }
            }
            s = 0;
        }

        // LED matrix scanning at 1kHz
        l++;
        if (l >= 35)
        {
            led_matrix_update();
            l = 0;
        }

        // Clears the TIM3 interrupt pending bit
        TIM_ClearITPendingBit(TIM3, TIM_IT_Update);
    }
}

int main(void)
{
    init_adc();
    init_timer();
    init_pwm();
    init_led_matrix();

    while (1)
    {
        // Wait until sampling is done
        while (!n_done);
        fft();
        mag_to_buf();
        n_done = 0;
    }
}

You can see the full code for this project on my repository.