awulff-pico-playground/adc_fft/adc_fft.c

// Sample from the ADC continuously at a particular sample rate
// and then compute an FFT over the data
//
// much of this code is from pico-examples/adc/dma_capture/dma_capture.c
// the rest is written by Alex Wulff (www.AlexWulff.com)

#include <stdio.h>
#include <math.h>

#include "pico/stdlib.h"
#include "hardware/adc.h"
#include "hardware/dma.h"
#include "kiss_fftr.h"

// set this to determine sample rate
// 0     = 500,000 Hz
// 960   = 50,000 Hz
// 9600  = 5,000 Hz
#define CLOCK_DIV 960
#define FSAMP 50000

// Channel 0 is GPIO26
#define CAPTURE_CHANNEL 0
#define LED_PIN 25

// BE CAREFUL: anything over about 9000 here will cause things
// to silently break. The code will compile and upload, but due
// to memory issues nothing will work properly
#define NSAMP 1000

// globals
dma_channel_config cfg;
uint dma_chan;
float freqs[NSAMP];

void setup();
void sample(uint8_t *capture_buf);

int main() {
  uint8_t cap_buf[NSAMP];
  kiss_fft_scalar fft_in[NSAMP]; // kiss_fft_scalar is a float
  kiss_fft_cpx fft_out[NSAMP];
  kiss_fftr_cfg cfg = kiss_fftr_alloc(NSAMP,false,0,0);
  
  // setup ports and outputs
  setup();

  while (1) {
    // get NSAMP samples at FSAMP
    sample(cap_buf);
    // fill fourier transform input while subtracting DC component
    uint64_t sum = 0;
    for (int i=0;i<NSAMP;i++) {sum+=cap_buf[i];}
    float avg = (float)sum/NSAMP;
    for (int i=0;i<NSAMP;i++) {fft_in[i]=(float)cap_buf[i]-avg;}

    // compute fast fourier transform
    kiss_fftr(cfg , fft_in, fft_out);
    
    // compute power and calculate max freq component
    float max_power = 0;
    int max_idx = 0;
    // any frequency bin over NSAMP/2 is aliased (nyquist sampling theorum)
    for (int i = 0; i < NSAMP/2; i++) {
      float power = fft_out[i].r*fft_out[i].r+fft_out[i].i*fft_out[i].i;
      if (power>max_power) {
	max_power=power;
	max_idx = i;
      }
    }

    float max_freq = freqs[max_idx];
    printf("Greatest Frequency Component: %0.1f Hz\n",max_freq);
  }

  // should never get here
  kiss_fft_free(cfg);
}

void sample(uint8_t *capture_buf) {
  adc_fifo_drain();
  adc_run(false);
      
  dma_channel_configure(dma_chan, &cfg,
			capture_buf,    // dst
			&adc_hw->fifo,  // src
			NSAMP,          // transfer count
			true            // start immediately
			);

  gpio_put(LED_PIN, 1);
  adc_run(true);
  dma_channel_wait_for_finish_blocking(dma_chan);
  gpio_put(LED_PIN, 0);
}

void setup() {
  stdio_init_all();

  gpio_init(LED_PIN);
  gpio_set_dir(LED_PIN, GPIO_OUT);

  adc_gpio_init(26 + CAPTURE_CHANNEL);

  adc_init();
  adc_select_input(CAPTURE_CHANNEL);
  adc_fifo_setup(
		 true,    // Write each completed conversion to the sample FIFO
		 true,    // Enable DMA data request (DREQ)
		 1,       // DREQ (and IRQ) asserted when at least 1 sample present
		 false,   // We won't see the ERR bit because of 8 bit reads; disable.
		 true     // Shift each sample to 8 bits when pushing to FIFO
		 );

  // set sample rate
  adc_set_clkdiv(CLOCK_DIV);

  sleep_ms(1000);
  // Set up the DMA to start transferring data as soon as it appears in FIFO
  uint dma_chan = dma_claim_unused_channel(true);
  cfg = dma_channel_get_default_config(dma_chan);

  // Reading from constant address, writing to incrementing byte addresses
  channel_config_set_transfer_data_size(&cfg, DMA_SIZE_8);
  channel_config_set_read_increment(&cfg, false);
  channel_config_set_write_increment(&cfg, true);

  // Pace transfers based on availability of ADC samples
  channel_config_set_dreq(&cfg, DREQ_ADC);

  // calculate frequencies of each bin
  float f_max = FSAMP;
  float f_res = f_max / NSAMP;
  for (int i = 0; i < NSAMP; i++) {freqs[i] = f_res*i;}
}