RAK 18000 (stereo mic) code example

I use a RAK 18000 sensor to acquire some audio data. I follow the given code example and the easiest case works fine. It remains, however, unclear to me how to properly costumize the code as to my wishes. Here is the original example sketch:

 * @file RAK18000_Stereo.ino
 * @author rakwireless
 * @brief  This example reads audio data from the on-board PDM microphones, and prints
 * out the samples to the Serial console. The Serial Plotter built into the
 * Arduino IDE can be used to plot the audio data (Tools -> Serial Plotter)
 * @version 0.1
 * @date 2021-03-07
 * @copyright Copyright (c) 2020
 * @note RAK4631 GPIO mapping to nRF52840 GPIO ports
   RAK4631    <->  nRF52840
   WB_IO1     <->  P0.17 (GPIO 17)
   WB_IO2     <->  P1.02 (GPIO 34)
   WB_IO3     <->  P0.21 (GPIO 21)
   WB_IO4     <->  P0.04 (GPIO 4)
   WB_IO5     <->  P0.09 (GPIO 9)
   WB_IO6     <->  P0.10 (GPIO 10)
   WB_SW1     <->  P0.01 (GPIO 1)
   WB_A0      <->  P0.04/AIN2 (AnalogIn A2)
   WB_A1      <->  P0.31/AIN7 (AnalogIn A7)

#include <PDM.h>

#define PDM_DATA_PIN  21
#define PDM_CLK_PIN   4
#define PDM_PWR_PIN   -1

// buffer to read samples into, each sample is 16-bits
short sampleBuffer[256] = {0};

// number of samples read
volatile int samplesRead = 0;

void setup() {

	time_t timeout = millis();
	while (!Serial)
		if ((millis() - timeout) < 5000)

  // configure the data receive callback
  // optionally set the gain, defaults to 20

  // initialize PDM with:
  // - 2 means stereo, 1 means single
  // - a 16 kHz sample rate
  if (!PDM.begin(2, 16000)) 
    Serial.println("Failed to start PDM!");
    while (1);

void loop() 
    for (int i = 0; i < samplesRead; i++) 
        //Simple filtering, if less than 200, it will be regarded as noise
        if(sampleBuffer[i] <= 200 && sampleBuffer[i] >= -200)
            sampleBuffer[i] = 0;
    // clear the read count
	  samplesRead = 0;

void onPDMdata() {
  // query the number of bytes available
  int bytesAvailable = PDM.available();

  // read into the sample buffer
  PDM.read((uint8_t *)sampleBuffer, bytesAvailable);
  // 16-bit, 2 bytes per sample
  samplesRead = bytesAvailable / 2;

What I wonder is:

  • when is the callback “onPDMdata()” actually called during the loop()? Using print statements, it seems the callback is called randomly in the middle of executing the main loop, which makes no sense to me as the variable samplesRead will then be overwritten
  • why does data acquistion fail when I add a simple delay(1000); at the end of the loop?
  • how could I make use of the deep sleep function (I have a code for that) and the microphone, e.g., sleep 10 minutes and then record data for 5 seconds - repeat

Note: on RAK GitHub, the link to the RAK 18000 sensor is broken

Thank you for any hints, e.g., more advanced code examples or some insighs as to the code understanding.


Sorry for the late reply.

The RAK18000 is now part of the WisBlock Audio product line. Examples for the RAK18000 are now in the RAKwireles Audio Library which can be installed with the ArduinoIDE library manager.

onPDMdata() callback is called when the PDM buffer is full (256 samples).

If you add a delay(1000), you might loose data as the application buffer is overwritten by callback.

I did not try sleep functions, but I guess you have to use PDM.stop() before you sleep, then on wakeup start the PDM again with PDM.begin(channels, PCM_16000));