Let’s add a mic to our BLE application and receive audio frames into a buffer. That way later we can apply some fun signal processing to the audio.

What is I2S and how do we use it in zephyr.

Inter-Intergrated Circuit Sound I2S is a serial interface protocal for transmitting two channel digital audio as Pulse Code Modulation PCM betwen IC components of an electronic device.

A bus separates clock and serial data signals which means the receivers are simpler. Since the clock does not need to bre recovered from the data stream.

DMA Segue

DMA (Direct Memory Access) lets data move directly between devices and memory without the CPU handling each piece. It’s useful for streams like audio because it allows continuous data capture or playback efficiently, freeing the CPU to work on other tasks instead of manually moving every sample.

  • Explore k_mem_slab and why this would be key.

#### Memory Slabs

  • In zephyr a memory slab is a fixed memory allocator, Imagine a stack of blocks that one can check out use and return.
  • Blocks are all the same size since it is predefined
  • Allocated at compile time so we dont run a risk of running over
  • They are thread safe and this guards us from leaking.
  • Since blocks are all the same size it prevents us from memory fragmentation.

We just define these blocks and i2s_read basically gets a block filled by DMA and if we are ever transmitting the i2s_write function will pass a block to DMA for transmission. Typically we have 2 Blocks so we can do double buffering i.e fill this one while we process the other one.

Pins Used

  • Serial Clock: SCK also known as BCLK
  • Word Select: WS also known as LRCLK or FS
  • Serial Data: SD also known as SDATA, SDIN, SDOUT, DACDATA, ADCDATA
Zephyr I²S API
  • Zephyr I²S
  • Focus on key APIs such as i2s_configure(), i2s_trigger(), and i2s_read().
  • Understand the i2s_config struct and how it defines word size, channels, format, and buffers.
Check nRF52840 I²S capabilities
  • Verify that CONFIG_I2S_NRFX is available in Kconfig.
  • Check the board’s .dts file for an i2s0 node.

  • If missing, plan to overlay the device tree for your specific pins.

    Audio flow plan

    • Configure I²S for the mic.
    • Allocate RX buffers.
    • Start reception.
    • Read audio frames into buffers for verification.

Device Tree + Configuration

  1. Device Tree overlay (20 min)
    • Create a board overlay to define I²S pins: SCK, LRCK (WS), SDIN (data).
    • Use the nrf52840 Reference Manual
    • Use the Sipeed I2S_Mic Datasheet
    • Reference the nRF52840 pin control
  2. Project configuration (10 min)
    • Enable I²S in prj.conf using CONFIG_I2S=y
    • Adjust stack and heap sizes to accommodate audio buffers.

Application Flow

  1. Initialization
    • Obtain the I²S device handle using Zephyr device API.
    • Configure it for mono 24-bit I²S audio with an appropriate sample rate (16 kHz or 48 kHz).
    #define I2S_RXNODE DT_NODELABEL(some_i2s_rx)

#define SAMPLE_BIT_WIDTH 24
#define SAMPLE_FREQUENCY 16000
#define CHANNEL_NUMBER 1
#define BLOCK_SIZE 256 
#define BLOCK_COUNT 2
#define TIMEOUT_MS 
   
K_MEM_SLAB_DEFINE_STATIC(mem_slab, BLOCK_SIZE, BLOCK_COUNT, 4);
  2. Start capture (10 min)
    • Trigger the I²S peripheral to start receiving audio frames.
    • Queue buffers to hold incoming audio, use the mem slab mentioned earlier
    • Read frames into memory buffers.
    • Verify sample values for activity (non-zero, fluctuating with sound).
    • Use UART or logging to monitor audio frames.
    • Ensure the mic outputs valid audio data and buffers are being correctly consumed.
    ...
i2c_configure(i2s_dev, I2S_DIR_RX, &config); 

i2s_trigger(i2s_dev, I2S_DIR_RX, I2S_TRIGGER_START); 
/* Capture Frames*/
i2s_trigger(i2s_dev, I2S_DIR_RX, I2S_TRIGGER_STOP);

Validation & Debug

  1. Sanity checks
    • Confirm pins are wired correctly (SCK, WS, SDIN).
    • Check I²S word size and alignment.
    • Ensure buffers are being filled as expected.

What have we managed to do

  • Have the MSM261S4030H0 I²S microphone wired and configured in the device tree.
  • Initialize and configure the I²S peripheral in Zephyr.
  • Capture audio frames into memory buffers.
  • Validate that the microphone is producing correct audio data.

References:

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