Chapter 78: MEMS microphones¶

What: digital MEMS microphones, the chips that replaced analog electret mics in everything from phones to wearables. We focus on I²S mics (TDK InvenSense INMP441, TDK ICS-43434) and contrast with PDM mics. Plus the ASoC machine-driver pattern needed to wire one to the i.MX6ULL SAI, since this is one case where you really do write a small “machine driver” but not a chip driver. ASoC: ALSA System-on-Chip, the embedded audio layer that connects CPU audio ports, codecs, and board wiring.

Why: every smart speaker, voice-assistant, voice-controlled IoT device, dashcam, drone for FAA-broadcast, they all have one or more digital microphones. The mic outputs already-digitized PCM (or PDM). No separate ADC or codec required. The driver structure is unusual: there’s no codec chip with registers, just a simple I²S DAI. The ASoC simple-card machine driver handles this exact pattern.

Focus: A digital MEMS mic is, from Linux’s view, an I²S DAI without any control interface, clocks in, samples out. It samples audio internally, outputs PCM on SD when WS/LR-clock + BCLK are running. To the SAI driver, the mic is just an I²S slave. Wire SAI to the mic in DT via simple-audio-card, then arecord captures the audio. That is the whole audio-input pipeline.

Tooling. This chapter uses alsa-utils (arecord, aplay), i2c-tools. ALSA: Linux’s kernel and user-space audio stack.

Ubuntu-base (target): apt install alsa-utils i2c-tools

Buildroot: BR2_PACKAGE_ALSA_UTILS=y BR2_PACKAGE_I2C_TOOLS=y Buildroot: a configuration-driven build system that produces a complete root filesystem and related images.

Full per-tool reference: Userspace tooling appendix. MCU bridge: Think of the rootfs as the firmware image’s file-backed runtime environment. On an MCU you link everything into flash. On Linux, programs and config live in this mounted tree. rootfs: root filesystem, the directory tree mounted at / that contains /bin, /etc, /dev, and libraries.

78.1 Sensor comparison¶

	TDK InvenSense INMP441	TDK ICS-43434	TDK ICS-41350 (PDM)
Interface	I²S	I²S	PDM (1-bit pulse-density)
Sample rate	7.19, 52.7 kHz	7.19, 51.6 kHz	1.024, 3.072 MHz PDM (decimates to ~48 kHz)
Resolution	24-bit	24-bit	1-bit out, 16-bit after decimation
SNR	61 dBA	65 dBA	64 dBA
Sensitivity	-26 dBFS @ 94 dB SPL	-26 dBFS	-26 dBFS
Acoustic overload (AOP)	120 dB SPL	116 dB SPL	130 dB SPL
Idle current	0.5 mA	0.6 mA	0.6 mA
Stereo (L/R select)	LR pin selects L or R channel	LR pin	LR pin
Package	6-pin LGA	6-pin LGA	5-pin LGA
Volume price	$1.50–2.50	$2–3.50	$1.50–2.50

Pick guide:

INMP441: cheap, common, fine for voice. Default choice.
ICS-43434: lower noise. Better for music or low-volume signal.
PDM: more compact wiring (1-bit data). But needs the SoC’s PDM-decoder hardware. I.MX6ULL’s SAI has only I²S, no native PDM. So PDM mics are awkward on i.MX6ULL, skip.

78.2 I²S protocol primer¶

I²S is a 3-wire serial audio standard:

BCLK (bit clock): one transition per bit.
LRCLK (word/frame clock): toggles once per sample, marking left vs right channel.
SD (serial data): the audio bits.

For a 48 kHz / 24-bit stereo stream:

LRCLK = 48 kHz square wave (low = L word, high = R word).
BCLK = 48 kHz × 64 = 3.072 MHz (64 bits per stereo sample, conventional even for 24-bit data).
SD = MSB-first, 24 bits per word, padded with zeros to fill the 32-bit slot.

The “master” generates BCLK + LRCLK. The “slave” follows. For a microphone, the SoC is master, mic is slave. The mic drives SD whenever LRCLK matches its programmed channel.

   LRCLK:  ──────────┐         ┌─────────────┐
                     │  L word │             R word ...
                     └─────────┘
   BCLK:   ┐_┌─┐_┌─┐_┌─┐_┌─┐_┌─┐ ...  (free-running)

   SD:     ────── 24 bits of L sample ──────── 24 bits of R sample ────

The mic outputs its sample MSB-first during its assigned LRCLK phase. Single mic → mono. Two mics with LR-select pin strapped opposite → stereo.

78.3 How the data flows in Linux¶

   Microphone (INMP441) ──[I²S]──► SAI (CPU DAI on i.MX6ULL)
                                       ↓ DMA
                                  DDR ring buffer
                                       ↓
                                  ALSA core
                                       ↓
                              /dev/snd/pcmC0D0c  ← user-space arecord reads

There’s no I²C control, the mic has no registers. The wires alone (BCLK, LRCLK, SD, LR-select strap) determine its behavior.

What the kernel needs¶

The kernel needs an ASoC sound card consisting of:

CPU DAI: the i.MX SAI driver (mainline, no work).
Codec DAI: a stub representing the mic. The mainline driver for this is called dmic (digital mic) or, for I²S mics specifically, there’s no chip driver because the mic has no registers. The ASoC machine driver uses a fake codec.
Machine driver: wires the two DAIs together. Use simple-audio-card for this.

simple-audio-card (in sound/soc/generic/simple-card.c) is a generic ASoC machine driver. You describe the audio topology in DT. The driver builds a working sound card from the description. No coding required.

78.4 Device tree for INMP441 → SAI2¶

Template warning: This block contains placeholder values. Replace compatible strings, GPIO numbers, addresses, and paths with values from your board before using it.

&sai2 {
    pinctrl-names = "default";
    pinctrl-0 = <&pinctrl_sai2>;
    assigned-clocks = <&clks IMX6UL_CLK_SAI2_SEL>,
                      <&clks IMX6UL_CLK_SAI2>;
    assigned-clock-parents = <&clks IMX6UL_CLK_PLL4_AUDIO_DIV>;
    assigned-clock-rates = <0>, <24576000>;
    status = "okay";
};

/* The "codec" is a fake — INMP441 has no control interface */
dmic_codec: dmic-codec {
    compatible = "dmic-codec";
    #sound-dai-cells = <0>;
};

sound {
    compatible = "simple-audio-card";
    simple-audio-card,name = "imx-inmp441";
    simple-audio-card,format = "i2s";
    simple-audio-card,bitclock-master = <&cpu_dai>;
    simple-audio-card,frame-master = <&cpu_dai>;

    cpu_dai: simple-audio-card,cpu {
        sound-dai = <&sai2>;
    };

    simple-audio-card,codec {
        sound-dai = <&dmic_codec>;
    };
};

Three nodes:

&sai2: enable the i.MX SAI2 peripheral. The mainline fsl_sai.c handles it.
dmic_codec: a fake codec node. The mainline sound/soc/codecs/dmic.c driver (compatible = "dmic-codec") is a generic “digital microphone” placeholder, no registers, no control, just claims to be an ASoC codec DAI.
sound: the machine. simple-audio-card reads this and builds the sound card by wiring sai2’s DAI to dmic_codec’s DAI. bitclock-master = <&cpu_dai> tells the framework that the SoC generates BCLK.

After boot:

[root@pa-mini:~]# arecord -l
**** List of CAPTURE Hardware Devices ****
card 0: imxinmp441 [imx-inmp441], device 0: 308b000.sai-dmic-hifi dmic-hifi-0 []

The mic appears as a capture device. Now record:

[root@pa-mini:~]# arecord -D plughw:0,0 -f S32_LE -r 48000 -c 1 -d 5 test.wav
Recording WAVE 'test.wav' : Signed 32 bit Little Endian, Rate 48000 Hz, Mono

The S32_LE format is required because INMP441 outputs 24 bits in a 32-bit slot. ALSA can downconvert to 16-bit via plughw (the plug prefix). Writing directly to hw:0,0 requires S32_LE.

Listen back on the host machine: it’ll be quiet (single mic, low input) but speech should be audible.

78.5 How the SAI + DMA + ALSA pipeline works¶

Worth understanding even when you do not write the drivers yourself:

SAI is the i.MX’s I²S peripheral. The mainline fsl_sai.c driver:
- Configures BCLK and LRCLK from a parent clock (the PLL4_AUDIO_DIV in our DT, at 24.576 MHz, exactly 512 × 48 kHz, a “perfect” rate for audio).
- Programs its TX/RX FIFO settings.
- Sets up an SDMA channel to copy RX FIFO → DDR (Ch 51).

DDR: external DRAM that must be configured and trained before most software can run from it.

Implements the ASoC snd_soc_dai_ops: set_fmt (I²S vs left-justified), hw_params (sample rate, channels), trigger (start/stop).

ASoC core binds the SAI DAI (CPU side) to the dmic-codec DAI (codec side). At hw_params time, both DAIs negotiate format and sample rate.
PCM substream: ALSA creates a substream backed by a DMA-coherent ring buffer in DDR. The SAI’s SDMA channel writes into it cyclically (Ch 51.5).

MCU bridge: Think of DMA like the MCU DMA controller you used for UART or SPI, but with cache coherency, scatter-gather descriptors, and kernel ownership rules added. DMA: Direct Memory Access. Hardware moves data to or from memory without the CPU copying each byte.

User-space: opens /dev/snd/pcmC0D0c, configures format/rate via ioctl, reads samples. Reads block until enough data is available. ALSA copies from the ring buffer.

In the data path: mic → BCLK timing → SD bits → SAI’s RX FIFO → SDMA → DDR ring buffer → memcpy → user-space buffer. Zero CPU between mic and SDMA. One memcpy per read().

78.6 A “machine driver” you might write¶

simple-audio-card handles most use cases. But for unusual topologies (multiple mics with different formats. On-the-fly clock-rate changes. DAPM widgets representing mute relays), you may write a custom machine driver. The shape:

/* sound/soc/fsl/my-mic-machine.c — sketch */

static struct snd_soc_dai_link my_dai = {
    .name           = "mic-link",
    .stream_name    = "mic-capture",
    .cpus           = SND_SOC_DAILINK_REGn(cpu, "30030000.sai"),
    .codecs         = SND_SOC_DAILINK_REGn(codec, "dmic-codec"),
    .platforms      = SND_SOC_DAILINK_REGn(platform, "30030000.sai"),
    .dai_fmt        = SND_SOC_DAIFMT_I2S
                    | SND_SOC_DAIFMT_NB_NF
                    | SND_SOC_DAIFMT_CBC_CFC,
    .ops            = &my_dai_ops,
};

static struct snd_soc_card my_card = {
    .name      = "imx-inmp441-custom",
    .owner     = THIS_MODULE,
    .dai_link  = &my_dai,
    .num_links = 1,
};

static int my_probe(struct platform_device *pdev)
{
    my_card.dev = &pdev->dev;
    return devm_snd_soc_register_card(&pdev->dev, &my_card);
}

Roughly 70 lines. Same shape as Ch 53’s WM8960 machine driver, only without the codec controls, DAPM widgets, and jack-detection.

For most cases: Most projects do not need a custom machine driver. Use simple-audio-card in DT.

78.7 Stereo with two mics¶

Two INMP441s on the same bus, one strapped LR=GND (Left), one strapped LR=VDD (Right):

   ┌── SD ─────────────────► both mics share SD
   ├── BCLK ───────────────► both
   ├── LRCLK ──────────────► both
   ┌── INMP441 #1, LR → GND  (drives SD during left phase)
   └── INMP441 #2, LR → VDD  (drives SD during right phase)

Both mics monitor LRCLK. Each drives SD only during its assigned phase. The SoC sees stereo without any extra wires.

DT: change simple-audio-card,routing to declare two channels. The kernel doesn’t need to know how many mics, that’s a wiring choice.

[root@pa-mini:~]# arecord -D plughw:0,0 -f S32_LE -r 48000 -c 2 -d 5 stereo.wav

The captured file plays back in stereo on the host.

78.8 Key Notes¶

An I2S microphone supplies digital samples through a clocked audio interface. ASoC joins that interface to the CPU DAI and DMA path without inventing a register-controlled codec.
Sample width, slot width and channel selection are separate parts of the stream format. A 32-bit container does not mean the microphone has 32 meaningful audio bits.
The clock source and microphone channel straps must agree. Two physical microphones are required for genuinely different left and right signals.
Buffering protects against scheduling variation, while acoustic placement, enclosure and noise determine the useful captured signal.

78.9 Pitfalls¶

Wrong format. INMP441 outputs 24-bit MSB-first I²S. Configure for S32_LE in ALSA. The 24 audio bits sit in the high 24 bits of the 32-bit slot. The bottom 8 bits are zero. S24_LE may work depending on ASoC version.
MCLK not provided. Some I²S mics need an MCLK in addition to BCLK + LRCLK. INMP441 does not. ICS-43434 also doesn’t. But other I²S codecs do. Verify against datasheet.
Master/slave mismatch. Both SoC and mic configured as slaves → no clock generated. INMP441 is always a slave (chip can’t generate clocks).
WS polarity wrong. Some chips expect LRCLK = HIGH for left. Others use LOW. simple-audio-card,format = "i2s" defaults to “left = LOW” (LJ vs I²S subtly differ).
LR-strap floating. INMP441 reads middle state. Outputs nothing or noisy data. Strap explicitly.
Single mic, stereo requested. ALSA gives you a stereo stream with the second channel duplicated (or zero, depending on plug setup). For real stereo, wire two physical mics.
DC-offset / wind noise. MEMS mics pick up low-frequency rumble. Apply a high-pass filter in user-space (sox has a highpass 100 effect).
Self-noise floor not what’s specified. A 61 dBA SNR INMP441 in a quiet room measures ~33 dB SPL noise floor, anything quieter is masked. To capture whispers, choose ICS-43434 (4 dB lower noise).

MCU bridge: Think of SPL like the tiny early startup code that runs from internal SRAM before DDR is usable. SPL: Secondary Program Loader, a tiny first U-Boot stage that fits in OCRAM and initializes DDR.

dmic-codec compatible string. Must be exactly "dmic-codec". If the kernel was built without CONFIG_SND_SOC_DMIC=y, the placeholder driver is missing and the card fails to probe.

78.10 Going deeper¶

sound/soc/generic/simple-card.c: read the parsing logic to understand what simple-audio-card accepts.
sound/soc/codecs/dmic.c: the dmic placeholder driver (~150 lines).
sound/soc/fsl/fsl_sai.c: the i.MX SAI driver.
Documentation/devicetree/bindings/sound/simple-card.yaml: DT binding reference.
INMP441 datasheet (TDK InvenSense): timing diagram, LR-strap behavior.
ICS-43434 datasheet: similar, with the SNR improvement.
ALSA documentation at https://www.alsa-project.org/ for advanced topics (ringbuffer tuning, sample-rate conversion).

Next chapter: Chapter 79: Health sensors (MAX30100 / MAX30102). PPG-based heart-rate and SpO₂ measurement. The I²C interface with FIFO + the user-space DSP to extract HR/SpO₂.