The STM32MP15 provides three Inter-IC sound Interfaces (I2S).

On the ConnectCore MP15 Development Kit, the I2S2 interface is connected to a Maxim 98089 low-power stereo codec with the following features:

  • Analog inputs: line-in A, line-in B, microphone:

    • Two line-in audio inputs through the LINE_IN_A_ and LINE_IN_B_ lines of the AUDIO and SPK connectors.

    • Microphone audio input through the MIC_ lines of the AUDIO connector.

  • Analog outputs: line-out, headphone, speakers:

    • Line-out audio output through the LINE_OUT_ lines of the AUDIO connector.

    • Headphone audio output through the on-board jack connector.

    • Speaker audio output through the SPKL_ and SPKR_ lines of the SPK connector.

  • Digital input/out multi-format audio interface.

  • Digital processing, filters, volume control, amplifiers.

The codec is a slave chip that the CPU controls via the I2S2 port of the ConnectCore MP15 system-on-chip. The CPU drives audio data using the inter-IC sound bus standard (I2S).

All audio output comes out through this codec and can be reproduced using a pair of speakers or headphones.

Kernel configuration

You can manage the audio driver support through the following kernel configuration options:

  • SoC Audio for STM32 I2S interface (SPI/I2S block) support (CONFIG_SND_SOC_STM32_I2S)

  • SoC Audio support for STM32 boards with max98088/max98089 (CONFIG_SND_SOC_STM32_MAX98088)

These options are enabled as built-in on the default ConnectCore MP15 kernel configuration file.

Kernel driver

The drivers for the audio interface and MAX98089 codec are located at:

File Description

sound/soc/stm/stm32_i2s.c

I2S driver

sound/soc/generic/simple-card.c

ASoC simple sound card driver

sound/soc/codecs/max98088.c

MAX98088/9 codec driver

Device tree bindings and customization

The I2S interface is documented at Documentation/devicetree/bindings/sound/st,stm32-i2s.yaml.

The device tree must contain entries for:

  • The I2S interface

  • The interface between the SAI and the audio codec

  • The audio codec

Example: I2S2 on ConnectCore MP15 Development Kit

Definition of the I2S

STM32MP15 device tree
[...]

i2s2: audio-controller@4000b000 {
	compatible = "st,stm32h7-i2s";
	#sound-dai-cells = <0>;
	reg = <0x4000b000 0x400>;
	interrupts = <GIC_SPI 36 IRQ_TYPE_LEVEL_HIGH>;
	dmas = <&dmamux1 39 0x400 0x01>,
	       <&dmamux1 40 0x400 0x01>;
	dma-names = "rx", "tx";
	status = "disabled";
};

[...]
ConnectCore MP15 Development Kit device tree
&i2s2 {
        #clock-cells = <0>;
        #sound-dai-cells = <0>;
        clocks = <&rcc SPI2>, <&rcc SPI2_K>, <&rcc PLL3_Q>, <&rcc PLL3_R>;
        clock-names = "pclk", "i2sclk", "x8k", "x11k";
        pinctrl-names = "default", "sleep";
        pinctrl-0 = <&ccmp15_i2s2_pins_a>;
        pinctrl-1 = <&ccmp15_i2s2_sleep_pins_a>;
        digi,i2s-master;
        status = "okay";
};

[...]
&pinctrl {
	[...]

	ccmp15_i2s2_pins_a: i2s2-0 {
		pins {
		pinmux = <STM32_PINMUX('I', 3, AF5)>, /* I2S2_SDO */
				 <STM32_PINMUX('I', 2, AF5)>, /* I2S2_SDI */
				 <STM32_PINMUX('I', 0, AF5)>, /* I2S2_WS */
				 <STM32_PINMUX('C', 6, AF5)>, /* I2S2_MCK */
				 <STM32_PINMUX('I', 1, AF5)>; /* I2S2_CK */
			slew-rate = <1>;
			drive-push-pull;
			bias-disable;
		};
	};

	ccmp15_i2s2_sleep_pins_a: i2s2-sleep-0 {
		pins {
			pinmux = <STM32_PINMUX('I', 3, ANALOG)>, /* I2S2_SDO */
				 <STM32_PINMUX('I', 2, ANALOG)>, /* I2S2_SDI */
				 <STM32_PINMUX('I', 0, ANALOG)>, /* I2S2_WS */
				 <STM32_PINMUX('C', 6, ANALOG)>, /* I2S2_MCK */
				 <STM32_PINMUX('I', 1, ANALOG)>; /* I2S2_CK */
		};
        };
};

Interface between I2S and audio codec

ConnectCore MP15 Development Kit device tree
[...]

sound_max98089: sound-max98089 {
	compatible = "simple-audio-card";
	simple-audio-card,name = "stm32max98088";
	simple-audio-card,format = "i2s";
        simple-audio-card,bitclock-master = <&cpu_dai>;
	simple-audio-card,frame-master = <&cpu_dai>;
	routing =
		"Headphone Jack", "HPL",
		"Headphone Jack", "HPR",
		"Ext Spk", "SPKL",
		"Ext Spk", "SPKR",
		"LineOut", "RECL",
		"LineOut", "RECR",
		"Mic1", "MIC1",
		"Mic2", "MIC2",
		"LineInA", "INA1",
		"LineInA", "INA2",
		"LineInB", "INB1",
		"LineInB", "INB2";

	cpu_dai: simple-audio-card,cpu {
		sound-dai = <&{i2s_port}>;
		system-clock-direction-out;
		clocks = <&{clocks_spi_k}>;
	};

	codec_dai: simple-audio-card,codec {
		sound-dai = <&max98089>;
	};
};

[...]

Audio codec (i2c2 slave)

ConnectCore MP15 Development Kit device tree
&i2c2 {
	[...]

	max98089: codec@10 {
                compatible = "maxim,max98089";
                reg = <0x10>;
                clocks = <&rcc SPI2_K>;
                clock-names = "mclk";
                #sound-dai-cells = <0>;
                #clock-cells = <0>;
                status = "disabled";
        };

	[...]
};

Using the audio interface

Audio controls

The audio codec is controlled from Linux via the ALSA framework. Digi Embedded Yocto provides an initial configuration that enables:

  • Line in A

  • Line in B

  • Microphone

  • Headphones

  • Speakers

  • Line out

You can use the command line application alsamixer to manage these interfaces and their associated controls (volumes, gains, filters).

Audio playback

Since all output routes are enabled by default, any sound will play on headphones, speakers, and line-out.

To play a wav file:

# aplay sound.wav

To play an mp3 file:

# mpg123 sound.mp3

Audio recording

Since microphone, line-in A, and line-in B input routes are enabled by default, any recorded sound will mix the audio coming from these inputs.

To record a stereo WAV audio file with 10 seconds duration from line-in:

# arecord -f cd sound.wav --duration 10

To record a mono WAV audio file from the microphone:

# arecord --format=S16_LE --channels=1 --rate=44100 micro.wav --duration=10