DFSDM device tree configuration

Applicable for

STM32MP13x lines, STM32MP15x lines

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1. Article purpose

The purpose of this article is to explain how to configure the DFSDM internal peripheral when the peripheral is assigned to Linux^® OS, and in particular:

• How to configure the DFSDM peripheral to enable filters and associated channels
• How to configure the board, e.g. serial interface input/output pins

The configuration is performed using the device tree mechanism.

It is used by the DFSDM Linux driver which registers the relevant information in IIO and ALSA frameworks.

If the peripheral is assigned to another execution context, refer to How to assign an internal peripheral to an execution context article for guidelines on peripheral assignment and configuration.

2. DT bindings documentation

The DFSDM maybe used as various functions: ADC and DMIC (for audio).

Each one is represented by a separate compatible string, documented in STM32 DFSDM device tree bindings.

The external analog frontend (e.g. sigma-delta modulator) is documented in Device-Tree bindings for sigma delta modulator

3. DT configuration

This hardware description is a combination of STM32 microprocessor and board device tree files. See Device tree for more explanations on device tree file split.

The STM32CubeMX can be used to generate the board device tree. Refer to How to configure the DT using STM32CubeMX for more details.

3.1. DT configuration (STM32 level)

DFSDM nodes are declared in stm32mp151.dtsi for STM32MP15x lines , and stm32mp131.dtsi for STM32MP13x lines .

• DT root node ('dfsdm') describes the ADC hardware block parameters such as registers area, clocks.
• DT child nodes ('dfsdm0', 'dfsdm1', ...) describe each filter independently: compatible string, interrupts, DMAs.

dfsdm: dfsdm@4400d000 {
	compatible = "st,stm32mp1-dfsdm";
	...                                                                            /* common resources in 'dfsdm' root node. */
	dfsdm0: filter@0 {
		compatible = "st,stm32-dfsdm-adc";                                     /* can either be st,stm32-dfsdm-(adc or dmic) */
		...                                                                    /* private resources in 'dfsdm0' child node. */
	}
	dfsdm1: filter@1 {
		...
	}

Warning

This device tree part is related to STM32 microprocessors. It should be kept as is, without being modified by the end-user.

3.2. DT configuration (board level)

Follow the sequences described in the below chapters to configure and enable the DFSDM on your board.

3.2.1. Common resources for all DFSDM filters

Configure the 'dfsdm' DT root node:

• Enable the DT root node for the DFSDM, by setting status = "okay".
• Configure the pins in use via pinctrl, by setting pinctrl-0, pinctrl-1 and pinctrl-names.
• Configure the SPI clock output frequency, by setting spi-max-frequency (optional: only for SPI master mode).
• Configure the audio clock to be used, by setting clocks and clock-names (optional: to use more accurate clock for audio).

&dfsdm {
	pinctrl-names = "default", "sleep";
	pinctrl-0 = <&dfsdm_clkout_pins_a &dfsdm_data1_pins_a>;                        /* default pins */
	pinctrl-1 = <&dfsdm_clkout_sleep_pins_a &dfsdm_data1_sleep_pins_a>;            /* sleep pins for low-power mode */
	spi-max-frequency = <2048000>;                                                 /* desired maximum clock rate */
	clocks = <&rcc DFSDM_K>, <&rcc ADFSDM_K>;
	clock-names = "dfsdm", "audio";
	status = "okay";
 };

3.2.2. Private resources for each DFSDM filter

Configure the filter(s) DT child node(s):

• Enable the DT child node(s) for the DFSDM filter(s) in use, by setting status = "okay".
• Override the compatible string by setting "st,stm32-dfsdm-dmic" (optional: only for audio digital microphone).
• Enable the channel(s), by setting st,adc-channels = <0 1 2...>.
• Configure the channel(s) by setting st,adc-channel-names, st,adc-channel-types (e.g. SPI or manchester) and st,adc-channel-clk-src (e.g. external or internal).
• Configure the filter order, by setting st,filter-order.

3.2.3. Additional configuration for DFSDM ADC

The DFSDM ADC device has an external analog front-end, the sigma delta modulator.

Configure the external sigma delta modulator for each channel (optional, not needed for audio digital microphone):

• Add your_sd_modulator DT node in the board dts file (see the generic sd-modulator example here after).
• Add io-channels = <&your_sd_modulator> to the DFSDM filter child node in order to assign it to the filter channel(s).

3.2.4. Additional configuration for DFSDM audio

Additional child nodes must be added for audio soundcard configuration.

3.3. DT configuration examples

3.3.1. DFSDM ADC DT configuration example

The example below shows how to configure the DFSDM ADC channel 1, assigned to DFSDM filter 0:

• Add sd-modulator in the board dts file.

sd_adc1: adc-1 {
	compatible = "sd-modulator";
	#io-channel-cells = <0>;
};

• Configure and enable DFSDM, configure channel 1 to use SPI (rising edge), associate it to filter0.

&dfsdm {
	/* Also configure common resources for all DFSDM filters */
	dfsdm0: filter@0 {
		st,adc-channels = <1>;                                                 /* Assign channel 1 to this filter */
		st,adc-channel-names = "in1";                                          /* Give it a name */
		st,adc-channel-types = "SPI_R";                                        /* SPI data on rising edge */
		st,adc-channel-clk-src = "CLKOUT_F";                                   /* internal clock source used for conversion */
		io-channels = <&sd_adc1>;                                              /* phandle to the external sd-modulator */
		st,filter-order = <1>;
		status = "okay";
	};
};

3.3.2. DFSDM audio DT configuration example

In this example two microphones are multiplexed on the same DFSDM input line. One microphone is sampled on the SPI clock rising edges, and the other on the SPI clock falling edges. By convention the rising edge corresponds to the left channel and the falling edge to the right channel. The st,adc-channel-types property allows to select either the left or the right channel on the input data line.

A channel y is connected to both the input lines DATAy and DATAy+1. For each channel, an input selector allows to select either the data from DATAy or DATAy+1 input line. The st,adc-alt-channel DT property is used to configure this channel input selector. Refer to the DFSDM "Serial channel transceivers" chapter of the STM32MP15 reference manuals for more details on the channel input selection.

• st,adc-alt-channel = <0> the channel x is fed by the input line DATAx
• st,adc-alt-channel = <1> the channel y is fed by the input line DATAy+1. If y=x-1, the channel x-1 is fed by the input line DATAx

The following device tree sample is an extraction of the STM32MP15 evaluation board device tree, highlighting the channels configuration for multiplexed data on input line. In this exemple the input line DATA1 provides the left audio samples to the channel 1 and the right audio samples to the channel 0.

&dfsdm {
	/* Also configure common resources for all DFSDM filters */
	dfsdm1: filter@1 {
		st,adc-channels = <0>; 	 	 	/* Use channel 0 fed by mic U2 signal wired to input 1 */
		st,adc-alt-channel = <1>; 	 	/* Connect channel 0 to next input (input 1) */
		st,adc-channel-types = "SPI_F"; 	/* mic U2 signal available on input 1 Falling edge (Right channel)  */
	};

	dfsdm3: filter@3 {
		st,adc-channels = <1>; 	 	 	/* Use channel 1 fed by mic U4 signal wired to input 1 */
	        st,adc-alt-channel = <0>;               /* Optional as set to 0 by default */
		st,adc-channel-types = "SPI_R"; 	/* mic U4 signal available on input 1 Rising edge (Left channel)  */
	};
};

Refer to STM32MP15 evaluation board device tree for the full DFSDM audio device tree configuration example.

4. How to configure the DT using STM32CubeMX

The STM32CubeMX tool can be used to configure the STM32MPU device and get the corresponding platform configuration device tree files.
The STM32CubeMX may not support all the properties described in the above DT bindings documentation paragraph. If so, the tool inserts user sections in the generated device tree. These sections can then be edited to add some properties and they are preserved from one generation to another. Refer to STM32CubeMX user manual for further information.

5. References

Please refer to the following links for additional information:

Linux^® is a registered trademark of Linus Torvalds.

Operating system

Digital filter for sigma-delta modulator

Device tree

Analog-to-digital converter. The process of converting a sampled analog signal to a digital code that represents the amplitude of the original signal sample.

Digital microphone

Serial peripheral interface

Linux^® is a registered trademark of Linus Torvalds.

Operating system

Digital filter for sigma-delta modulator

Device tree

Analog-to-digital converter. The process of converting a sampled analog signal to a digital code that represents the amplitude of the original signal sample.

Digital microphone

Serial peripheral interface