ADC 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 analog-to-digital converter ADC internal peripheral using the device tree mechanism, relying on the bindings documentation that is the description of the required and optional device-tree properties.

The peripheral can be assigned to different contexts/software components, depending on the final product needs. Refer to How to assign an internal peripheral to a runtime context for guidelines on this configuration.

The device tree file to configure depends on the context/component to which the ADC peripheral is assigned:

• U-Boot (Boot time context): refer to U-Boot overview.
• Linux® OS (Runtime context): refer to the IIO framework.

2 DT bindings documentation

U-Boot, Linux® OS: STM32 ADC device tree bindings: Documentation/devicetree/bindings/iio/adc/st,stm32-adc.yaml

3 DT configuration

This hardware description is a combination of the STM32 microprocessor device tree files (.dtsi extension) and board device tree files (.dts extension). See the Device tree for an explanation of the device tree file split.

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

ADC DT configuration

3.1 DT configuration (STM32/SoC level)

An ADC peripheral provides one or more ADCs. Some hardware common resources are managed in the ADC peripheral, whatever the number of ADC sub-blocks. In the following chapters, the ADC peripheral is called "ADC block", while "ADC" refers to an ADC peripheral sub-block.

The ADC DT nodes are structured as follows:

• The DT root node describes the ADC block parameters such as register areas, clocks and interrupts.
• The DT child nodes ('adc1' and/or 'adc2') describe the ADCs independently.

3.1.1 STM32MP13x lines 

The ADC nodes are declared in stm32mp131.dtsi and in stm32mp133.dtsi.

adc_1: adc@address {
	compatible = "st,stm32mp13-adc-core";
	...                                      /* common resources in 'adc_1' root node. */
	adc1: adc@0 {
		compatible = "st,stm32mp13-adc";
		...                              /* private resources in 'adc1' child node. */
	};
};
adc_2: adc@address {
	compatible = "st,stm32mp13-adc-core";
	...                                      /* common resources in 'adc_2' root node. */
	adc2: adc@0 {
		compatible = "st,stm32mp13-adc";
		...                              /* private resources in 'adc2' child node. */
	};
};

Warning

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

3.1.2 STM32MP15x lines 

The ADC nodes are declared in stm32mp151.dtsi.

adc: adc@address {
	compatible = "st,stm32mp1-adc-core";
	...                                      /* common resources in 'adc' root node. */
	adc1: adc@0 {
		compatible = "st,stm32mp1-adc";
		...                              /* private resources in 'adc1' child node. */
	};
	adc2: adc@100 {
		compatible = "st,stm32mp1-adc";
		...                              /* private resources in 'adc2' child node. */
	};
};

3.2 DT configuration (board level)

To configure and enable the ADC on your board, follow the sequences described in the following chapters. The objective of this chapter is to explain how to enable and configure the ADC internal peripheral DT nodes for a board. This board DT is overloaded on top of the SoC DT. It is usually composed of several DT files: at least one "board" dts file, that includes the "SoC" dtsi file, plus one or several other dts files.

3.2.1 Common resources for all ADCs

The DT root node must be filled in:

• Enable the ADC block by setting status = "okay".
• Configure the pins in use via pinctrl, through pinctrl-0 and pinctrl-names.
• Configure the analog supply voltage regulator by setting vdda-supply = <&your_vdda_regulator>.
• Configure the analog reference voltage regulator by setting vref-supply = <&your_vref_regulator>.

Information

The ADC block can use the internal VREFBUF or any other external regulator wired to VREF+ pin.

3.2.2 Resources dedicated to an ADC

The DT child nodes ('adc1' and 'adc2') must be filled in with the ADC and channels properties.

3.2.2.1 ADC properties

• Enable ADC by setting status = "okay".
• Set ADC resolution by setting assigned-resolution-bits = <12> (optional).

3.2.2.2 Channel properties (for ecosystem release ≥ v3.1.0 )

Each channel is described by a dedicated DT sub node, filled in with properties documented in Generic IIO bindings for ADC channels.

• Define channel index 'x' by setting reg = <x>.
• Identifiy the channel by setting its name with label = "xxx" (optional).
  Reserved labels allow to identify the internal channels.
  • "vrefint", "vddcore": These internal channels are defined in the SoC DT. They can be removed in the board DT if they are not used.
  • "vbat": This internal channel can be defined in the board DT. It is not defined by default in the SoC DT to save current consumption.
• Enable differential channel pair (<vinp vinn>) by setting diff-channels = <x y> (optional).
• Set the minimum sampling time (optional):
  • by setting st,min-sample-time-ns = <10000>, for ecosystem release ≥ v4.0.0
  • by setting st,min-sample-time-nsecs = <10000>, for ecosystem release v3.1.0

3.2.2.3 Legacy channel properties (for ecosystem release ≤ v3.0.0 )

These properties are deprecated, but are still supported for backward compatibility. The properties introduced in v3.1.0 should be used instead, whenever possible.

• Enable single-ended channel(s) (<vinp...>) by setting st,adc-channels = <0 1 2...>.
• Enable differential channel(s) pairs (<vinp vinn>, ...) by setting st,adc-diff-channels = <1 0>, <2 6>, ....
• Set the minimum sampling time for each or all channels by setting st,min-sample-time-nsecs = <10000> (optional).

3.3 DT configuration examples

The example below shows how to configure ADC1:

• Input pin: use Pinctrl device tree configuration to configure PF12 as analog input.
• Analog supply: it is provided by one of the PMIC LDO regulators.
• Voltage reference: it is provided by the VREFBUF internal regulator.
• Input channel: configure ADC1_IN6 (e.g on PF12).
• Sampling time: the minimum sampling time is 10 µs.

# part of pin-controller dt node
adc1_in6_pins_a: adc1-in6 {
	pins {
		pinmux = <STM32_PINMUX('F', 12, ANALOG)>; /* configure 'PF12' as ANALOG */
	};
};

3.3.1 For ecosystem release ≥ v3.1.0

Warning

For ecosystem release v3.1.0 use "st,min-sample-time-nsecs", instead of "st,min-sample-time-ns", to configure the sampling time.

&adc {
	/* ADC block common resources */
	pinctrl-names = "default";
	pinctrl-0 = <&adc1_in6_pins_a>;              /* Use PF12 pin as ANALOG */
	vdda-supply = <&vdda>;                       /* Example to supply vdda pin by using a PMIC regulator
	vref-supply = <&vrefbuf>;                    /* Example to use VREFBUF (It needs to be enabled as well) */
	status = "okay";                             /* Enable ADC block */
	adc1: adc@0 {
	        #address-cells = <1>;
        	#size-cells = <0>;
		/* private resources for ADC1 */
		status = "okay";                     /* Enable ADC1 */
		channel@6 {
			reg = <6>;               /* ADC1 in6 channel is used */
			st,min-sample-time-ns = <10000>;  /* 10µs sampling time */
		};
	};
};

3.3.2 For ecosystem release ≤ v3.0.0

&adc {
	/* ADC common resources */
	pinctrl-names = "default";
	pinctrl-0 = <&adc1_in6_pins_a>;              /* Use PF12 pin as ANALOG */
	vdda-supply = <&vdda>;                       /* Example to supply vdda pin by using a PMIC regulator
	vref-supply = <&vrefbuf>;                    /* Example to use VREFBUF (It needs to be enabled as well) */
	status = "okay";                             /* Enable ADC block */
	adc1: adc@0 {
		/* private resources for ADC1 */
		st,adc-channels = <6>;               /* ADC1 in6 channel is used */
		st,min-sample-time-nsecs = <10000>;  /* 10µs sampling time */
		status = "okay";                     /* Enable ADC1 */
	};
};

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.
STM32CubeMX may not support all the properties described in DT binding files listed 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

For additional information, refer to the following links: