STM32MP2 VREFBUF internal peripheral

Applicable for

STM32MP23x lines  STM32MP25x lines

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1. Article purpose

The purpose of this article is to:

• briefly introduce the VREFBUF peripheral and its main features,
• indicate the peripheral instances assignment at boot time and their assignment at runtime (including whether instances can be allocated to secure contexts),
• list the software frameworks and drivers managing the peripheral,
• explain how to configure the peripheral.

2. Peripheral overview

The VREFBUF peripheral is an internal voltage regulator.

The VREFBUF is supplied via the VDDA pin. When enabled, it can provide a reference voltage in the range of: 1,21V or 1,5V. The VREFBUF can be used to provide an analog voltage reference for:

• ADC internal peripheral
• External components through the dedicated VREF+ pin.

The VREFBUF can be left unused. In this case, an external voltage regulator can provide reference voltage to VREF+ pin.

Refer to the STM32 MPU reference manuals for the complete list of features, and to the software frameworks and drivers, introduced below, to see which features are implemented.

3. Peripheral usage

This chapter is applicable in the scope of the OpenSTLinux BSP running on the Arm^® Cortex^®-A processor(s), and the STM32CubeMPU Package running on the Arm^® Cortex^®-M processor.

3.1. Boot time assignment

3.1.1. On STM32MP2 series

Click on to expand or collapse the legend...

• ☐ means that the peripheral can be assigned to the given boot time context.
• ☑ means that the peripheral is assigned by default to the given boot time context and that the peripheral is mandatory for the Yocto-based OpenSTLinux Embedded Software.
• ⬚ means that the peripheral can be assigned to the given boot time context, but this configuration is not supported in Yocto-based OpenSTLinux Embedded Software.
• ✓ is used for system peripherals that cannot be unchecked because they are hardware connected in the device.

The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possibilities might be described in STM32 MPU reference manuals.

Domain	Peripheral	Boot time allocation				Comment
		Instance	Cortex-A35 secure (ROM code)	Cortex-A35 secure (TF-A BL2)	Cortex-A35 nonsecure (U-Boot)
Analog	VREFBUF	VREFBUF			⬚	OP-TEE offers SCMI regulator service to manage VREFBUF

3.2. Runtime assignment

Information

As soon as VREFBUF client peripherals (ADC1 or ADC2 or ADC3) are assigned to different execution contexts, the VREFBUF should be assigned to main processor secure context with RIFSC. In Cortex-A35 main processor mode, OpenSTlinux offers only VREFBUF support by OP-TEE secure OS through SCMI even if all ADC are assigned to a unique execution context.

3.2.1. On STM32MP21x lines 

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STM32MP21 internal peripherals

Check boxes illustrate the possible peripheral allocations supported by Yocto-based OpenSTLinux Embedded Software:

• ☐ means that the peripheral can be assigned to the given runtime context.
• ☑ means that the peripheral is assigned by default to the given runtime context and that the peripheral is mandatory for the Yocto-based OpenSTLinux Embedded Software.
• ⬚ means that the peripheral can be assigned to the given runtime context, but this configuration is not supported in Yocto-based OpenSTLinux Embedded Software.
• ✓ is used for system peripherals that cannot be unchecked because they are hardware connected in the device.

Refer to How to assign an internal peripheral to an execution context for more information on how to assign peripherals manually or via STM32CubeMX.
The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possibilities might be described in STM32MP21 reference manuals.

Domain	Peripheral	Runtime allocation					Comment
		Instance	Cortex-A35 secure (OP-TEE / TF-A BL31)	Cortex-A35 nonsecure (Linux)	Cortex-M33 secure (TF-M)	Cortex-M33 nonsecure (STM32Cube)
Power & Thermal	VREFBUF	VREFBUF	☑OP-TEE	⬚	⬚	⬚	OP-TEE offers SCMI regulator service to manage VREFBUF

3.2.2. On STM32MP23x lines 

Click on to expand or collapse the legend...

STM32MP23 internal peripherals

Check boxes illustrate the possible peripheral allocations supported by Yocto-based OpenSTLinux Embedded Software:

• ☐ means that the peripheral can be assigned to the given runtime context.
• ☑ means that the peripheral is assigned by default to the given runtime context and that the peripheral is mandatory for the Yocto-based OpenSTLinux Embedded Software.
• ⬚ means that the peripheral can be assigned to the given runtime context, but this configuration is not supported in Yocto-based OpenSTLinux Embedded Software.
• ✓ is used for system peripherals that cannot be unchecked because they are hardware connected in the device.

Refer to How to assign an internal peripheral to an execution context for more information on how to assign peripherals manually or via STM32CubeMX.
The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possibilities might be described in STM32MP23 reference manuals.

Domain	Peripheral	Runtime allocation					Comment
		Instance	Cortex-A35 secure (OP-TEE / TF-A BL31)	Cortex-A35 nonsecure (Linux)	Cortex-M33 secure (TF-M)	Cortex-M33 nonsecure (STM32Cube)
Power & Thermal	VREFBUF	VREFBUF	☑OP-TEE	⬚	⬚	⬚	OP-TEE offers SCMI regulator service to manage VREFBUF

3.2.3. On STM32MP25x lines 

Click on to expand or collapse the legend...

STM32MP25 internal peripherals

Check boxes illustrate the possible peripheral allocations supported by Yocto-based OpenSTLinux Embedded Software:

• ☐ means that the peripheral can be assigned to the given runtime context.
• ☑ means that the peripheral is assigned by default to the given runtime context and that the peripheral is mandatory for the Yocto-based OpenSTLinux Embedded Software.
• ⬚ means that the peripheral can be assigned to the given runtime context, but this configuration is not supported in Yocto-based OpenSTLinux Embedded Software.
• ✓ is used for system peripherals that cannot be unchecked because they are hardware connected in the device.

Refer to How to assign an internal peripheral to an execution context for more information on how to assign peripherals manually or via STM32CubeMX.
The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possibilities might be described in STM32MP25 reference manuals.

Domain	Peripheral	Runtime allocation						Comment
		Instance	Cortex-A35 secure (OP-TEE / TF-A BL31)	Cortex-A35 nonsecure (Linux)	Cortex-M33 secure (TF-M)	Cortex-M33 nonsecure (STM32Cube)	Cortex-M0+ (STM32Cube)
Power & Thermal	VREFBUF	VREFBUF	☑OP-TEE	⬚	⬚	⬚		OP-TEE offers SCMI regulator service to manage VREFBUF

4. Software frameworks and drivers

Below are listed the software frameworks and drivers managing the VREFBUF peripheral for the embedded software components listed in the above tables.

• SCMI
• OP-TEE driver: core/drivers/regulator/stm32_vrefbuf.c

5. How to assign and configure the peripheral

The peripheral assignment can be done via the STM32CubeMX graphical tool (and manually completed if needed).
This tool also helps to configure the peripheral by generating:

• partial device trees (pin control and clock tree) for the OpenSTLinux software components,
• HAL initialization code for the STM32CubeMPU Package.

The configuration is applied by the firmware running in the context in which the peripheral is assigned.

• For the Linux kernel configuration, please refer to device internal regulator. An example can be found also in ADC DT configuration examples

5.1. DT configuration example

In OP-TEE device tree:

 &etzpc {
 	st,decprot = <
 		DECPROT(STM32MP1_ETZPC_VREFBUF_ID, DECPROT_S_RW, DECPROT_UNLOCK)
 	>;
 };
 &scmi_regu {
 	scmi_vrefbuf: voltd-vrefbuf {
 		reg = <VOLTD_SCMI_VREFBUF>;
 		voltd-supply = <&vrefbuf>;
 	};
 };
 &vrefbuf {
 	vdda-supply = <&vdd>;
 	status = "okay";
 };

In Linux device tree:

 &adc_1 {
 ...
 	vref-supply = <&scmi_vrefbuf>;
 	status = "okay";
 };

 &scmi_regu {
 	scmi_vrefbuf: voltd-vrefbuf {
 		reg = <VOLTD_SCMI_VREFBUF>;
 		regulator-name = "vrefbuf";
 		regulator-min-microvolt = <1500000>;
 		regulator-max-microvolt = <1500000>;
 	};
 };

Voltage reference buffer (STM32 specific)

Board support package

Arm^® is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere.

Cortex^®

Read only memory

Trusted firmware for Arm^® Cortex^®-A

Boot loader stage 2

Das U-Boot -- the universal boot loader (see U-Boot)

Open portable trusted execution environment

System control and management interface

Operating system

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.

Linux^® is a registered trademark of Linus Torvalds.

Hardware abstraction layer

Device tree

Extended trustzone protection controller

Voltage reference buffer (STM32 specific)

Board support package

Arm^® is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere.

Cortex^®

Read only memory

Trusted firmware for Arm^® Cortex^®-A

Boot loader stage 2

Das U-Boot -- the universal boot loader (see U-Boot)

Open portable trusted execution environment

System control and management interface

Operating system

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.

Linux^® is a registered trademark of Linus Torvalds.

Hardware abstraction layer

Device tree

Extended trustzone protection controller