STM32MP15 VREFBUF internal peripheral

Applicable for

STM32MP15x 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: 1,5V, 1,8V, 2,048V or 2,5V.

The VREFBUF can be used to provide an analog voltage reference for:

• ADC internal peripheral
• DAC 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 STM32MP15 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 STM32MP15x lines 

The VREFBUF is not used at boot time.

3.2. Runtime assignment

3.2.1. On STM32MP15x lines 

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STM32MP15 internal peripherals

Check boxes illustrate the possible peripheral allocations supported by STM32 MPU Embedded Software:

• ⬚ means that the peripheral can be assigned to the given runtime context, but this configuration is not supported in STM32 MPU Embedded Software distribution.
• ☐ 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 STM32 MPU Embedded Software distribution.
• ✓ 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 possiblities might be described in STM32MP15 reference manuals.

Domain	Peripheral	Runtime allocation				Comment
		Instance	Cortex-A7 secure (OP-TEE)	Cortex-A7 non-secure (Linux)	Cortex-M4 (STM32Cube)
Analog	VREFBUF	VREFBUF		☐	☐	Assignment (single choice)

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.

• Linux^®: regulator framework
• Linux driver drivers/regulator/stm32-vrefbuf.c

Information

The VREFBUF is a system resource which needs to be also controlled by the resource manager in case its consumers (e.g. ADC, DAC or an external device connected to VREF+ pin) are spread across: the Arm® Cortex®-A7 non-secure context the Arm® Cortex®-M4 context For this reason, the direct control of VREFBUF from the Arm® Cortex®-M4 is not recommended in STM32Cube by default. It's recommended to implement it in STM32Cube only if all consumers and the VDDA supply pin are controlled in the Arm® Cortex®-M4 context.

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:

• partial device trees (pin control and clock tree) generation for the OpenSTLinux software components,
• HAL initialization code generation 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
• In case the control of VREFBUF consumers are spread across the various cores, see also Resource manager for coprocessing