VREFBUF internal peripheral





1 Article purpose[edit]

The purpose of this article is to

  • briefly introduce the VREFBUF peripheral and its main features
  • indicate the level of security supported by this hardware block
  • explain how each instance can be allocated to the three runtime contexts and linked to the corresponding software components
  • explain, when needed, how to configure the VREFBUF peripheral.

2 Peripheral overview[edit]

The VREFBUF peripheral is an internal voltage regulator.

2.1 Features[edit]

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[1]
  • DAC internal peripheral[2]
  • 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 components, introduced below, to know which features are really implemented.

2.2 Security support[edit]

The VREFBUF is a non-secure peripheral.

3 Peripheral usage and associated software[edit]

3.1 Boot time[edit]

The VREFBUF is usually not used at boot time. But it may be needed by the SSBL (see Boot chains overview), to supply the internal ADC[1] for example.

3.2 Runtime[edit]

3.2.1 Overview[edit]

The VREFBUF can be allocated to the Arm® Cortex®-A7 non-secure to be used under Linux® with regulator framework[3].

Info.png The VREFBUF is a system resource[4] which needs to be also controlled by the resource manager[4] in case its consumers (e.g. ADC[1], DAC[2] 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[5] 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.

The Peripheral assignment chapter describes which peripheral instance can be assigned to which context.

3.2.2 Software frameworks[edit]

Domain Peripheral Software frameworks Comment
Cortex-A7
secure
(OP-TEE)
Cortex-A7
non-secure
(Linux)
Cortex-M4

(STM32Cube)
Analog VREFBUF Linux regulator framework

3.2.3 Peripheral configuration[edit]

The configuration is applied by the firmware running in the context to which the peripheral is assigned. The configuration by itself can be performed via the STM32CubeMX tool for all internal peripherals. It can then be manually completed (especially for external peripherals) according to the information given in the corresponding software framework article.

3.2.4 Peripheral assignment[edit]

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.
  • is used for system peripherals that cannot be unchecked because they are statically connected in the device.

Refer to How to assign an internal peripheral to a runtime 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 References[edit]

voltage reference buffer (STM32 specific)

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-to-analog converter

Second Stage Boot Loader

Open Portable Trusted Execution Environment

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