STM32MP13 VREFBUF internal peripheral

Applicable for

STM32MP13x 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: 1,65V, 1,8V, 2,048V or 2,5V. 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 STM32MP13 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 STM32MP13x lines 

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

Expand

Click on the right to expand the legend...

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

• ☐ means that the peripheral can be assigned (☑) to the given boot time context.
• ⬚ means that the peripheral can be assigned to the given boot time context, but this configuration is not supported in STM32 MPU Embedded Software distribution.
• ✓ is used for system peripherals that cannot be unchecked because they are statically 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-A7 secure (ROM code)	Cortex-A7 secure (TF-A BL2)	Cortex-A7 non-secure (U-Boot)
Analog	VREFBUF	VREFBUF			⬚	OP-TEE offers SCMI regulator service to manage VREFBUF

3.2. Runtime assignment

3.2.1. On STM32MP13x lines 

Expand

Click on the right to expand the legend...

STM32MP13 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.
• ⬚ means that the peripheral can be assigned to the given runtime context, but this configuration is not supported in STM32 MPU Embedded Software distribution.
• ✓ 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 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 STM32MP13 reference manuals.

Domain	Peripheral	Runtime allocation			Comment
		Instance	Cortex-A7 secure (OP-TEE)	Cortex-A7 non-secure (Linux)
Analog	VREFBUF	VREFBUF	☐	⬚	Assignment (single choice) OP-TEE offers SCMI regulator service to manage VREFBUF

Information

As soon as one VREFBUF client peripheral (ADC1 or ADC2) is set secure, the VREFBUF should be set secure with ETZPC configuration. In OpenSTlinux, VREFBUF is only managed by OP-TEE exported through SCMI even if all ADC are non-secured.

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
• Linux^®: regulator framework
  • drivers/regulator/stm32-vrefbuf.c (deprecated as SCMI is used in OpenSTLinux)
• 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:

• 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

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 = <2500000>;
 		regulator-max-microvolt = <2500000>;
 	};
 };

Voltage reference buffer (STM32 specific)

Board support package

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