STM32MP25 VREFBUF internal peripheral

Applicable for

1. Article purpose[edit source]

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[edit source]

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[edit source]

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[edit source]

3.1.1. On STM32MP2 series[edit source]

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.

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 STM32 MPU Embedded Software distribution.
⬚ 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 hardware connected in the device.

The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possibilities might be described in STM32MP25 reference manuals.

Domain	Peripheral	Boot time allocation				Comment
Domain	Peripheral	Instance	Cortex-A35 secure (ROM code)	Cortex-A35 secure (TF-A BL2)	Cortex-A35 non-secure (U-Boot)	Comment
Analog	VREFBUF	VREFBUF			☐

3.2. Runtime assignment[edit source]

3.2.1. On STM32MP25x lines [edit source]

Click on to expand or collapse the legend...

STM32MP25 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 is assigned by default to the given runtime context and that the peripheral is mandatory for the STM32 MPU Embedded Software distribution.
⬚ 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 hardware connected in the device.

The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possibilities might be described in STM32MP25 reference manuals.

Domain	Peripheral	Runtime allocation						Comment
Domain	Peripheral	Instance	Cortex-A35 secure (OP-TEE / TF-A BL31)	Cortex-A35 non-secure (Linux)	Cortex-M33 secure (TF-M)	Cortex-M33 non-secure (STM32Cube)	Cortex-M0+ (STM32Cube)	Comment
Analog	VREFBUF	VREFBUF	☑^OP-TEE	☐	☐	☐		OP-TEE offers SCMI regulator service to manage VREFBUF

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.

4. Software frameworks and drivers[edit source]

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

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

5. How to assign and configure the peripheral[edit source]

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[edit source]

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>;
 	};
 };