16. Voltage reference buffer (VREFBUF)

This section applies to STM32U073xx and STM32U083xx devices only.

16.1 Introduction

The devices embed a voltage reference buffer which can be used as voltage reference for the on-chip ADC, and also as voltage reference for external components through the VREF+ pin.

16.2 VREFBUF functional description

The internal voltage reference buffer supports two voltages ^(a) , which are configured with VRS bits in the VREFBUF_CSR register:

The internal voltage reference can be configured in four different modes depending on ENVR and HIZ bits configuration. These modes are provided in the table below:

Table 86. VREF buffer modes

After enabling the VREFBUF by setting ENVR bit and clearing HIZ bit in the VREFBUF_CSR register, the user must wait until VRR bit is set, meaning that the voltage reference output has reached its expected value.

a. The minimum V _DDA voltage depends on VRS setting, refer to the product datasheet.

16.3 VREFBUF registers

16.3.1 VREFBUF control and status register (VREFBUF_CSR)

Address offset: 0x00

Reset value: 0x0000 0002

Bits 31:4 Reserved, must be kept at reset value.

Bit 3 VRR : Voltage reference buffer ready

0: the voltage reference buffer output is not ready.

1: the voltage reference buffer output reached the requested level.

Bit 2 VRS : Voltage reference scale

This bit selects the value generated by the voltage reference buffer.

0: Voltage reference set to \( V_{REF\_OUT1} \) (around 2.048 V).

1: Voltage reference set to \( V_{REF\_OUT2} \) (around 2.5 V).

Bit 1 HIZ : High impedance mode

This bit controls the analog switch to connect or not the \( V_{REF+} \) pin.

0: \( V_{REF+} \) pin is internally connected to the voltage reference buffer output.

1: \( V_{REF+} \) pin is high impedance.

Refer to Table 86: VREF buffer modes for the mode descriptions depending on ENVR bit configuration.

Bit 0 ENVR : Voltage reference buffer mode enable

This bit is used to enable the voltage reference buffer mode.

0: Internal voltage reference mode disable (external voltage reference mode).

1: Internal voltage reference mode (reference buffer enable or hold mode) enable.

16.3.2 VREFBUF calibration control register (VREFBUF_CCR)

Address offset: 0x04

Reset value: 0x0000 00XX

Bits 31:6 Reserved, must be kept at reset value.

16.3.3 VREFBUF register map

The following table gives the VREFBUF register map and the reset values.

Table 87. VREFBUF register map and reset values

Refer to Section 2.2: Memory organization for the register boundary addresses.