Brown-out Detector (BOD)

Introduction

Brown-out detector (BOD) is a voltage monitoring circuit whose functions include BOD Reset and BOD Interrupt. BOD Reset circuit puts the system into reset state when the supply falls below the Brown-Out Reset threshold. BOD Interrupt circuit is used to indicate the application when the supply voltage drops below the configured interrupt threshold. BOD Interrupt circuit will be prompted in two stages. When the supply voltage drops below the configured interrupt threshold, BOD_ASSERT_IRQn is triggered. When the voltage rises above the sum of the hysteresis voltage and the configured interrupt threshold, BOD_DEASSERT_IRQn is triggered.

Main Features

Programmable BOD interrupt levels
Hysteresis voltage prevents repeated interruptions
Voltage drops and rises with separate interruptions

Functional Description

Architecture

In the SoC, the core of the BOD lies in the use of two voltage comparators, as shown in the figure below, one for interrupts and one for resets. When the VBATL voltage is lower than the VREF voltage, a falling edge interrupt or reset will be generated. The BOD Reset is enabled by default. The BOD Interrupt is optional and the Interrupt threshold level is programmable, so that developers need to configure the BOD Interrupt through corresponding API integrated in the SDK. Developers can also configure the BOD through specific registers.

In sleep mode, BOD interrupt is not available, whereas BOD reset is available in any power modes. Refer to 表 30 for details.

A hysteresis voltage (V_HYST) of approximately 50 mV prevents voltage fluctuations from causing interrupts to be continuously triggered. Developers also need to configure appropriate interrupt threshold to prevent interrupts from being triggered frequently.

Note that the BOD interrupt and Reset level thresholds will not change in different PMU modes.

Programming

The BOD Reset can be enabled by configuring RF_REG_3.BOD_EN. The BOD Reset threshold (V_BODRS) is fixed, and related parameters are listed in 表 35.

The BOD Interrupt can be enabled by configuring RF_REG_3.BOD2_EN. The BOD Interrupt threshold (V_BODIT) is a variable to be configured through RF_REG_3.BOD2_LVL_CTRL_LV. The voltage range can be found in 表 36.

The BOD Interrupt is also generated when the supply voltage is below the threshold at the time that the BOD interrupt is enabled, or if the threshold is reconfigured to a higher value than the current voltage. As shown in the following figure, at T=0, V_BATL has already fallen below V_BODIT (for example, when the start-up voltage is stable and VBATL is below the interruption threshold), and then BOD_ASSERT_IRQn is generated. The BOD_ASSERT_IRQn will also be generated when the threshold is reset from V_BODIT to V_BODIT2 above V_BATL.

Electrical Specifications

表 35 BOD specifications
Parameter	Symbol	Min.	Typ.	Max.	Unit	Remarks
Brown-Out reset threshold	TH_BOD		1.60		V

表 36 BOD2 specifications
Parameter	Symbol	Min.	Typ. (BOD_LVL: Voltage Value)	Max.	Unit	Remarks
Brown-Out interrupt threshold	TH_BOD2		0x0: 1.64 0x1: 1.73 0x2: 1.83 0x3: 1.94 0x4: 2.07 0x5: 2.24 0x6: 2.44 0x7: 2.67		V