Introduction

The BOD provides a wide range of voltage detection function to prevent unexpected behavior or damage caused by insufficient voltage, thus ensuring the stability of the IC.

The threshold voltage can be selected according to the actual situation to compensate for limitations of the low and non-adjustable reset voltage of the POR circuit.

The BOD supports debounce function that could filter voltage jitters. Debounce time can be adjusted from 0 to 16ms, which is 100us by default.

The fundamental principal of BOD is illustrated in the figure below:

../../rst_rtos/8_bod/figures/bod_schematic.svg

BOD schematic

Among them, VDH_IO0 is the supply voltage being detected, BOD_DET is the detection signal and BOD_RST/INTR is the debounced detection signal.

Vbod_L and Vbod_H are the low threshold voltage and high threshold voltage, respectively.

  • When VDH_IO0 drops below Vbod_L, the BOD circuit sends out BOD_DET signal to the system.

  • When VDH_IO0 rises from Vbod_L to above Vbod_H , BOD_DET signal is released.

Note

  • Both Vbod_L and Vbod_H are configurable.

  • Vbod_H needs to be set higher than Vbod_L. It is recommended to reserve about 200mV or higher hysteresis window between Vbod_H and Vbod_L.

Operating Modes

BOD supports 2 modes:

  • Interrupt mode: When the supply voltage drops below the low threshold voltage, BOD detects it and sends an interrupt notification to the CPU.

  • Reset mode: When the supply voltage drops below the low threshold voltage and subsequently rises above the high threshold voltage, the chip will reboot.

Note

  • BOD cannot work in reset mode and interrupt mode simultaneously

  • After BOD reset, BOOT REASON: BOR will be displayed in the reboot log.

BOD is enabled by default and works in reset mode. The low threshold voltage and the high threshold voltage of diffrent chips are shown in the table below:

BOD Reset Threshold Voltage

Ameba SoC

RTL8721Dx

RTL8720E

RTL8726E

RTL8713E

RTL8730E

RTL8721F

Vbod_L(V)

2.57

2.64

2.64

2.64

2.63

2.56

Vbod_H(V)

2.7

2.74

2.74

2.74

2.74

2.7

Note

  • The supported supply voltage range of RTL8721Dx is 1.72V to 3.63V. When the chip’s normal operating voltage is below 2.97V, the BOD threshold voltage needs to be adjusted, otherwise system may hang. Please contact RealTek for specific modification.

  • The BOD in RTL8721F is enabled by hardware after power on, not by software. If the boot voltage rises too slowly, it may trigger a BOD reset, and users will see BOOT REASON: BOR in the boot log.