AN3216 datasheet(14/33 Pages) STMICROELECTRONICS

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Power supplies

AN3216

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DocID17496 Rev 7

Figure 6. PVD thresholds

2.3.3

Brownout reset (BOR)

During power on, the brownout reset (BOR) keeps the device under reset until the supply

voltage reaches the specified VBOR threshold.

For devices operating from 1.65 to 3.6 V, the BOR option is not available and the power

supply is monitored by the POR/PDR. As the POR/PDR thresholds are at 1.5 V, a “grey

zone” exists between the VPOR/VPDR thresholds and the minimum product operating voltage

1.65 V.

For devices operating from 1.8 to 3.6 V, the BOR is always active at power on and its

threshold is 1.8 V.

When the system reset is released, the BOR level can be reconfigured or disabled by option

byte loading.

If the BOR level is kept at the lowest level, 1.8 V at power-on and 1.65 V at power down, the

system reset is fully managed by the BOR and the product operating voltages are within

safe ranges.

When the BOR option is disabled by option byte, the power down reset is controlled by the

PDR and a “grey zone” exists between the 1.65 V and VPDR.

VBOR is configured through device option bytes. By default, level 4 threshold is activated.

Five programmable VBOR thresholds can be selected (see product datasheets for actual

VBOR0 to VBOR4 thresholds).

When the supply voltage (VDD) drops below the selected VBOR threshold, a device reset is

generated. When the VDD is above the VBOR upper limit the device reset is released and the

system can start.

BOR can be disabled by programming the device option bytes. To disable the BOR function,

VDD must have been higher than VBOR0 to start the device option byte programming

sequence. The power-on and power-down is then monitored by the POR and PDR (see

power-on reset (POR)/power-down reset (PDR) section in the product datasheets).

The BOR threshold hysteresis is ~100 mV (between the rising and the falling edge of the

supply voltage).

VDD/VDDA

PVD output

100 mV

hysteresis

PVD threshold

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