Power supplies AN3216
12/30 Doc ID 17496 Rev 5
Figure 5. 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 V
BOR
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 V
POR
/V
PDR
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 V
PDR
.
V
BOR
is configured through device option bytes. By default, level 4 threshold is activated.
Five programmable V
BOR
thresholds can be selected (see product datasheets for actual
VBOR0 to VBOR4 thresholds).
When the supply voltage (V
DD
) drops below the selected V
BOR
threshold, a device reset is
generated. When the V
DD
is above the V
BOR
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,
V
DD
must have been higher than V
BOR0
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).
V
DD
/V
DDA
PVD output
100 mV
hysteresis
PVD threshold
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