Example of computation of the energy provided to the MCU during the power-down phase
If the sum of decoupling capacitors on V
DDX
is 10 μF and V
DD
drops below 1 V while V
DDX
is still at 3.3 V, the
energy remaining in the decoupling capacitors is:
E = ½ C x V
2
= ½ x 10
-5
x 3.3
2
= 0.05 mJ
The energy remaining in the decoupling capacitors is below 1 mJ, so it is acceptable for the MCU to absorb it.
2.4
Reset and power-supply supervisor
2.4.1
Brownout reset (BOR)
The devices have a Brownout reset (BOR) circuitry. The BOR is active in all power modes except Shutdown
mode, and cannot be disabled. The BOR monitors the Backup domain supply voltage, that is V
DD
when present,
V
BAT
otherwise.
Five BOR thresholds can be selected through option bytes.
During power-on, the BOR keeps the device under reset until the supply voltage V
DD
reaches the specified V
BORx
threshold. When V
DD
drops below the selected threshold, a device reset is generated. When V
DD
is above the
V
BORx
upper limit, the device reset is released and the system can start.
For more details on the Brownout reset thresholds, refer to the electrical characteristics section in the datasheet.
Figure 6.
Brownout reset waveform
Hysteresis
Reset
V
DD
V
BOR
(rising edge)
V
BOR
(falling edge)
t
RSTTEMPO
temporization
(1)
(1) The reset temporization t
RSTTEMPO
is present only for the BOR lowest threslhold (V
BOR0
)
2.4.2
System reset
A system reset sets all registers to their reset values except the reset flags in RCC_CSR register and the
registers in the Backup domain.
A system reset is generated when one of the following events occurs (refer to reference manual for more details):
•
a low level on the NRST pin (external reset)
•
a window watchdog event (WWDG reset)
•
an independent watchdog event (IWDG reset)
•
a software reset
•
a low-power mode security reset
•
an option byte loader reset
•
a Brownout reset
These sources act on the NRST pin, that is always kept low during the delay phase. The reset service routine
vector is selected via the boot option bytes.
AN5373
Reset and power-supply supervisor
AN5373
-
Rev 1
page 12/37
