
Power control (PWR)
RM0453
222/1454
RM0453 Rev 2
Warning:
During
t
RSTTEMPO
(temporization at V
DD
startup) or after a PDR
is detected, the power switch between V
BAT
and V
DD
remains
connected to V
BAT
.
During the startup phase, if V
DD
is established in less than
t
RSTTEMPO
(refer to the datasheet for the value of t
RSTTEMPO
)
and V
DD
> V
BAT
+ 0.6 V, a current can be injected into V
BAT
through an internal diode connected between V
DD
and the
power switch (V
BAT
).
If the power supply/battery connected to the VBAT pin cannot
support this current injection, it is strongly recommended to
connect an external low-drop diode between this power
supply and the VBAT pin.
If no external battery is used in the application, it is recommended to connect VBAT
externally to V
DD
with a 100 nF external ceramic decoupling capacitor.
When the Backup domain is supplied by V
DD
(analog switch connected to V
DD
), the
following pins are available:
•
PC13, PC14 and PC15, that can be used as GPIO pins
•
PC13, PC14 and PC15, that can be configured by RTC, TAMP or LSE (refer to
Section 32: Real-time clock (RTC)
and
Section 33: Tamper and backup registers
)
•
PA0/TAMP_IN2 and PB3/TAMP_IN3 when they are configured by the TAMP as tamper
pins
Note:
Due to the fact that the analog switch can transfer only a limited amount of current (3 mA),
the use of GPIO PC13 to PC15 in output mode is restricted: the speed must be limited to
2 MHz with a maximum load of 30 pF and these I/Os must not be used as current source
(e.g. to drive a LED).
When the Backup domain is supplied by V
BAT
(analog switch connected to V
BAT
because
V
DD
is not present), the following functions are available:
•
PC13, PC14 and PC15 can be controlled only by RTC, TAMP or LSE (refer to
Section 32: Real-time clock (RTC)
and
Section 33: Tamper and backup registers
)
•
PA0/TAMP_IN2 and PB3/TAMP_IN3, when they are configured by the TAMP as
tamper pins
Backup domain access
After a system reset, the Backup domain (RTC and TAMP backup registers) is protected
against possible unwanted write accesses. The DBP bit must be set in the
to enable access to the Backup domain
VBAT battery charging
When V
DD
is present, It is possible to charge the external battery on VBAT through an
internal resistance.
The VBAT charging is done either through a 5 k
Ω
resistor or through a 1.5 k
Ω
resistor,
depending on the VBRS bit value in the
PWR control register 4 (PWR_CR4)
.