Battery Backup Domain - STMicroelectronics STM32F05 series Reference Manual

Power control (PWR)
When V
DDA
ensure this condition, also during power-up/power-down transitions, an external Shottky
diode may be used between V
6.1.2

Battery backup domain

To retain the content of the Backup registers and supply the RTC function when V
turned off, V
by another source.
The V
BAT
the RTC to operate even when the main power supply is turned off. The switch to the V
supply is controlled by the Power Down Reset embedded in the Reset block.
Warning:
If no external battery is used in the application, it is recommended to connect V
externally to V
to AN2586).
When the backup domain is supplied by V
following functions are available:
●
PC13, PC14 and PC15 can be used as GPIO pins
●
PC13, PC14 and PC15 can be configured by RTC or LSE (refer to
functional description on page
Note: Due to the fact that the switch only sinks a limited amount of current (3 mA), the use of
GPIOs PC13 to PC15 in output mode is restricted: the speed has to be limited to 2 MHz with
a maximum load of 30 pF and these IOs must not be used as a current source (e.g. to drive
an LED).
When the backup domain is supplied by V
V is not present), the following functions are available:
DD
●
PC13, PC14 and PC15 can be controlled only by RTC or LSE (refer to
RTC functional description on page
68/742
is different from V
DD
DD
pin can be connected to an optional standby voltage supplied by a battery or
BAT
pin powers the RTC unit, the LSE oscillator and the PC13 to PC15 IOs, allowing
During
t
RSTTEMPO
is detected, the power switch between V
connected to V
During the startup phase, if V
t (Refer to the datasheet for the value of t
RSTTEMPO
and V > V
DD
through an internal diode connected between V
power switch (V
If the power supply/battery connected to the V
support this current injection, it is strongly recommended to
connect an external low-drop diode between this power
supply and the V
with a 100 nF external ceramic decoupling capacitor (for more details refer
DD
Doc ID 018940 Rev 1
, it must always be higher or equal to V
and V .
DDA
(temporization at V
.
BAT
DD
+ 0.6 V, a current may be injected into V
BAT
).
BAT
pin.
BAT
(analog switch connected to V
DD
536)
(analog switch connected to V
BAT
536)
. In order to
DD
startup) or after a PDR
DD
and V
BAT DD
is established in less than
RSTTEMPO
and the
DD
pin cannot
BAT
DD
Section 24.3: RTC
RM0091
is
DD
BAT
remains
)
BAT
BAT
), the
because
BAT
Section 24.3: