Battery Backup Domain - STMicroelectronics STM32F405 Reference Manual

Power controller (PWR)
5.1.2

Battery backup domain

Backup domain description
To retain the content of the RTC backup registers, backup SRAM, and supply the RTC when
V is turned off, V
DD
battery or by another source.
To allow the RTC to operate even when the main digital supply (V
pin powers the following blocks:
•
The RTC
•
The LSE oscillator
•
The backup SRAM when the low-power backup regulator is enabled
•
PC13 to PC15 I/Os, plus PI8 I/O (when available)
The switch to the V
block.
Warning:
If no external battery is used in the application, it is recommended to connect the V
V with a 100 nF external decoupling ceramic capacitor in parallel.
DD
When the backup domain is supplied by V
following functions are available:
•
PC14 and PC15 can be used as either GPIO or LSE pins
•
PC13 can be used as a GPIOas the RTC_AF1 pin (refer to
more details about this pin configuration)
Note: Due to the fact that the switch only sinks a limited amount of current (3 mA), the use of PI8
and PC13 to PC15 GPIOs in output mode is restricted: the speed has to be limited to 2 MHz
with a maximum load of 30 pF and these I/Os must not be used as a current source (e.g. to
drive an LED).
118/1731
pin can be connected to an optional standby voltage supplied by a
BAT
supply is controlled by the power-down reset embedded in the Reset
BAT
During (temporization at V
t
RSTTEMPO
is detected, the power switch between V
connected to V .
BAT
During the startup phase, if V
t (Refer to the datasheet for the value of t
RSTTEMPO
and V > V + 0.6 V, a current may be injected into V
DD BAT
through an internal diode connected between V
power switch (V ).
BAT
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
BAT
DocID018909 Rev 11
startup) or after a PDR
DD
BAT
is established in less than
DD
pin.
(analog switch connected to V
DD
RM0090
) is turned off, the V
DD
and V remains
DD
)
RSTTEMPO
BAT
and the
DD
pin cannot
BAT
BAT
), the
DD
Table 37: RTC_AF1 pin
BAT
pin to
for