ST STM32F105 series User Manual

Power control (PWR) RM0008
69/1128 DocID13902 Rev 15
5.1.2 Battery backup domain
To retain the content of the Backup registers and supply the RTC function when V
DD
is
turned off, V
BAT
pin can be connected to an optional standby voltage supplied by a battery
or by another source.
The V
BAT
pin powers the RTC unit, the LSE oscillator and the PC13 to PC15 IOs, allowing
the RTC to operate even when the main digital supply (V
DD
) is turned off. The switch to the
V
BAT
supply is controlled by the Power Down Reset embedded in the Reset block.
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 may 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 V
BAT
pin cannot
support this current injection, it is strongly recommended to
connect an external low-drop diode between this power
supply and the V
BAT
pin.
If no external battery is used in the application, it is recommended to connect V
BAT
externally to V
DD
with a 100 nF external ceramic decoupling capacitor (for more details refer
to AN2586).
When the backup domain is supplied by V
DD
(analog switch connected to V
DD
), the
following functions are available:
• PC14 and PC15 can be used as either GPIO or LSE pins
• PC13 can be used as GPIO, TAMPER pin, RTC Calibration Clock, RTC Alarm or
second output (refer to Section 6: Backup registers (BKP) on page 81)
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
BAT
(analog switch connected to V
BAT
because
V
DD
is not present), the following functions are available:
• PC14 and PC15 can be used as LSE pins only
• PC13 can be used as TAMPER pin, RTC Alarm or Second output (refer to section
Section 6.4.2: RTC clock calibration register (BKP_RTCCR) on page 83).

ST STM32F105 series Specifications

General

Brand	ST
Model	STM32F105 series
Category	Microcontrollers
Language	English