RM0090: Reference manual - STMicroelectronics

RM0090 Power control (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
VDD is turned off, VBAT pin can be connected to an optional standby voltage supplied by a
battery or by another source.
To allow the RTC to operate even when the main digital supply (VDD ) is turned off, the VBAT 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 VBAT 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 the V BAT pin to
V DD with a 100 nF external decoupling ceramic capacitor in parallel.
When the backup domain is supplied by VDD (analog switch connected to VDD), the
following functions are available:
● PC14 and PC15 can be used as either GPIO or LSE pins
● PC13 can be used as a GPIO or as the RTC_AF1 pin (refer to Table 29: RTC_AF1 pin
for 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
GPIOs PI8 and PC13 to PC15 are restricted: only one I/O at a time can be used as an
output, 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).
Doc ID 018909 Rev 3 92/1416