AN4661 datasheet(7/33 Pages) STMICROELECTRONICS

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AN4661

Power supplies

If no external battery is used in the application, it is recommended to connect the VBAT pin

to VDD 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 and PI8 can be used as a GPIO pin (refer to RM0385 RTC PC13 pin

configuration table and RTC PI8 pin configuration table, for more details about these

pin configurations).

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 is 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).

When the backup domain is supplied by VBAT (analog switch connected to VBAT because

VDD 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 (TAMP1).

•

PI8 can be used as tamper pin (TAMP2).

1.1.4

Voltage regulator

The voltage regulator is always enabled after reset. It works in three different modes

depending on the application modes.

•

In Run mode, the regulator supplies full power to the 1.2 V domain (core, memories

and digital peripherals).

•

In Stop mode, the regulator supplies low power to the 1.2 V domain, preserving the

contents of the registers and SRAM.

•

In Standby mode, the regulator is powered down. The contents of the registers and

SRAM are lost except for those concerned with the standby circuitry and the backup

domain.

Note:

Depending on the selected package, there are specific pins that should be connected either

to VSS or VDD to activate or deactivate the voltage regulator. Refer to the voltage regulator

section in the datasheet for more details.

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