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pull or open-drain). At this time, the input driver is configured to input floating mode.
In the output alternate function mode, the pin is disconnected from the output data register and connected with the
output signal of the on-chip peripheral. If the software configures a GPIO pin as output alternate function, but the
peripheral is not activated, then its output will be uncertain.
Clock output MCO
The microcontroller allows the clock signal to be output to the external MCO pin (PA8). PA8 must be configured to
push-pull alternate output function mode. The following four clock signals can be selected as MCO clocks, and the
selection of the clock is controlled by RCC_CFG.MCO[2:0] bits:
SYSCLK divided clock
HSI
HSE
PLL divided clock
Software remapping I/O alternate function
In order to expand the flexibility of alternate peripheral functions under different device packages, some peripheral
alternate functions can be remapped to other pins. You can configure the corresponding register (AFIO_RMP_CFG,
AFIO_RMP_CFGx) by software. At this time, the alternate function is disconnected from its original pin and
remapped to the new pin.
Functional remapping of backup domain pins PC13~PC15
7.2.5.3.1
Functional description of backup power domain pins PC13~PC15
PC13~PC15 pins are located in the backup domain and can be used in GPIO mode or alternate function mode:
When the backup domain is powered by VDD (internal analog switch connected to VDD), the following
functions are available:
PC14 and PC15 can be used for GPIO or LSE pins.
PC13 can be used as a general-purpose I/O port, TAMPER pin, RTC calibration clock, RTC alarm clock or
second output (see backup register 5.4 chapter)
Note: Because the analog switch can only pass low current (3mA), the functions of I/O ports of PC13, PC14, PC15
in the output mode are limited: the speed must be limited below 2MHz, the maximum load is 30pF, and these I/O
ports must not be used as current sources (such as driving LEDs).
When the backup domain is powered by VBAT (the analog switch is connected to VBAT after VDD is off), the
following functions can be used:
PC14 and PC15 can only be used for LSE pins.
PC13 can be used as a TAMPER pin, RTC alarm clock or second output (see 14.2.8), and output RTC
calibration clock, RTC alarm clock pulse or second pulse on PC13 pin (when this pin is not used for
intrusion detection). For the convenience of measurement, the RTC clock can be divided by 64 and output
to the intrusion detection pin TAMPER. Turn on this function by setting the RTC_CTRL.COEN bit.