Using The Hse Or Lse Oscillator Pins As Gpios - ST STM32L4x6 Reference Manual

RM0351
7.3.13

Using the HSE or LSE oscillator pins as GPIOs

When the HSE or LSE oscillator is switched OFF (default state after reset), the related
oscillator pins can be used as normal GPIOs.
When the HSE or LSE oscillator is switched ON (by setting the HSEON or LSEON bit in the
RCC_CSR register) the oscillator takes control of its associated pins and the GPIO
configuration of these pins has no effect.
When the oscillator is configured in a user external clock mode, only the pin is reserved for
clock input and the OSC_OUT or OSC32_OUT pin can still be used as normal GPIO.
7.3.14 Using the GPIO pins in the RTC supply domain
The PC13/PC14/PC15 GPIO functionality is lost when the core supply domain is powered
off (when the device enters Standby mode). In this case, if their GPIO configuration is not
bypassed by the RTC configuration, these pins are set in an analog input mode.
For details about I/O control by the RTC, refer to
on page 1065.
DocID024597 Rev 3
General-purpose I/Os (GPIO)
Section 34.3: RTC functional description
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