ST STM32L4+ Series Reference Manual page 337

RM0432
There is no need for the software to disable interrupts when programming the GPIOx_ODR
at bit level: it is possible to modify one or more bits in a single atomic AHB write access.
8.3.6 GPIO locking mechanism
It is possible to freeze the GPIO control registers by applying a specific write sequence to
the GPIOx_LCKR register. The frozen registers are GPIOx_MODER, GPIOx_OTYPER,
GPIOx_OSPEEDR, GPIOx_PUPDR, GPIOx_AFRL and GPIOx_AFRH.
To write the GPIOx_LCKR register, a specific write / read sequence has to be applied. When
the right LOCK sequence is applied to bit 16 in this register, the value of LCKR[15:0] is used
to lock the configuration of the I/Os (during the write sequence the LCKR[15:0] value must
be the same). When the LOCK sequence has been applied to a port bit, the value of the port
bit can no longer be modified until the next MCU reset or peripheral reset. Each
GPIOx_LCKR bit freezes the corresponding bit in the control registers (GPIOx_MODER,
GPIOx_OTYPER, GPIOx_OSPEEDR, GPIOx_PUPDR, GPIOx_AFRL and GPIOx_AFRH.
The LOCK sequence (refer to
(GPIOx_LCKR) (x = A to
GPIOx_LCKR register due to the fact that GPIOx_LCKR bit 16 has to be set at the same
time as the [15:0] bits.
For more details refer to LCKR register description in
lock register (GPIOx_LCKR) (x = A to
8.3.7 I/O alternate function input/output
Two registers are provided to select one of the alternate function inputs/outputs available for
each I/O. With these registers, the user can connect an alternate function to some other pin
as required by the application.
This means that a number of possible peripheral functions are multiplexed on each GPIO
using the GPIOx_AFRL and GPIOx_AFRH alternate function registers. The application can
thus select any one of the possible functions for each I/O. The AF selection signal being
common to the alternate function input and alternate function output, a single channel is
selected for the alternate function input/output of a given I/O.
To know which functions are multiplexed on each GPIO pin, refer to the device datasheet.
8.3.8 External interrupt/wakeup lines
All ports have external interrupt capability. To use external interrupt lines, the port can be
configured in input, output or alternate function mode (the port must not be configured in
analog mode).
Refer to
Wakeup event management.
I)) can only be performed using a word (32-bit long) access to the
Section 16: Extended interrupts and events controller (EXTI)
Section 8.4.8: GPIO port configuration lock register
I).
RM0432 Rev 6
General-purpose I/Os (GPIO)
Section 8.4.8: GPIO port configuration
and to Section 16.3.2:
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