RM0365 Datasheet, PDF (219/1076 Pages) STMicroelectronics – This reference manual targets application developers

General-purpose I/Os (GPIO)
RM0365
11.3.2
11.3.3
All GPIO pins have weak internal pull-up and pull-down resistors, which can be activated or
not depending on the value in the GPIOx_PUPDR register.
I/O pin alternate function multiplexer and mapping
The device I/O pins are connected to on-board peripherals/modules through a multiplexer
that allows only one peripheral alternate function (AF) connected to an I/O pin at a time. In
this way, there can be no conflict between peripherals available on the same I/O pin.
Each I/O pin has a multiplexer with up to sixteen alternate function inputs (AF0 to AF15) that
can be configured through the GPIOx_AFRL (for pin 0 to 7) and GPIOx_AFRH (for pin 8 to
15) registers:
• After reset the multiplexer selection is alternate function 0 (AF0). The I/Os are
configured in alternate function mode through GPIOx_MODER register.
• The specific alternate function assignments for each pin are detailed in the device
datasheet.
In addition to this flexible I/O multiplexing architecture, each peripheral has alternate
functions mapped onto different I/O pins to optimize the number of peripherals available in
smaller packages.
To use an I/O in a given configuration, you have to proceed as follows:
• Debug function: after each device reset these pins are assigned as alternate function
pins immediately usable by the debugger host
• GPIO: configure the desired I/O as output, input or analog in the GPIOx_MODER
register.
• Peripheral alternate function:
– Connect the I/O to the desired AFx in one of the GPIOx_AFRL or GPIOx_AFRH
register.
– Select the type, pull-up/pull-down and output speed via the GPIOx_OTYPER,
GPIOx_PUPDR and GPIOx_OSPEEDER registers, respectively.
– Configure the desired I/O as an alternate function in the GPIOx_MODER register.
• Additional functions:
– For the configure the desired I/O in analog mode in the GPIOx_MODER register
and configure the required function in the registers.
– For the additional functions like RTC, WKUPx and oscillators, configure the
required function in the related RTC, PWR and RCC registers. These functions
have priority over the configuration in the standard GPIO registers.
Please refer to the “Alternate function mapping” table in the device datasheet for the
detailed mapping of the alternate function I/O pins.
I/O port control registers
Each of the GPIO ports has four 32-bit memory-mapped control registers (GPIOx_MODER,
GPIOx_OTYPER, GPIOx_OSPEEDR, GPIOx_PUPDR) to configure up to 16 I/Os. The
GPIOx_MODER register is used to select the I/O mode (input, output, AF, analog). The
GPIOx_OTYPER and GPIOx_OSPEEDR registers are used to select the output type (push-
pull or open-drain) and speed. The GPIOx_PUPDR register is used to select the pull-
up/pull-down whatever the I/O direction.
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