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General-purpose I/Os (GPIO) RM0367

240/1043 RM0367 Rev 7

When the pin is configured as output, the value written to the output data register

(GPIOx_ODR) is output on the I/O pin. It is possible to use the output driver in push-pull

mode or open-drain mode (only the low level is driven, high level is HI-Z).

The input data register (GPIOx_IDR) captures the data present on the I/O pin at every AHB

clock cycle.

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.

9.3.2 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, the user has 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

• Peripheral alternate function:

– Connect the I/O to the desired AFx in one of the GPIOx_AFRL or GPIOx_AFRH

– 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 ADC, DAC and COMP, configure the desired I/O in analog mode in the

GPIOx_MODER register and configure the required function in the ADC, DAC and

COMP 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.

Refer to the “Alternate function mapping” table in the device datasheet for the detailed

mapping of the alternate function I/O pins.

ST STM32L0x3 User Manual

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