AN3422 datasheet(45/52 Pages) STMICROELECTRONICS

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AN3422

Firmware migration using the library

Doc ID 018976 Rev 2

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5.4.4

Alternate function mode

In STM32F1 series

The configuration to use an I/O as alternate function depends on the peripheral mode

used, for example the USART Tx pin should be configured as alternate function push-

pull while the USART Rx pin should be configured as input floating or input pull-up.

To optimize the number of peripheral I/O functions for different device packages, it is

possible by software to remap some alternate functions to other pins, for example the

USART2_RX pin can be mapped on PA3 (default remap) or PD6 (by software remap).

In STM32L1 series

Whatever the peripheral mode used, the I/O must be configured as alternate function,

then the system can use the I/O in the proper way (input or output).

The I/O pins are connected to onboard peripherals/modules through a multiplexer that

allows only one peripheral’s alternate function to be connected to an I/O pin at a time.

In this way, there can be no conflict between peripherals sharing the same I/O pin.

Each I/O pin has a multiplexer with sixteen alternate function inputs (AF0 to AF15) that

can be configured through the GPIO_PinAFConfig () function:

–

After reset all I/Os are connected to the system’s alternate function 0 (AF0)

–

The peripherals’ alternate functions are mapped by configuring AF1 to AF13

–

Cortex-M3 EVENTOUT is mapped by configuring AF15

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 peripheral I/O

functions for different device packages, for example the USART2_RX pin can be

mapped on PA3 or PD6 pin

Configuration procedure

–

Connect the pin to the desired peripherals' Alternate Function (AF) using

GPIO_PinAFConfig() function

–

Use GPIO_Init() function to configure the I/O pin:

- Configure the desired pin in alternate function mode using

GPIO_InitStructure->GPIO_Mode = GPIO_Mode_AF;

- Select the type, pull-up/pull-down and output speed via

GPIO_PuPd, GPIO_OType and GPIO_Speed members

The example below shows how to remap USART2 Tx/Rx I/Os on PD5/PD6 pins in

STM32F1 series:

/* Enable APB2 interface clock for GPIOD and AFIO (AFIO peripheral is used

to configure the I/Os software remapping) */

RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOD | RCC_APB2Periph_AFIO, ENABLE);

/* Enable USART2 I/Os software remapping [(USART2_Tx,USART2_Rx):(PD5,PD6)] */

GPIO_PinRemapConfig(GPIO_Remap_USART2, ENABLE);

/* Configure USART2_Tx as alternate function push-pull */

GPIO_InitStructure.GPIO_Pin = GPIO_Pin_5;

GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;

GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;

GPIO_Init(GPIOD, &GPIO_InitStructure);

/* Configure USART2_Rx as input floating */

GPIO_InitStructure.GPIO_Pin = GPIO_Pin_6;

GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;

GPIO_Init(GPIOD, &GPIO_InitStructure);

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