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General-purpose and alternate-function I/Os (GPIOs and AFIOs) RM0008

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7.1.3 External interrupt/wakeup lines

All ports have external interrupt capability. To use external interrupt lines, the port must be

configured in input mode. For more information on external interrupts, refer to:

● Section 8.2: External interrupt/event controller (EXTI) on page 126 and

● Section 8.2.3: Wakeup event management on page 127.

7.1.4 Alternate functions (AF)

It is necessary to program the Port Bit Configuration Register before using a default

alternate function.

● For alternate function inputs, the port must be configured in Input mode (floating, pull-

up or pull-down) and the input pin must be driven externally.

Note: It is also possible to emulate the AFI input pin by software by programming the GPIO

controller. In this case, the port should be configured in Alternate Function Output mode.

And obviously, the corresponding port should not be driven externally as it will be driven by

the software using the GPIO controller.

● For alternate function outputs, the port must be configured in Alternate Function Output

mode (Push-Pull or Open-Drain).

● For bidirectional Alternate Functions, the port bit must be configured in Alternate

Function Output mode (Push-Pull or Open-Drain). In this case the input driver is

configured in input floating mode

If you configure a port bit as Alternate Function Output, this disconnects the output register

and connects the pin to the output signal of an on-chip peripheral.

If software configures a GPIO pin as Alternate Function Output, but peripheral is not

activated, its output is not specified.

7.1.5 Software remapping of I/O alternate functions

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

possible to remap some alternate functions to some other pins. This is achieved by

software, by programming the corresponding registers (refer to AFIO registers on page 115.

In that case, the alternate functions are no longer mapped to their original assignations.

7.1.6 GPIO locking mechanism

The locking mechanism allows the IO configuration to be frozen. When the LOCK sequence

has been applied on a port bit, it is no longer possible to modify the value of the port bit until

the next reset.

ST STM32F102 series User Manual

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