General-purpose I/Os (GPIO) RM0390
176/1328 RM0390 Rev 4
7 General-purpose I/Os (GPIO)
7.1 GPIO introduction
Each general-purpose I/O port has four 32-bit configuration registers (GPIOx_MODER,
GPIOx_OTYPER, GPIOx_OSPEEDR and GPIOx_PUPDR), two 32-bit data registers
(GPIOx_IDR and GPIOx_ODR), a 32-bit set/reset register (GPIOx_BSRR), a 32-bit locking
register (GPIOx_LCKR) and two 32-bit alternate function selection register (GPIOx_AFRH
and GPIOx_AFRL).
7.2 GPIO main features
• Up to 16 I/Os under control
• Output states: push-pull or open drain + pull-up/down
• Output data from output data register (GPIOx_ODR) or peripheral (alternate function
output)
• Speed selection for each I/O
• Input states: floating, pull-up/down, analog
• Input data to input data register (GPIOx_IDR) or peripheral (alternate function input)
• Bit set and reset register (GPIOx_BSRR) for bitwise write access to GPIOx_ODR
• Locking mechanism (GPIOx_LCKR) provided to freeze the I/O configuration
• Analog function
• Alternate function input/output selection registers (at most 16 AFs per I/O)
• Fast toggle capable of changing every two clock cycles
• Highly flexible pin multiplexing allows the use of I/O pins as GPIOs or as one of several
peripheral functions
7.3 GPIO functional description
Subject to the specific hardware characteristics of each I/O port listed in the datasheet, each
port bit of the general-purpose I/O (GPIO) ports can be individually configured by software in
several modes:
• Input floating
• Input pull-up
• Input-pull-down
• Analog
• Output open-drain with pull-up or pull-down capability
• Output push-pull with pull-up or pull-down capability
• Alternate function push-pull with pull-up or pull-down capability
• Alternate function open-drain with pull-up or pull-down capability
Each I/O port bit is freely programmable, however the I/O port registers have to be
accessed as 32-bit words, half-words or bytes. The purpose of the GPIOx_BSRR register is
to allow atomic read/modify accesses to any of the GPIO registers. In this way, there is no
risk of an IRQ occurring between the read and the modify access.
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