STM32W108HB datasheet(91/278 Pages) STMICROELECTRONICS

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STM32W108HB STM32W108CC STM32W108CB STM32W108CZ

General-purpose input/output

Doc ID 16252 Rev 14

91/278

8.1.5

nBOOTMODE

nBOOTMODE is a special alternate function of PA5 that is active only during a pin reset

(NRST) or a power-on-reset of the always-powered domain (POR_HV). If nBOOTMODE is

asserted (pulled or driven low) when coming out of reset, the processor starts executing an

embedded serial boot loader instead of its normal program.

While in reset and during the subsequent power-on-reset startup delay (512 high-frequency

RC oscillator periods), PA5 is automatically configured as an input with a pull-up resistor. At

the end of this time, the STM32W108 samples nBOOTMODE: a high level selects normal

startup, and a low level selects the boot loader. After nBOOTMODE has been sampled, PA5

is configured as a floating input. The BOOTMODE bit in the GPIO_DBGSR register

captures the state of nBOOTMODE so that software may act on this signal if required.

Note:

To avoid inadvertently asserting nBOOTMODE, PA5's capacitive load should not exceed

252 pF.

8.1.6

GPIO modes

Analog mode

Analog mode enables analog functions, and disconnects a pin from the digital input and

output logic. Only the following GPIO pins have analog functions:

●

PA4, PA5, PB5, PB6, PB7, and PC1 can be analog inputs to the ADC.

●

PB0 can be an external analog voltage reference input to the ADC, or it can output the

internal analog voltage reference from the ADC.

●

PC6 and PC7 can connect to an optional 32.768 kHz crystal.

Note:

When an external timing source is required, a 32.768 kHz crystal is commonly connected to

PC6 and PC7. Alternatively, when PC7 is configured as a digital input, PC7 can accept a

digital external clock input.

When configured in analog mode:

●

The output drivers are disabled.

●

The internal pull-up and pull-down resistors are disabled.

●

The Schmitt trigger input is connected to a high logic level.

●

Reading GPIOx_IDR returns a constant 1.

Input mode

Input mode is used both for general purpose input and for on-chip peripheral inputs. Input

floating mode disables the internal pull-up and pull-down resistors, leaving the pin in a high-

impedance state. Input pull-up or pull-down mode enables either an internal pull-up or pull-

down resistor based on the GPIOx_ODR register. Setting a bit to 0 in GPIOx_ODR enables

the pull-down and setting a bit to 1 enables the pull up.

When configured in input mode:

●

The output drivers are disabled.

●

An internal pull-up or pull-down resistor may be activated depending on GPIOx_CRH/L

and GPIOx_ODR.

●

The Schmitt trigger input is connected to the pin.

●

Reading GPIOx_IDR returns the input at the pin.

●

The input is also available to on-chip peripherals.

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