
General-purpose input/outputs
STM32W108CB, STM32W108HB
Doc ID 16252 Rev 3
8.1.4
Reset
A full chip reset is one due to power on (low or high voltage), the NRST pin, the watchdog, or
the SYSRESETREQ bit. A full chip reset affects the GPIO configuration as follows:
●
The GPIO_PxCFGH/L configurations of all pins are configured as floating inputs.
●
The GPIO_EXTREGEN bit is set in the GPIO_DBGCFG register, which overrides the
normal configuration for PA7.
●
The GPIO_DEBUGDIS bit in the GPIO_DBGCFG register is cleared, allowing Serial
Wire/JTAG access to override the normal configuration of PC0, PC2, PC3, and PC4.
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 GPIO_BOOTMODE bit in the GPIO_DBGSTAT 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.
Table 8.
GPIO forced functions
GPIO
Override condition
Forced function
Forced signal
PA7
GPIO_EXTREGEN bit set in the
GPIO_DBGCFG register
Open-drain output
REG_EN
PC0
Debugger interface is active in JTAG mode
Input with pull up
JRST
PC2
Debugger interface is active in JTAG mode
Push-pull output
JTDO
PC3
Debugger interface is active in JTAG mode
Input with pull up
JDTI
PC4
Debugger interface is active in JTAG mode
Input with pull up
JTMS
PC4
Debugger interface is active in Serial Wire
mode
Bidirectional (push-pull
output or floating input)
controlled by debugger
interface
SWDIO