I am designing an UltraLow-Power product around the MCU STM32L412. I need to help clarifying the GPIO states during Standby. Specifically, if the Schmitt trigger is ON or OFF during Standby. This will impact my electrical design if I need to have external Pullup/Pulldowns on some pins to prevent current leakage during Standby. The Reference Manual {which Overall is fantastic} has what appears to be inconsistent info regarding this issue. I am hoping for an ST employee to clarify.
QUESTION 1
FROM: RM0394 Reference Manual STM32L41xxx/........
5.3.10 Shutdown mode
I/O states in Shutdown mode
In the Shutdown mode, are by default in floating state. If the APC bit of PWR_CR3 register
has been set, the I/Os can be configured either with a pull-up (refer to PWR_PUCRx
registers (x=A,B,C,D,E,F,G,H), or with a pull-down (refer to PWR_PDCRx registers
(x=A,B,C,D,E,F,G,H)), or can be kept in analog state if none of the PWR_PUCRx or
PWR_PDCRx register has been set. The pull-down configuration has highest priority over
pull-up configuration in case both PWR_PUCRx and PWR_PDCRx are set for the same IO.
However this configuration is lost when exiting the Shutdown mode due to the power-on
reset.
It appears the authors stated that by default pins are floating state {which to me implies Schmitt trigger is ON} then later in paragraph states that if PWR_CR3 register is not used AND if no pullup or pulldown set then pins are kept in analog state {which to me implies Schmitt trigger is OFF}
If an ST employee can please clarify if I can indeed set GPIOs in Standby with Schmitt trigger OFF as I have several pins, I prefer to have no external components and would leave floating voltage on the pins during standby.
QUESTION2: Similar question but specific for PB3
from Reference manual I understand the Debug pins in Standby will be
• PA15: JTDI in pull-up
• PA14: JTCK/SWCLK in pull-down
• PA13: JTMS/SWDAT in pull-up
• PB4: NJTRST in pull-up
• PB3: JTDO in floating state no pull-up/pull-down
For PB3 will the Schmitt trigger be ON or OFF by default in Standby? And is there any option to turn OFF if it is ON by default?
I understand if Schmitt trigger is always ON for PB3 in Standby then I will need to add an external pulldown/pullup resistor to prevent current leakage due to the floating volts affecting the trigger. I would prefer to avoid adding external components if not necessary.
Thanks,
John Russell
Design Engineer
