STM32 wake up from standby by WKUP pin rising edge

Question

I want to wake up my stm32 controller from standby mode by giving a rising edge on WKUP pin, but there is a problem. When I press the switch on a WKUP pin for more than 10 seconds, then and then only my controller should wake up not by just pressing and releasing the switch.

Answer 1

Depending on which low power mode the STM32 is in (sleep, stop, standby), there are a couple of ways I see right now to do this:

• Software: Wake the MCU up on the rising edge on your WKUP pin immediately. Then wait for 10 seconds and keep polling the state of the pin (in a busy loop) or check if a falling edge interrupt on the pin has happened (the GPIO would probably need to be reconfigured for that). Depending on the low power mode that was used, only the required peripherals would need to be reactivated during the waiting period, and if an IRQ is used for the falling edge, the core would not even need to be clocked during that time (a timestamp could be recorded after wake up). If the WKUP pin is released before 10 seconds have passed, the program could signal that by flashing an LED or a beep, and go to sleep again.

• Hardware: An external circuit could be used to wait for 10 seconds until it actually signals the MCU, which isn't bothered at all before the actual wake up event happens and no special software would be required. If accuracy isn't that important, a simple RC circuit could be used. There also exist specialized, accurate delay ICs that do exactly that (e.g. the TimerBlox series by Linear, e.g. LTC6994, you can set the delay time with a resistor).

• MCU Peripheral: Use an STM32 peripheral to implement the delay: maybe try to set up an RTC or TIMER/COUNTER interrupt after WKUP, so that an interrupt will be triggered after a certain amount of time, and go to sleep again. However, you would also need to setup an interrupt on the WKUP pin to cancel the action when the pin goes low before the waiting period ends.

Which approach would be the best certainly depends on the application requirements (accuracy, power usage, simplicity etc.). - The first one is the easiest and most straightforward IMHO, since power usage usually isn't an issue for 10 seconds of busy waiting. And accuracy also isn't that important for waking up the MCU after sleeping, right? - So the other solutions are probably overkill.