How to drive an output GPIO as HIGH even during sleep / suspend ?

[Dinesh K]

Hi ST Community,

We are using STM32MP157C-DK2 board and OpenSTlinux Firmware (v3.1-Openstlinux-5-4-dunfell-mp1-20-11-12) for our development. We have a requirement to drive the Blue LED (LD8) always ON even during sleep / suspend. Currently the GPIO PD11 connected to the LED becomes LOW when we issue "systemctl suspend" command in Linux.

Below is the DTS snippet:

     led {

        compatible = "gpio-leds";

        led-blue {

            label = "heartbeat";

            gpios = <&gpiod 11 GPIO_ACTIVE_HIGH>;

            linux,default-trigger = "heartbeat";

            default-state = "off";

            retain-state-suspended;

        };

    };

[Uwe Bonnes]

Use an external transistor to drive the LED. Static switch on the transistor by a pullup. To switch off, use the GPIO to short the Base/Gate to ground with GPIO as open Drain, switched low.

[Tesla DeLorean]

Is it actually low or just floating. You could reverse the sense of the diode and connect to VCC rather than GND, and then it would illuminate in the LOW case.

​

People tend not to have LEDs on or pull ups as these leech current, which you're typically looking to avoid.​

[PatrickF]

Hi,

Above answers from @Uwe Bonnes​ and @Community member​ are valid, an external pull-up resistor and a transistor are usually required if you want to have a LED forced ON by default. Note that LED will also be ON during platform reset/startup (otherwise, might need a much more complex circuitry which sound overkilling for just a LED).

I confirm that on most STM32 (including the STM32MP1 series), whenever the VDDCORE is shutdown, all the GPIOs are forced in High-Z (except obviously those which do system control and those related to backup domain such are TAMP, RTC, etc... when enabled).

To avoid a transistor, it might be possible to add a second drive pin for the LED (up to few mA like other GPIOs) by ANDing PWR_ON pin and the heartbeat GPIO using diode(s). PWR_ON signal is driven LOW in system Standby mode, even if VDDCORE is shutdown.

Need to have the LED active-low, i.e. connected to VDD.

This is just an idea, to be checked for potential side effects.

Regards,

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