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A Low Power Startup/Shutdown Button for RPi 4B

The "one-button-startup-shutdown" (OBSS) feature for Raspberry Pi has been around for a while, and works on almost all RPi hardware versions. This nifty innovation is the result of Matthijs Kooijman's development of the dtoverlay for gpio-shutdown. His blog post: "Raspberry pi powerdown and powerup button" discusses his motivation, and remains active as of this writing. I do not comprehend the code implementing the gpio-shutdown overlay, but as a user, was impressed by the simplicity of this feature. I implemented it immediately on my Raspberry Pi 4B:

  1. Add the following dtoverlay in /boot/config.txt:
dtoverlay=gpio-shutdown,gpio_pin=3
  1. 1 push-button and 2 wires: Connect the push-button between GPIO 3 and GND.

As Matthijs pointed out in his blog, the OBSS doesn't remove power, and so the RPi continues to draw power (at a lower level) after being shutdown. This is the same as all RPis do after receiving a halt, shutdown or poweroff command from the terminal. In my case, the RPi 4B draws approximately 270mA using the default set of bootloader configuration properties, or approximately 1.35 watts of power. This same RPi 4B draws approximately 470mA / 2.35 watts when it is running. From my reading, this seems fairly typical . Consequently, the energy savings incentive for using the OBSS is not huge. Despite the convenience of OBSS, shutdown is not a "sleep mode".

One of the features introduced in the RPi 4B is a "Low Power Mode" (LPM):

If [POWER_OFF_ON_HALT=] 1 and WAKE_ON_GPIO=0 then sudo halt will switch off all PMIC outputs. This is lowest possible power state for halt but may cause problems with some HATs because 5V will still be on. GLOBAL_EN must be shorted to ground to boot.

And so LPM is set up by changing two default values in the bootloader configuration: WAKE_ON_GPIO and POWER_OFF_ON_HALT. According to my experiment, doing so reduces power consumption in shutdown to approximately 0.2 watts - a current draw of about 40mA. AFAIK, this is the lowest achievable power consumption for the RPi 4B without "pulling the plug".

Unfortunately, as mentioned above, this shutdown state is not a sleep mode, and a reboot is required to return the system to an operational state. The necessary reboot from LPM shutdown is only possible by either: 1. pulling down the GLOBAL_EN node, or 2. "pulling and re-inserting the plug". GLOBAL_EN is not a GPIO, nor can it (at least currently) be assigned to a GPIO. This likely due to the fact that the 3V3 bus is turned off in LPM - thus disabling all GPIO. Regardless of the cause, the net effect is this: OBSS, as currently implemented, does not work when the RPi 4B is configured for Low Power Mode.

One work-around to this is shown in the following schematic. The concept is simple: We connect the pushbutton to the trigger inputs on a pair of "one-shots". We take advantage of the fact that the RPi 4B's 3V3 bus is powered down during LPM, and wire the 3V3 line to enable only one of the one-shot's outputs. One of these outputs is wired to GPIO 3 for shutdown, the other is wired to GLOBAL_EN for startup. The present design has the drawback that the RPi must be in LPM - it does not work in the bootloader default configuration forhalt/shutdown mode. This could be addressed by an improvement to the hardware design below, but a better solution - if possible - might be available in another dtoverlay?

schematic