nRF52840 Power Management when idle

Question

Greetings, 
 
 I am looking into power management options for our custom board using the nrf52840 with the nRF Connect SDK. 
 
 I wanted to ask for a clarification if possible. 
 Do I have to enable anything other than CONFIG_PM=y option to enable the system to go into low power mode when the idle thread is running? 
 Do I have to use the CAF for that to happen? 
 
 Thank you! 
 
 Best Regards, 
 Stavros

Stian Røed Hafskjold · Accepted Answer

Hi Savros,

You don't have to do anything to actively put the chip into System ON idle mode. After all threads are finished (or threads are put to sleep by k_msleep() or similar), the CPU enters the idle thread, and wakes up on the next event. This happens regardless of the CONFIG_PM state.

CONFIG_PM is set to =y by default in the samples, but actively turning this off does not prevent the CPU from entering the idle thread. What you want to do is to use CONFIG_PM_DEVICE to turn off peripherals that are still active after the CPU enters the idle thread.

Here is an example using the zephyr/sample/basic/blinky:

When you run this out of the box on a nRF52 DK you can see that the idle current is around 500-800 uA (depending on which chip you use).
The main loop enters k_msleep() after toggling the GPIO.
k_msleep() lets the CPU enter the idle thread, and the CPU will now be in system ON idle until the timer runs out after SLEEP_TIME_MS milliseconds (or another thread wakes up the CPU)

But why does the example still consume 500-800 uA? This is because logging is enabled, and logging uses the UART peripheral.

There are two ways to turn off logging.

The easiest way is to put CONFIG_SERIAL=n in the prj.conf file as this will shut down all the UART peripherals. But then you can not use UART later.
Use the power management API:
- Add CONFIG_PM_DEVICE=y to prj.conf
- Add #include <zephyr/pm/device.h> to main.c
- The following code will turn off the logging module (and UART):
```
const struct device *uart0_dev = DEVICE_DT_GET(DT_CHOSEN(zephyr_console));
pm_device_action_run(uart0_dev, PM_DEVICE_ACTION_SUSPEND);
```

The resulting main.c for the zephyr/samples/basic/blinky example will look something like this:

#include <zephyr/kernel.h>
#include <zephyr/drivers/gpio.h>
#include <zephyr/pm/device.h>


/* 1000 msec = 1 sec */
#define SLEEP_TIME_MS   1000

/* The devicetree node identifier for the "led0" alias. */
#define LED0_NODE DT_ALIAS(led0)

/*
 * A build error on this line means your board is unsupported.
 * See the sample documentation for information on how to fix this.
 */
static const struct gpio_dt_spec led = GPIO_DT_SPEC_GET(LED0_NODE, gpios);
const struct device *uart0_dev = DEVICE_DT_GET(DT_CHOSEN(zephyr_console));

void main(void)
{
	int ret;

	ret = pm_device_action_run(uart0_dev, PM_DEVICE_ACTION_SUSPEND);
	if (ret < 0) {
		return;
	}

	if (!device_is_ready(led.port)) {
		return;
	}

	ret = gpio_pin_configure_dt(&led, GPIO_OUTPUT_ACTIVE);
	if (ret < 0) {
		return;
	}

	while (1) {
		ret = gpio_pin_toggle_dt(&led);
		if (ret < 0) {
			return;
		}
		k_msleep(SLEEP_TIME_MS);
	}
}

And prj.conf:

CONFIG_GPIO=y
CONFIG_PM_DEVICE=y

I tested the code above on a DK and the idle current is around 2 uA. If I set CONFIG_PM to =n the idle current is still around 2 uA.

Best regards,
Stian

nRF52840 Power Management when idle

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