How to put the UART to sleep (low power mode)

[GameCodingNinja]

I believe it started with SDK 2.0.0 but the board config files now define different ways to specify how hardware is to run.

&pinctrl {

    uart1_default: uart1_default {
		group1 {
			psels = <NRF_PSEL(UART_TX, 0, 26)>,
				<NRF_PSEL(UART_RTS, 0, 29)>;
		};
		group2 {
			psels = <NRF_PSEL(UART_RX, 0, 27)>,
				<NRF_PSEL(UART_CTS, 0, 28)>;
			bias-pull-up;
		};
	};

	uart1_sleep: uart1_sleep {
		group1 {
			psels = <NRF_PSEL(UART_TX, 0, 26)>,
				<NRF_PSEL(UART_RX, 0, 27)>,
				<NRF_PSEL(UART_RTS, 0, 29)>,
				<NRF_PSEL(UART_CTS, 0, 28)>;
			low-power-enable;
		};
	};
	
&uart1 {
	status = "okay";
	pinctrl-0 = <&uart1_default_alt>;
	pinctrl-1 = <&uart1_sleep_alt>;
	pinctrl-names = "default", "sleep";
};

I took this to mean that I can dynamically toggle these configurations from "uart1_default" to "uart1_sleep" from perhaps a function call. Did find the pinctrl_apply_state() function call but it's return a -2 that state id doesn't exist even though it is defined. Here's my code.

uart = device_get_binding("UART_1");

int err = pinctrl_apply_state(uart->config, PINCTRL_STATE_SLEEP); // Returns -2

Any ideas?

[Vidar Berg]

Hello,

You are correct, 'pin control' was introduced in v2.0.0. However, it only controls the pin state. To get the UART to a low power state, you need to use it via the power management subsystem. The code snippet below shows an example of how you can use the API to power down your UART.

prj.conf

# Device power management
CONFIG_PM_DEVICE=y

Code 

#include <zephyr/pm/device.h>

...

void some_function(void) 
{
    int err;
    /* Power down the UART device */
    err = pm_device_action_run(<uart_dev>, PM_DEVICE_ACTION_SUSPEND);
    if (err) {
        printk("pm_device_action_run() failed (%d)\n", err);
    }
    
    ...
    /* Power it on again */
    err = pm_device_action_run(<uart_dev>, PM_DEVICE_ACTION_RESUME);
    if (err) {
        printk("pm_device_action_run() failed (%d)\n", err);
    }

Best regards,

Vidar

[GameCodingNinja]

Sweet, I got it to compile! Will let you know. Big thanks!

[GameCodingNinja]

On the 52840, will "pm_device_action_run(<uart_dev>, PM_DEVICE_ACTION_SUSPEND);" together with nrf_power_system_off(NRF_POWER); reduce the power to 0.95uA, per the datasheet

[Vidar Berg]

Yes, you should be able to reduce the sleep current to around 0.95 uA by entering System OFF mode. The important thing to ensure before entering this mode is that you don't have any floating inputs, as the GPIO configurations are preserved in this mode. Using the command "pm_device_action_run(<uart_dev>, PM_DEVICE_ACTION_SUSPEND);" will handle the pin configuration assigned to the UART interface. That is, set the pins as "input, disconnect" to prevent current leakge.

[GameCodingNinja]

My apologies, I don't quite follow. Are you saying I should make a change to the below to indicate "input, disconnect"? I tried replacing low-power-enable; with input,disconnect but that doesn't compile.

&pinctrl {

    uart0_default: uart0_default {

        group1 {

            psels = <NRF_PSEL(UART_TX, 0, 26)>,

                <NRF_PSEL(UART_RTS, 0, 27)>;

        };

        group2 {

            psels = <NRF_PSEL(UART_RX, 0, 0)>,

                <NRF_PSEL(UART_CTS, 0, 6)>;

            bias-pull-up;

        };

    };

    uart0_sleep: uart0_sleep {

        group1 {

            psels = <NRF_PSEL(UART_TX, 0, 26)>,

                <NRF_PSEL(UART_RX, 0, 0)>,

                <NRF_PSEL(UART_RTS, 0, 27)>,

                <NRF_PSEL(UART_CTS, 0, 6)>;

            low-power-enable;

        };

    };

[Vidar Berg]

Sorry for not being clearer. No, you don't need to change anything for the UART. What I meant to point out is that there may be other pins not assigned to the UART that need to be configured as "input disconnect" as well. The easiest way to get an overview of all the pin configurations is probably by using the peripheral viewer in VS Code when you debug the application. You can place a breakpoint at the function where you enter system off to check what the GPIO configurations are at that point.

The 'low-power-enable' property will make the pin control module configure the UARTE pins as input disconnect when the uart0_sleep group is applied via pm_device_action_run()->uarte_nrfx_pm_action() -> pinctrl_configure_pins()

GPIO configurations viewed in VS code

0x2 means the pin is configured as input disconnect and can safely be left floating. 

[GameCodingNinja]

Except for the few pins setup for BLE Reset, none of the other GPIO pins are pinned out. In VS Code debugger, GPIO P0 and P1 pins have the value of 0x00000000. I assume that's ok.