nRF91 UART Sleep Current – AT Host deinit & pm_device_action_run() for Low Power

[Akshay]

Hi support team,

We are using nRF9151 with NCS v2.7.0 for our IoT product. During power optimization, we observed:

• Even after disabling UART RX interrupt using uart_irq_rx_disable(), the sleep current does not reduce.

• We confirmed CONFIG_PM_DEVICE=y, CONFIG_PM_DEVICE_RUNTIME=y, and other PM settings are enabled.

• Disabling logging or the AT Host library does reduce current significantly, but we need the AT Host for modem communication.

Our Current Understanding:

• The AT Host library keeps UART RX active via continuous reception tasks to handle AT command responses and URCs.

• Calling uart_irq_rx_disable() only disables the interrupt but does not stop RX DMA or UARTE reception, so the peripheral remains fully powered.

• We tried using:

pm_device_action_run(uart_dev, PM_DEVICE_ACTION_SUSPEND);

But we are unsure if the nRF91 UARTE driver implements the required PM hooks to allow suspend/resume.

Current Measurements:

• Average current in PSM with UART active: ~630 µA

• Target sleep current: <10 µA

When we disable the AT Host or deinitialize UART completely, we observe significantly lower sleep current, but we lose modem communication capability.

Our Goal:

We want to:

1. Fully disable or suspend UART (UARTE) during deep sleep to achieve ~4–10 µA sleep current.

2. Reinitialize or resume it before the next modem communication cycle.

Questions:

1. Does the nRF91 UARTE driver support PM device suspend/resume using pm_device_action_run() in NCS v2.7.0?

2. If not, is there any recommended approach to fully deinitialize UART while using AT Host, and reinitialize it safely before modem communication?

Modem firmware: 2.0.1

NCS Version : 2.7.0

[dejans]

Hi,

Does the nRF91 UARTE driver support PM device suspend/resume using pm_device_action_run() in NCS v2.7.0?

Yes, use of pm_device_action_run() is supported by UARTE driver in NCS v2.7.0.

Best regards,
Dejan

[Akshay]

Hi Dejan,

Apparently that didn't work for me, it's still drawing 630uA. Only CONFIG_SERIAL=n is bringing down the current to 40uA(as I have SAADC and some other peripherals enabled).

Regards,

[dejans]

Hi,

You can check if you have more than one UARTE instance enabled.

Best regards,
Dejan

[Akshay]

hi,

I have disabled both uart0 and uart1 in code using rx_irq_disable function and suspended them using power management api. Still it is drawing 630uA. the AT host library must be enabling another instance of uart0. How do I disable the AT host library in runtime?!

regards,

[dejans]

Hi,

CONFIG_SERIAL=y activates all uart instances that have status ok in the devicetree.
You can use ENABLE register to check if UARTE is enabled or disabled.
If you do not need peripheral and you want to put the system in the lowest power state, you can follow low power documentation and first stop and then disable the peripheral. You can also look at this Device PM and nrfx UARTE discussion.

Best regards,
Dejan