NRF9160 Deep Sleep Power Consumption Help

[TylerS]

Hello,

I am using mfw1.3.0 and NCS 1.7.0 developing an application on a custom board implementing the NRF9160 and NRF52820 (The 52820 is put into very low power using nrf_pwr_mgmt_shutdown(NRF_PWR_MGMT_SHUTDOWN_STAY_IN_SYSOFF);). 

There is a state in the application where we would like the device to go into a deep sleep mode where the device ideally consumes <200uA of current (Lower is even better). The device is awoken by a state change on a configured Sense pin. 

In the current stage of development, everything is working decently well with the modem powering off and then the device being set to deep sleep. The issue is that we are seeing around 900uA of average current draw (It seems to be fluctuating between about 500uA and 1200uA). I know some of this current is due to on-board peripheral parts drawing a bit of power but it really should only be accounting for around 150uA or so and is something we are working on separately.

I have read pretty much every post out there about low power mode on the NRF9160 and it seems that the first thing to check is if CONFIG_SERIAL is disabled in both the application config as well as the SPM config. Our application will be using serial but we want to shut it down before going into deepsleep with this function: 

pm_device_state_set(uart_0_dev, PM_DEVICE_STATE_OFF);

This behavior is found in the serial_lte_modem example. The problem is that using this line to disable uart seems to have no effect on current draw.

I also tried just not using serial at all in the application and ensuring that CONFIG_SERIAL is not set in either the application config or SPM config. This also did not have any affect which was surprising to me and leads me to believe that there is another peripheral that we have enabled which is causing the same current draw that having the serial peripheral does.

My questions that can't be answered by other tickets are:

1. What is the real reason that turning serial off saves power in deep sleep? From looking at other tickets I think that the root cause of the power draw is keeping the HF clock active in deep sleep which can draw a decent amount of current.

2. Are there other peripherals that could be causing the same issue for us still? We use I2C, SPI, UART, PWM, and GPIO in the application.

3. What is the best way to have serial enabled in the application but turn the peripheral off before sleeping in NCS1.7.0 and mfw1.3.0?

Also, we have tried with mfw1.3.1 which supposedly has a fix for a deep sleep power issue but that has no affect. 

Thanks!

Tyler

[TylerS]

Wanted to add a bit more flavor here, and a quick update. We have gotten the average current draw down to around 500uA by powering down some other SOC peripherals (Accelerometer etc.)  I am using a "Current Ranger" to measure the current and hooked up my scope to grab the waveform while it is in deep sleep.

It seems like some internal system is periodically pulling maybe around 600uA or so at a period of about 25ms. Is this typical of some internal peripheral on the 9160?

We have also learned that mfw1.3.1 is what will be supported soon by the US carriers so we will be developing on this version going forward.