nrf9160-dk reset with external supply at 3V

[Anthony Ambuehl]

I'm performing power measurements with nrf9160-DK running the Serial LTE Modem application. 

The example SLM has been modified to add support for HTTP, MQTT and also to relocate the UART pins.

To measure power I apply a test voltage to external power supply and a DMM (34465A) measures current at the nRF current measurement header.

When testing with 5V the system works as expected, however when lowering the voltage to 3.3V the nrf9160 emits "READY" on the serial port shortly after activating LTE.  I believe this is an indication the nrf9160 has reset.

My power supply is configured to limit current to 500 mA.  The DMM is operating in 1A current measurement mode.

Testing at 5V - normal operation

Test at 4.5V - normal operation

Test at 3.7V - normal operation

Test at 3.5V - "READY" received after CFUN=21 

Test at 3.3V - "READY" received after CFUN=21

Do I need to make any modification to SLM or nrf9160-DK to operate at 3.3V ?

[Albrecht Markus Schellenberger]

Hello Anthony,

Modem operations at 3.3V could be on the limit to what is possible. The current shown in this table are measured with a power supply of 3.7V.  I guess you get a noticeable voltage drop before the DK is resetting? You might wanna remove the current limit and see if the behaviour changes.

Otherwise I wonder if there could be any series resistance in your measurement set-up that is enough for a voltage drop resetting the DK?

There is an AT command for Power off warnings from the modem which starts at 3300mV and thus considered as low power.

Regards,

Markus

[Anthony Ambuehl]

The problem seems to come from the nrf9160 DK "power supply circuitry"  If I inject 3.3V at both the external supply header and the high side of the nRF9160 current measurement header, the system operates without reset.