Problem with UARTE in direct supply mode

[TSonono]

Hello,

I encounter a problem when I am trying to measure the current consumption of the nRF52840 on the nRF52840-DK. I have set the board in direct supply mode as described here. I am feeding the external supply pins with 3.3V using an otii arc. However, it seems that if I do not connect a USB cable to the J2 connector, I cannot send data over the UART pins I have configured (0.27 and 0.26). What could be the cause of this? The application works fine when I'm not measuring current OR when I'm measuring current as long as there is also a USB cable connected from my PC to J2.

Best regards,
Tofik

[Simonr]

Hi again

As stated in the direct supply documentation, it is recommended to supply the board from a different source to prevent the pins of the SoC to be connected to unpowered devices (which I assume is what's happening here). The voltage follower also requires 5V to be present on the DK when SW10 is in the ON position. You can alternatively disconnect the voltage follower from the external supply by cutting SB58. If you do so, it's recommended to set the DK in nRF only mode by setting SW6 to nRF only.

Best regards,

Simon

[TSonono]

Thanks for the reply Simon.

I see! It also seems that if I put the DK in nRF only mode, the application works without any USB cable connected to J2 (without cutting SB58). Would the measurements I get with this configuration be incorrect for some reason? Should I cut SB58 to get more "correct" measurements?

Regards,

Tofik

[Simonr]

The nRF only mode disconnects the power supply of the interface MCU, the external memory, and the LEDs as well as disconnects the signal lines between the SoC and the MCU using analog switches. This is done to isolate the SoC as much as possible, and can be of use when measuring currents on low-power applications. Cutting SB58 should provide a more realistic current reading, as the voltage follower will consume some current, although not a lot.

Best regards,

Simon

[TSonono]

Thanks Simon, this seems to be the most viable configuration. UART is working fine now and measurements are hopefully accurate

Best regards,

Tofik