How to use VEXT->nRF reliably for a 1.8v external power source along with the USB IF MCU

[Christopher Hunt]

Hi there,

I've set up an nRF52840 DK with an external power supply of 1.8v and also connected the USB IF MCU to my laptop. I've positioned the VEXT->nRF switch to "on" and observed that the VDD output on the P1 connector is 1.8v. The nRF power supply switch is set to VDD.

I'm *sometimes* able to see LED5 illuminate weakly with 1.8v connected, and able to communicate via nrfjprog and the USB modem from my laptop. Othertimes (most times), I am not.

If I shift the VEXT->nRF to off then I'm *always* able to see the LED5 illuminate, and it does so fully. I'm also then able to connect via the USB modem from my laptop.

Is there some required sequence of connecting the 1.8v supply, connecting the USB IF MCU, and setting the VEXT->nRF switch to on? So far, I've not been able to determine the sequence.

Is it even supported to be able to use the USB modem from my laptop when the DK is using external power?

Thanks for any help.

Kind regards,
Christopher

[Christopher Hunt]

Hi again,

Just wondering, if I can get a reply to the following, and then I think I've got the answers I need. Thanks!

Thanks again for the replies. Meanwhile, I remain curious as to the function of the VEXT-nRF button. My understanding from the doc was that I could use the 5v power to the MCU IF (USB) and supply the SoC with external power. There is mention of voltage leaking in the doc, but I’d now like to understand the purpose of VEXT-nRF. 

[Jared]

Hi,

Some developers would like to only power the SoC when they are developing their application. For instance, some would like to measure the power consumption of their application, powering the whole board would not give a realistic result. 

Christopher Hunt said:

My understanding from the doc was that I could use the 5v power to the MCU IF (USB) and supply the SoC with external power.

The concern in your case is that the nRF52840 and the Interface chip are operating at two different voltage levels, 1.8 V for the former versus 3.0 V for the latter. 

regards

Jared