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

[Jared]

Hi,

 The are two issues with this approach:

1. You will introduce leakage current. The leakage current will be corrected if you set SW6 to nRF only, but the issue is then that you can't communicate with the interface chip. You would therefore have to set the SW6 to default while debugging and setting it back nRF only when you're done flashing/debugging the application.
2. This is the more serious concern. There will be a significant difference between the voltage that the interface chip is operating on and the voltage of the nRF. The nRF has a maximum voltage on I/O pins to be 0.3 + VDD. If VDD is 1.8 V then the maximum voltage you can expose the nRF pins with is 2.1 V, and that is only over a short period. The interface chip will operate at 3 V. This will result in a failing IC over time. As such, you would have to increase the voltage that you use on the external supply so that it's within the maximum operating conditions. 

Why exactly do you want to configure the DK this way?

regards

Jared 

[Christopher Hunt]

Thanks for the reply. My goal is to run the SoC at 1.8v as the peripherals connected to it via the GPIO pins of the DK operate at 1.8v. At the same time, I’d like to flash and debug my app and view its logging.

[Jared]

Hi

By default the nRF52840 VDD will be regulated to 1.8V if you supply it via the high voltage regulator (USB or LiPo). See this thread. You can try supplying the board via the USB port closest to the nRF and then use a external debugger to debug the SoC via P18. See documentation here.