nrf52840 high voltage mode

[srishtik]

I have created a bare chip circuit with nrf52840-QIAA-R. The requirements were
1) It has to be powered by USB standalone and also a backup battery for which I have used a BQ24075 IC.
2) USB functionality is required
3) Internal DC-DC converter has to be enabled.

For this I have created the circuit and have also got it manufactured as well.
I am attaching the schematic below.
I have used high voltage mode of nRF and have connected VBUS from usb to VBUS pin of nrf and also have connected VDDH to VBUS as well and left VDD floating (with decoupling).

Note that the VBUS is supplied from the output of the charger IC BQ24075 (which is on another sheet). I have basically shorted VBUS to VDDH, used
LC filters and have enabled DC-DC converter in zephyr config.

As I power on the board it functions correctly and 1.8V default VDD is detected by programmer. I was also able to program it with a sample USB example to print HELLO world sample program of zephyr which printed Hello world to serial terminal over onboard USB connector. I also tried USB mass storage example with FAT file system and the device got identified on windows and was able to transfer files as well.

This is a pcb that is a custom keyboard whuch would use ZMK firmware which is an application built on top of zephyr.
I have defined my own device tree and flashed the firmware. But I am getting an error saying "zmk: FAILED TO SEND OVER USB: -19 "

I wanted to know whether this could be caused due to connecting VDDH and VBUS to same VBUS? Because I got most of the zephyr samples from zephyr to work on this custom board.

Link to github repo of zmk that I have forked: repo
This is the link to the devicetree that I use to build and flash zmk: DTS ( which is in the repo mentioned above in app/boards/arm/tr60)
I am also providing link to kicad project: kicad_project

I also tried the same application with exactly same code and same devicetree as mine on nrf52840dk and it worked. I am not able to understand why other USB samples work on my board but
not zmk application. Any thoughts

Thanks in advance.

[Simonr]

Hi

Yes, it's fine to connect VBUS to VDDH. You can see circuit configuration 1 in the Infocenter which connects VBUS and VDDH. This is to let the device be powered from VBUS.

It's possible that you get a faulty event if the VBUS is powered from the battery circuit, so we would recommend a diode connection for example to connect VBUS to only the USB contact, so the battery circuit won't power the VBUS directly.

Best regards,

Simon

[srishtik]

Thanks for your reply. My circuit works as intended for now. I will keep this in mind while designing my next design Will get it reviewed here. We can close the ticket.

Thanks for you help.