Situation: I have D+ & D- connected to the boron's USB, VBUS is NOT connected via USB port or vusb breakout pin. The boron is powered via Li+. Can you program the boron? Or does the boron need to have 5V to run the internal USB driver?
Nevermind, found the answer in the NRF52840 datasheet.
As a consequence, VBUS and
either VDDH or VDD supplies are required for USB peripheral operation.
You are correct. 5V on the USB port is required for nRF52 devices (Boron, B SoM, Argon, Tracker) because the MCU does not power the processing block that handles USB when there is no USB power as a power-saving feature.
5V on USB is not required for RTL872x (P2 and Photon 2), or STM32 (Gen 2) to use USB.