BUG: USB icon stuck after cable disconnect on boards without VBUS detection

[paoloantinori]

Description

After flashing via USB and unplugging the cable, the display continues showing the USB icon instead of switching to the BT icon. The endpoint never switches to BLE.

Root Cause

Some nRF52840 boards (e.g. Corne-ish Zen) have no VBUS detection configured. When the USB cable is pulled, the nRF52840 USB peripheral detects bus idle (no SOF packets) and reports USB_DC_SUSPEND, but ZMK maps this to ZMK_USB_CONN_HID (still "connected") in app/src/usb.c. Without VBUS detection, the chip cannot distinguish "cable unplugged" from "host suspended the bus."

The status flow on cable unplug is:

USB_DC_CONFIGURED -> USB_DC_SUSPEND   (cable pulled)

While for a legitimate host suspend (e.g. closing a laptop lid):

USB_DC_CONFIGURED -> USB_DC_SUSPEND -> USB_DC_RESUME   (host wakes)

Both cases look identical to the USB peripheral without VBUS detection.

Related Issues

This is related to #841 (USB power-only cable detected as host), which was fixed by PR #2458. That fix addressed the case where a power bank is plugged in (no data), while this issue covers the case where the cable is removed entirely.

Environment

• Board: Corne-ish Zen (nRF52840)
• Branch: main
• Zephyr: v4.1.0+zmk-fixes

Workaround

A configurable suspend timeout can treat prolonged USB_DC_SUSPEND (e.g. >2 seconds without USB_DC_RESUME) as a disconnect. This is a software-only workaround for boards that lack VBUS detection hardware.

[caksoylar]

Does this only affect the status screen display, or does endpoint fallback not work? (e.g. USB output priority is selected, does not fallback to BLE when USB cable is disconnected.)

[SethMilliken]

I have not been able to reproduce this with my zen display patches branch (current main with some Zen-related patches).

1. Attach central to USB.
2. Explicitly select &out OUT_USB
3. Observe USB symbol on display.
4. Activate bootloader mode.
5. Flash firmware to Zen.
6. Observe USB symbol on display after reboot.
7. Detach USB cable.
8. Observe USB symbol change to Bluetooth + profile.

Also tried without 4-6.

I initially thought I had reproduced it when I had my power switch off. When I unplugged the board, the display (now without power) stayed on USB and lost the connection to the still-powered peripheral, so it looked like it was not falling back.

Edit: FWIW, using a Corne-ish Zen R3

[paoloantinori]

Thanks folks for looking into this!

@caksoylar just the screen . the keyboards connect correctly over BT, but it still shows the USB icon on the screen. the suggested fix, does indeed fix it in my case

@SethMilliken thanks . i will check your branch. I think I have some of the same patches as you do, I have been using @caksoylar builds for a while, until recently when AI helped with some custom patches:

https://github.com/paoloantinori/zmk-config-zen-2