T-Som cannot flash via USB and strange LED behavior

Hi, we have a custom T-SOM carrier board that reproduces the normal Particle LED functionality, and some of our boards have become impossible to flash. When reset, we get about 2 seconds each of magenta flashing, yellow flashing, rapid white flashing, white breathing, and then back to listening mode.

We notice that something is pulling the MODE pin to ground, even with a 3.3k pullup (there is nothing connected to the MODE pin except a button and the pullup).

Affected devices will appear when particle usb list is run, and the aforementioned LED sequence repeats, except for flashing yellow, when particle usb dfu is run. Particle USB dfu never finishes running.

Particle usb safe-mode causes the same behavior, and particle usb setup-done returns "Done", but nothing changes.

We are kind of at an impasse here as we don't want to flash any more of our boards in case we are bricking things. The MODE pin being pulled down seems important.

Additionally it is not possible to run particle usb-reset - it gets to about 2%, then sits there indefinitely.

Device OS will never set the MODE pin as an output, so I'd check for a physical cause - a physical connection to some other pin or GND. One of the adjacent pads is GND, so make sure there isn't a solder bridge. It could also be somewhere else on the board.

If you have a four-pin SMD tactile momentary switch make sure you didn't connect the wrong pins. I've done that on a board so RESET or MODE was always connected to GND instead of only when pressed.

Your blinking behavior is safe mode (blinking magenta), DFU mode (blinking yellow), clear credentials (blinking white). This is the behavior for the MODE button stuck down or connected to GND.

Hi Rick, on further investigation (cutting PCB trace to T-SOM) we see a voltage of ~70 mV on the MODE pin vs. the ground pin next to it. I am seeing a resistance of 266 ohms between the two pins.