I have a related query: When I power via 5V or 3.3V, the board does nothing, not even the blue LED. If I apply 5v via the SPI connector, I get the Arduino boot logo, but my preloaded (matrix) sketch doesn't work, matrix remains blank after the heart beats twice. (Ditto with power through a USB hub). It would appear that unless USB power directly from the laptop, is starts in SBC mode.
The board can't be powered on with just 3.3V; the MPU (Qualcomm Dragonwing) relies on the 5v to 3.8V buck converter or USB_VBUS_IN, so 3.3V alone won't let it boot.
5V on any of the 5V pins should start up the entire board, and if your sketch is set to run at startup then that will also start after the MCU is finished booting (Arduino logo finishes). I think UNO Q flashes the MCU (STM32) after boot, so there may be a little bit for it to do that too.
Looking at the Arduino UNO Q Power Tree from the UNO Q datasheet, I don't think only the STM32 can be booted, without the MCU. Not sure though. It definitely won't run any sketches even if it is powered on.
Edit: I did have an issue with 5v powering the board when trying it out with my bench power supply. If I plugged the 5V and GND pins of JANALOG in, then switched on the output of the power supply, it wouldn't boot but the green pwr led would light up. However, if I turned on the power supply's output and then plugged in the 5V pin, it had no issue turning on.
There might be a power sequencing issue if the voltage rise is too slow (my bench power supply seems to have a lot of output capacitance).
I take back what I said about being able to only boot the STM32, it is possible. I just tried it by powering the 3.3V pin on JANALOG. To get a sketch to run, you'd need to flash it using Arduino IDE, and set the Startup Mode to Immediate.
