32F413: Strange VCAP1/VCAP2 Behavior?

[JOLSO.11]

I'm debugging a hand-stuffed PCB that could have a component stuffing error I cannot discern. The board's artwork is a minor tweak on a prior known-good PCB using a 32F413 in the 100-pin package.

This board will not boot.

However, all VDD pins are at +3V as intended and the nRESET pin is high. VCAP1 and VCAP2 pins are equipped with their 2.2uF bypass caps and have ~+1.2 volts on them. The quartz crystal is oscillating.

Then strangely 3 seconds after application of VDD the voltage on both VCAP pins drops to zero and the crystal stops. nRESET remains high.

What would force the VCAP pins to collapse to VSS after 3 seconds?

TIA for any advice on this.

[JOLSO.11]

Additional piece of data is that my STLINK V3 programmer reported to me (but only once) that it could see my target board and program/verify it successfully with our known good code.

But thereafter the STLINK programmer reports "no target found" and the VCAP voltages are not stable.

[JOLSO.11]

Never mind. Our software wiz tells me the beast intentionally goes to sleep 3 seconds after the VDD rail comes up, explaining the loss of the VCAP voltages.

My observation that the device would not boot was clouded by a shorted pin on the LCD display.