My custom designed bare STM32L052C (QFP48) board refuses to talk to external debuggers (STLinkV2, JLink) when the power goes below 3.0 Volts on Vdd pins.
Only SWD interface and step-down power source are connected. The target voltage is detected correctly by both external debuggers. STLinkV2 is completely new, never connected before. I tried switching debuggers, connecting external PSU, checking all SWD pins, with the same results.
The SWD signals, with the exception of 22 Ohm serial resistors, are pretty much the same as in STM32L073Z-EVAL board, which by the way have been proven to work well down to 1.8 V, using the very same equipment. Reset and SWDIO are normally high when the debugger is connected. BOOT0 is grounded.
I did my best to follow AN4467 "Getting started with STM32L0xx hardware development" during HW design. Since this is an ultra low power application, I omitted 1uF and 4.7 uF blocking capacitors. The board works reliably at 3.3 V. The signal lines are as short as possible, with ground poured between pins and over bottom layer (2-layer PCB). The debug interface is passively connected over TagConnect2030 (RJ12), converted to standard 20-pin JTAG IDC connector over very short ribbon.
No LCD or USB is used in the design. Drop-in replacements are L053C and L063C.
Logic analyzer signal capture failed to reveal anything, since I have no SWD decoder available. The capture files are available at my web page.
Does anyone have an idea what else could be possibly checked?