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Cannot access STM32F051 via SWD after first download

Question asked by ostap on Nov 30, 2015
Latest reply on Dec 3, 2015 by ostap
I have self-designed board with STM32F051 (48-pin). After assembling I could detect the core using J-Link and downloaded my firmware by IAR debugger but debugger lost the CPU immediately and cannot access it anymore. J-Link itself also cannot detect CPU (however it finds SWD-DP). I also changed CPU phisically and same story - download ones and then it stops.
About HW: Vdd=2,9V (tried 3,3V with same result), Vdda connected to Vdd, BOOT0 pin is not connected (however similar boards run well with BOOT0 not connected), NRST connected to SWD connector only (no external pull-up). Unfortunately USART2 is not used in this project and pins are connected to LEDs so it is very difficult to use BOOT0 and USART and terminal. I'd rather change CPU on my board. 
About SW: I checked that GPIOA13 and 14 are not touched in the program. In general SW implements touch-keys (TSL library from ST) and few LEDs. I do not use any of power-down modes, CPU is always in RUN state.
So... what I missed? First I do not understand why I lose communication to the debugger. Second I do not understand why J-Link cannot detect core if I have nRST pin connected to the J-Link. What can I do to access the CPU again (USART2 and terminal solution is almost impossible) and what to look for in source code to find possible reason of such behabior?