Trying to flash STM32f030f4 (ARM, Cortex M0) - what is proper wiring?
At last I've found what was missing.

VDDA should be connected! e.g. to VDD. I believe if device has VSSA also, it also should be connected.

Otherwise the chip is in reset mode "thanks" to functionality which monitors both VDD and VDDA and simply does not allow chip to start.

So minimal connection is like this:


3.3 Volts to VDD, VDDA, BOOT0
GND to VSS (and VSSA if present)


(at this point you can check that PA9 yields strong high level - seems it at once becomes TX output)


PA9 (TX) to the RX of the cable (in my case no pull-up is needed as it is full-functional output)
PA10 (RX) to the TX of the cable.


By the way I use the cable with 5V levels and it is Ok as the pins are 5v-tolerant.