Hi Alex, Thank you for the helpful tips! The problematic board got a new problem today. It is randomly reseting. I probed RESET pin which is connected to 3.3v with a pull up resistor and discovered some intermittent 2.048MHz oscillations. I'm not very surprised because RESET pin and CLK pin (I have clock output enabled and this pin is not connected to anything) are also physically close to each other. If clock output is disabled the problem solves itself. Then the chip is replaced and all problems are gone. I'll be more careful handling the chips and pay more attention to the board layout next time!
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