Which of these marks signifies Pin 1 on the STM32F (LQFP64)?
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If there is still some doubt in your mind, use a continuity tester to make sure all the grounds are connected, since these will likely have unique locations you should be able to verify which of the pins are connected together. For example pins 27,28 and 49,50 are a unique grouping that doesn't occur on other sides.



It's very very likely that since this is a processor that the ground is common throughout the die.

Next step would be to use a diode checker and to test the power pins against group.

Use a little bit of correction fluid to the package body to mark your pin with a white (or yellow etc.) spot.