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Looking at the schematics of the STM32F4-Discovery board page 28, whose screenshot is attached below.. there is a Zero Ohms resistor 'R25' which is shown in the circuit with the Crystal. And there is an actual physical 0R resistor on the PCB board at this place. I wonder what is the purpose of a zero ohms resistor with Crystal?

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Probably to allow the crystal to be disconnected (by removing the 0R resistor) and an external clock signal to be input to OSC IN from elsewhere.

And actually, it's all explained (resistors and solder bridges) on page 18:

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