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MSP430FR5728 crystal oscillator external capacitors

Florian Neumeister
Prodigy 30 points
Other Parts Discussed in Thread: MSP430FR5728

Hello,

can anyone help me with the additional external capacitors for the MSP430FR5728 to get a stable oscillation?

I picked a 32.768kHz crystal with a load capacitance of Cl=9pF. After reading through several application reports and user guides, I am still not shure about the values of the required external capacitors.

Does anybody have experiences with this topic?

Thank you very much in advance!

Best regards

Flo

  • 0
    Guru 227245 points

    A load capacitance of 9pF means attaching two 18pF capacitors to the crystal. This includes and trace and pin stray capacitance.

    The available settings in the clock module usually mean the effective load capacitance and include the pin capacitance, but not any additional parasitic capacitances due to the traces.

    If there is no value that fits your crystal, set this to the lowest setting (actually just the pin capacitance) and attach two capacitors from each crystal pin to GND (in your case 18-2 = 16pF each, or less for longer traces).

    The traces should be shielded against other signals by a GND wire and should be as short and close as possible to the MSP (and no vias, as those are inductances)
    Also, the area beneath the crystal (bottom side of the PCB) should be a GND plane that is separated from the normal GND plane (if any) except for a single connection pint (so no GND currents can flow through it)

    The case of watch crystals should be soldered to GND too (for HF crystals this isn’t that important for clean operation, but may reduce HF emissions on higher frequencies >10MHz)

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