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MSP430F5419A EXTERNAL CRYSTAL NOT WORKING

rehana jose89
Prodigy 20 points
Other Parts Discussed in Thread: MSP430F5419A

Hi,

We are using MSP430f5419A microcontroller for one of our applications and an external crystal of frequency 32.768Khz is connected to XT1 pin. Using FLL it is multipled to produce 25Mhz internally.39pF is used as the external load capacitances.

The problem is for 2 of our PCB boards it is working fine and for other 2,msp is not detecting external crystal.We have checked the connections and crystal part and it seems ok.

Kindly suggest possible reasons and solutions for this.

  • 0
    Guru 303830 points

    Read SLAA322 (PDF).

    Show the schematics and the board layout.

  • 0 in reply to rehana jose89
    TI__Intellectual 1330 points
    Hi,
    Like described in SLAA322, you should also shield your circuit path with a GND ring, not just the crystal itself. Probably there is some coupling from your Vcc line.

    Can you please also share which crystal type your are using? Is 39pF the recommended cap value for this kind? Did you considered the parasitic capacitance of your board and subtracted this value?

    Can you accurately measure the frequency on the two working boards, this will give me an indication if the value of the used caps are fine.

    Best regards,
    Tobias
  • 0
    Guru 25210 points
    39pF seems awfully high. Most crystals I have worked with need capacitance in the 10pF range. Check the datasheet for the crystal and do the calculations to determine the capacitors.
  • 0 in reply to Brian Boorman
    Guru 227245 points
    Agreed. Typical load capacitances are 9 or 12pF, which means 2*18 or 24pF including pin capacitance and stray capacitance on the wires. Usually, the internal capacitors can be used and no external ones are required at all. If both are there, the load increases even more, so the crystal frequency might be far off and the resulting oscillation amplitude might be too low to trigger the digital thresholds on the input driver.

    Due to tolerances in the analog parts, some boards may work and others won't when you're already close to the edge.

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