nRF52840 LFXO

I don't fully understand datasheet for LFXO.

17.4.3

CL_LFXO Load Capacitance = 12.5pF. Is this the recommended CL for external crystal?

RS_LXFO ESR = 100k. Is this the max ESR in crystal OR is this a ESR on the chip?

PD_LFXO = 1uW. Is this the recommended minimum Drive Level for crystal OR is this max outout power to crystal?

ILFXO = 0.23uA. Is this the current for silicon driving the crystal or is this the max output current to crystal?

I must be sure to not "overdrive" crystal. The crystals Drive Level I'm thinking of can be 0.1uW.

Kan anybody enlighten me because the datasheet it's not obvious to me.

Parents
  • It seems that the specification "talk's" about the recommended crystal, not driveability of the driver.

    I thought that PD_LXFO of 1uW is drive capability but maybe it's recommended DL of the crystal.

    I assume then that CL_LFXO, RS_LXFO and PD_LXFO is only recommendations to select a crystal...

    Reason for my question is that I have a crystal with DL of 0.5uW. I must be sure not to exceed 0.5uW. Best would be 0.1uW.

    One way is to select crystal with lower CL which gives lower current.

    I

Reply
  • It seems that the specification "talk's" about the recommended crystal, not driveability of the driver.

    I thought that PD_LXFO of 1uW is drive capability but maybe it's recommended DL of the crystal.

    I assume then that CL_LFXO, RS_LXFO and PD_LXFO is only recommendations to select a crystal...

    Reason for my question is that I have a crystal with DL of 0.5uW. I must be sure not to exceed 0.5uW. Best would be 0.1uW.

    One way is to select crystal with lower CL which gives lower current.

    I

Children
  • Terho said:
    Reason for my question is that I have a crystal with DL of 0.5uW. I must be sure not to exceed 0.5uW. Best would be 0.1uW.

    Hi,

    You just have to make sure you don't exceed the maximum values. For the drive level, the maximum level the oscillator can drive the crystal is 0.5 µW. So the crystal must be able to handle at least this. If the crystal has a higher max tolerated drive strength, it can still be used with our chip.

    Best regards,

    Marjeris

  • Sorry but I looked in Product Specification v0.5.1. where PD_LXFO is 1uW. In v1.2 PD_LXFO is changed to 0.5uW.

    PD_LXFO is drive capability from oscillator to crystal and is 0.5uW.

    ILFXO (run current) is the current to oscillator (0.23uA). Is the current through crystal included in this?

    I guess if I select a crystal with "low" CL I can reduce DL lower than 0.5uW. And use crystal with DL 0.5uW max.

    Best Regards

    Terho

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