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Internal oscillator loading cap calculation questions

ScottBBBB

I'm having some difficulty understanding what the nRF53 datasheet is trying to tell us about the internal load capacitors and related items.  We are using both the LFXO and HFXO with external crystals.  The target load for the LFXO crystal is 9 pF and for the HFXO it is 8 pF.  The datasheet specifies them differently, which doesn't help with clarity.

LFXO:

There are two specs for Cpin: 4 pF when internal cap is disabled, and 6 to 9 pF "between" (i.e. across) the pads when enabled.  Questions:

1. Is the 4 pF to ground, and is it on each pad?  I assume yes to both.

2. Are the two values cumulative, i.e. the 4 pF is still there on each pin when the internal cap is enabled?

HFXO:

For this one there are Cpin parameters for when the internal cap is disabled (5 pF) and when enabled (2.5 pF). Questions:

1. If we have the internal caps enabled, we'd include in the calculation 2.5 pF on each pin, correct?

2. There is an internal capacitor on each pad, to ground, unlike for the LFXO which has one capacitor across the two pads, correct?

Thanks,

Scott

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  • +1

    Hi Scott,

    1. Is the 4 pF to ground, and is it on each pad?  I assume yes to both.

    Yes, the 4pF input capacitance for each pin to ground when the internal capacitor is disabled.

    2. Are the two values cumulative, i.e. the 4 pF is still there on each pin when the internal cap is enabled?

    When the internal capacitor is enabled the selected capacitance, CINT, is the capacitance seen across XL1 and XL2 pins , including the pin capacitance. This pin capacitance is not necessarily 4pF when the internal capacitor is enabled.

    1. If we have the internal caps enabled, we'd include in the calculation 2.5 pF on each pin, correct?

    Yes, the pin capacitance should be included in the calculations. The capacitance required for a given crystal CL can be calculated using:

    C=2CL-CPIN

    Where in this case CPIN = 2.5pF.

    2. There is an internal capacitor on each pad, to ground, unlike for the LFXO which has one capacitor across the two pads, correct?

    Yes, this is correct.

     

    Best regards,

    Bendik

Reply
  • +1

    Hi Scott,

    1. Is the 4 pF to ground, and is it on each pad?  I assume yes to both.

    Yes, the 4pF input capacitance for each pin to ground when the internal capacitor is disabled.

    2. Are the two values cumulative, i.e. the 4 pF is still there on each pin when the internal cap is enabled?

    When the internal capacitor is enabled the selected capacitance, CINT, is the capacitance seen across XL1 and XL2 pins , including the pin capacitance. This pin capacitance is not necessarily 4pF when the internal capacitor is enabled.

    1. If we have the internal caps enabled, we'd include in the calculation 2.5 pF on each pin, correct?

    Yes, the pin capacitance should be included in the calculations. The capacitance required for a given crystal CL can be calculated using:

    C=2CL-CPIN

    Where in this case CPIN = 2.5pF.

    2. There is an internal capacitor on each pad, to ground, unlike for the LFXO which has one capacitor across the two pads, correct?

    Yes, this is correct.

     

    Best regards,

    Bendik

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