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Is it possible to drive the on-chip oscillator of LPC1114FN28 using the clock output of a FOX924B TCXO? The Datasheet of the uC indicates that a 100 pF cap needs to be put between the oscillator and the XTALIN pin with an additional cap connected to grnd to keep the voltage limited to 1.8V. Considering that output load as per the FOX924B datasheet is limited to 25pF, the 100 pF cap recommended by NXP seems too much for the TCXO. Am I right?

Also, why can't I simply use a resistor based voltage divider to bring the TCXO output to 1.8V?

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  1. Since the 100 pF series cap between the TCXO and the uC input pin is in series with the (presumably) high-impedance input pin of the uC, that by itself does not violate the 15 pF output load limit of the TCXO.

  2. However if you try to make a capacitive divider to limit the uC input voltage to 1.8 V, you should calculate the equivalent capacitance of the two capacitors in series and compare that with the 15 pF limit.

  3. If you have a few extra pennies and enough board space, consider just finding a 1.8 V buffer with 3.3-V tolerant inputs and put that between your TCXO and the uC (for example 74AUC1G07 74AUC1G04).

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    \$\begingroup\$ I wouldn't use an '07 though, since you would probably want a push-pull output in order to actively drive the clock line at all times. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Feb 27, 2016 at 19:34
  • \$\begingroup\$ @Photon, Thanks!, yes considering the two caps to be in series will never match the 25 pF limit of the FOX924B. Your (3) is nice, although it will require a ~1.8V power source to pull up the output of 74AUC1G07. \$\endgroup\$
    – Abhishek
    Commented Feb 27, 2016 at 19:50
  • \$\begingroup\$ @Ignacio, Can you clarify please? \$\endgroup\$
    – Abhishek
    Commented Feb 27, 2016 at 19:50
  • \$\begingroup\$ And, why can't I use a simple resistive voltage divider? \$\endgroup\$
    – Abhishek
    Commented Feb 27, 2016 at 19:56
  • \$\begingroup\$ @Photon, I checked the datasheet of 74AUC1G07 (ti.com/lit/gpn/sn74auc1g07) the recommended input rise and fall times for Vcc of 3.3V is 100 ns/V, for a swing of 3.3V, the input frequency can be no more than (1/(3.3*100)) about 3 MHz. The TCXO I wish to use is a 10 MHz piece. So I guess 74AUC1G07 won't work. \$\endgroup\$
    – Abhishek
    Commented Feb 27, 2016 at 20:53

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