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We have an oscillator providing a clock signal for an MCU. It has oscillator in and out pins: XTAL-IN and XTAL-OUT.

We'd like to use the same clock signal as a digital clock input to another MCU. Can I simply tap the XTAL-IN signal and use that directly as the input to the other MCU, or will this interfere with the PLL?

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This can be done but it requires extremely careful analysis to make it work reliably.

You should generally tap your signal off the XTAL-OUT (a.k.a X2) pin as this is the output of the on chip oscillator circuit and drives a higher signal amplitude and the driver will have lower node impedance. This is particularly important if the XTAL-IN (a.k.a X1) and XTAL-OUT pins are connected to an actual crystal. Added capacitance, particularly at X1 pin, can change the operating frequency of the oscillator circuit or prevent it from starting up reliably.

Tapping either the X1 or X2 pins requires consideration that these are analogue signals and do not normally traverse between full digital logic levels. The tap circuit load needs to be high impedance and needs to be able to translate the low level analog signals to the type of signal needed for the load at the next circuit in line.

If you instead are driving the XTAL-IN pin with some type of external oscillator device it may already be providing a full logic swing signal. In that case the external device may very well be quite capable of driving to the XTAL-IN pins of multiple MCUs with no problems at all.

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In our case, the second MCU can't receive and oscillator- it has a digital clock input only. Could I connect XTAL-OUT to this? – user48956 Sep 18 '12 at 17:18
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@user48956 - It may be possible to connect it but you will really have to look closely at what the voltage signal range is on the XTAL-OUT pin is. Almost all of them that I've looked at when a crystal is connected do not have a signal that transitions as a reliable digital signal. You would thus probably need to find a buffer that can square up the signal and produce an output that is a legitimate digital signal for the 2nd MCU. - Continued - – Michael Karas Sep 19 '12 at 11:44
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- From Continued - If you are going to have to end up adding a chip in order to buffer the XTAL-OUT I would recommend that an overall better solution may be to just add a standalone oscillator to the design that has a good digital output and connect that to both of the MCUs. -------- You have not given much information related to the actual parts that you are using and frequency of operation so it is not feasible to recommend possible components that you could use for either approach. – Michael Karas Sep 19 '12 at 11:49

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