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When there are two integrated circuits clocked on the same frequency it should be possible clock both ICs from a single oscillator. My understanding is that the first chip is connected to the external circuitry (crystal or resonator) as usually via two pins - first is output from oscillator (inverting amplifier) that drives the crystal/resonator and second is input to the oscillator (feedback). The second chip has the oscillator output bypassed and only the input is connected to the crystal or resonator.

How can I verify that both ICs can be clocked reliably like this? What problems might arise?

Thank you,

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  • Follow the components' datasheets advice about clocking and of using external clocks, if any.
  • XTALIN of an IC can almost always be driven by an external clock; in this case the XTALOUT of the other IC.
  • Check, that the controller, that has the crystal, doesn't have power saving modes that shut down the crystal oscillator, or that your design can accept the loss of clock of the other IC as well in that condition.
  • If the phase relationship of the clocks between the ICs is critical (usually is not), note that the clocks will be in opposite phases and that the clock signal used for the ICs' internal logic is usually XTALOUT, not XTALIN. But, as said, this is usually not a problem.
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You can verify the internal outputs when driving both inputs.

The two pins are just an unbuffered inverter (1 stage) with a 10MOhm or so feedback resistor that a crystal or ceramic resonator Pi filter with 180 deg phase shift can resonant with enough gain (>=10) to create a square wave out. The Resistor feedback inside self biases to Vcc/2 to create 50% duty cycle.

When you drive the CLK input externally , the internal feedback (is shunted by your active clock and) has no effect.

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