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Pretty simple question, please point me in the right direction if this has already been asked.

I am designing a board around freescale's IMX23 microprocessor which runs from an external crystal at 24 MHz. I know that I need to keep the crystal close to the proc, but does it matter if the crystal is on the other side of the board (4 layers)? I know that for routing DRAM traces you want to avoid vias like the plague but would I be okay putting the crystal directly underneath the processor? The traces would be extremely short, but the vias would pass through a GND and VDD plane. The alternative is placing the crystal slightly farther from the proc, about 250 mil.

Thanks.

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    \$\begingroup\$ Are you using a crystal or a crystal oscillator? \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 15 '15 at 16:13
  • \$\begingroup\$ *Crystal sorry. I'll edit \$\endgroup\$ – Optox Jun 15 '15 at 18:03
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Either will work out fine. The XTAL is a high Q BPF so it does a good job of rejecting noise in most cases. You do want to try and limit high edge rate signals adjacent to those traces where you can, though. The XTAL also has very low emissions so won't induce noise into your planes. I've done it both ways and never noticed any difference in SI one way or the other. Place it where it makes sense for the remainder of your design.

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