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placement of crystal/routing of lines

Hi - I'm working on laying out a board with a Atmel AT91SAM7X256 (ARM microcontroller). I'm working on placing the crystal. I have it in my head somewhere that you should always keep crystals as close as possible to the chip that they're connected to, and vias should be avoided as much as possible with them. Is there any validity to this, or is this just something that I made up? Any guidelines to the placement of them? If it matters the crystal is ~20 MHz. Also - is there anything wrong with placing the crystal directly underneath the chip? Thanks,

M. Noone

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M. Noone
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The lines from the crystal to the micro should be as short as possible. The main reason in my experience is to reduce the possibility of outside RF interfering with the processor oscillator. The crystal is very sensitive to external noise and if your design is to be subjected to a radiated RF immunity you may find that if your leads act like antenna the micro may act strange at certain RF test frequencies.

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Dan Hollands
1120 S Creek Dr
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Dan Hollands

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