placement of crystal/routing of lines

[M. Noone]

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

 

[Dan Hollands]

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.

--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606
[email protected]
www.QuickScoreRace.com