I/O pins and Crystals

Philldapill · 2009-01-02 03:16

Just curious about routing traces on a PCB that come very close the the crystal. I'm designing a PCB for the propeller, and it would be great if I could move some traces closer to the crystal. My traces will be on the opposite side of the board as the crystal, but is there any sort of EM interference between the crystal and traces that could be a problem?

Oldbitcollector (Jeff) · 2009-01-02 03:30

I'm not an EE, but as I understand it the Crystal needs be as close as
possible to the pins to insure accurate function.

EE's correct me if I'm wrong here..

OBC

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mynet43 · 2009-01-02 03:36

I've built quite a few boards and never had a problem with the xtal location.

I attached a pic with the xtal pretty close to the Red VGA line.

Jim

Philldapill · 2009-01-02 03:40

Oh, the crystal itself is very close. I think total trace lengths are less than 1/2", probably closer to 1/4". I was just concerned about any sort of "cross-talk" scenario where a trace has alot of AC through it, e.g. an output toggling very quickly. Now that I've thought about it more, I doubt there would be any problem...

mirror · 2009-01-02 04:24

Jim,

That's a nice little crystal you've got. What is it?

The most practical crystal I've found in in an HC49 package which is about as long as the QFP propeller chip.

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Leon · 2009-01-02 04:24

You might be thinking about high-speed clock signals, which can couple energy into adjacent tracks.

Leon

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mynet43 · 2009-01-02 04:31

@mirror

Here's a pic of the XTAL, with the specs and P/N.

The body of the xtal is a little less than 3/8" long.

I've used it on several boards and it's great!

Jim

mirror · 2009-01-02 07:00

Jim,

Thanks for the info, it now makes much more sense.

The crystal is still as large as the ones that I use, it just gets to stand vertically instead of horizontally.

For the record I use DigiKey Part # 535-9087 which is a ABLS-5.000MHz-B2-T type crystal.

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Philldapill · 2009-01-02 08:17

Leon, yes, that is the sort of thing I was worried about. There's always the possibility that a highspeed device might be attached, which uses a high freq. clock signal. One particular device I have in mind is a TLV2543. It's an ADC that uses a 4MHz signal from an I/O pin. Let's say that this clock pin is the one that is riiiight next to the leads of the crystal(0.025"). Could this mess with the operation of the crystal? If so, I have some major re-routing to do...... ugh.

Leon · 2009-01-02 09:25

You shouldn't get any problems at that frequency. Running the track at right angles to the crystal tracks will minimise coupling, anyway. Or use another output.

Leon

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mynet43 · 2009-01-02 14:31

mirror,

Sorry to carry on two threads here.

Thanks for the info on the xtal you use. I like it much better than the usual through-hole type. I may order some.

You said they're about the same size, however, the Digikey part is about 45% bigger than the Newark part (0.5"/0.346").

The reason I like the Newark part is its very small footprint when you're trying to fit a lot on a little board.

The footprint is a circle 3 mm across, about 0.1", which isn't much space.

I may use yours for some apps because it's so close to the board, which I like.

Jim

I/O pins and Crystals

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