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very small clock sources?

Hi - I'm working on a board that I need to save absolutely every last square mm of space on. Normally I just use a crystal and two capacitors as a clock source for a microcontroller - but that actually takes up a good deal of board space. I looked at using all surface mount parts - but in the end - such parts will probabaly take up nearly as much board space as the AVR itself! (for some odd reason I couldn't find any small surface mount crystals)

So then I've been looking at using through hole crystals and probabaly surface mount caps - the crystal I'm looking at is 1.1mm pitch, so it would take up very little board space - definitely an improvement over the surface mount crystals (the ones I was looking at were 3x5mm)

But then I was looking in Eagle (a pcb design program) to see if they already had this crystal in the built in library. The only cylinder type had something very odd in it - there was a circle in the middle of the part showing the outline of the crystal. Then nearly tangent to the center circle was the two pads for the crystal. So it looked something like oOo if you can follow my crude drawing. What this means to me is that for some reason they felt the pins coming from the crystal needed to be seperated. This pretty much gets rid of the benefit of using the cylinder type crystal. Does anybody know why it would be drawn like this? Or would it be OK for me to have the pads with 1.1mm pitch? Are there any other really small 20Mhz clock sources?

Thanks!

-Michael J. Noone

Reply to
Michael Noone
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Joerg wrote in news:wAiAe.247$_% snipped-for-privacy@newssvr14.news.prodigy.com:

It is possible to run the uC without an external clock source - but the internal clock sources are slower and less accurate than what I need, unfortunately.

This Yoketan crystals definitely are small - but can they still be soldered by hand? They look alot like some of the other crystals I looked at in that it appears the pads are entirely isolated on the bottom of the crystal - so that it would be impossible to solder by hand without a hot air station.

Thanks,

-Michael

Reply to
Michael Noone

It's pretty common to separate very close-spaced leads on through-hole components (such as TO-92). It keeps the part from getting too close to the board, for one thing (the cylinder crystals probably have a hermetic glass-metal seal between the leads and the base), which could be affected by the thermal shock of soldering (but probably won't be) and it prevents sideways pressure from putting enormous tension on the rather thin leads. It also allows reasonable size pads, which is less of a factor with plated-through holes.

If you can space it off a bit, and find some way to keep the package in place if there is vibration, I don't see other any reason to use that footprint. Just make your own. A lot of the footprints included with PCB layout packages are sub-optimal in various ways.

For reliability, I'd worry a lot more about drive power issues in this case.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Hello Michael,

There are very tiny SAW filters but I don't know whether they are already commercially made:

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Check Yoketan in Taiwan. Some resonators look like you wouldn't ever find them after dropping them on the floor. Certainly much smaller than the 3mm by 5mm you had. Examples in the 2mm range:

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Question: Can't you run the uC free running without any external clocks, crystals or resonators? IIRC that can be done with some of the MSP430.

Regards, Joerg

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Reply to
Joerg

The electrical characteristics of what you describe aren't all that different than they would be with the holes closer - as long as you don't run the traces parallel from there. Could it be that whoever made that Eagle part was concerned about the case shorting the traces?

I wouldn't trust the Eagle pad layout to answer this question if I were you. I would look to the datasheets for the Crystal and uC first to see if they shed any light on the subject, then I would try a "parts hanging in the air" breadboard and try adding a small capacitance between the crystal pins to see if there is a problem.

--
Guy Macon
Reply to
Guy Macon

On Sun, 10 Jul 2005 18:40:20 -0500, Michael Noone wroth:

How about Linear's LTC6905? One SOT-23 and an 0402 sized frequency setting resistor. You can probably share the bypass capacitor of something already on the board.

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Jim

Reply to
jmeyer

Hello Michael,

That will be a problem with all the small stuff. It's all SMT without real pins. If you mount them sideways and lay out the board accordingly you can solder these by hand. But keep in mind that these are not crystals. They are resonators, meaning much higher tolerances.

Regards, Joerg

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Reply to
Joerg

Try:

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Do you need crystal accuracy?. If not, some ceramic resonators come up smaller than crystals. If you can accept even lower accuracies, a resistor, and capacitor can be used as an oscillator. This is why on some of the PIC products, the chips have internal calibrated oscillators. Not 'crystal' accuracies, but good enough for many applications. Are there perhaps AVR units with similar abilities?. There are mechanical limits on crystal sizes. This is exactly the same reason why larger value capacitors are bulky. The smallest packages I know of, are:
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The same manufacturer also does a version that is slightly shorter, and wider (4mm*2.5mm).

Best Wishes

Reply to
Roger Hamlett

Don't forget you can have double-sided surface mount.

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John Devereux
Reply to
John Devereux

[snip]

Very small crystals are made, for example for use in mobile phones. Getting samples of these might be a challenge.

Example:

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Reply to
Chris Jones

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