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very small clock sources?

M

Michael Noone

Jan 1, 1970
0
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
 
J

Joerg

Jan 1, 1970
0
Hello Michael,

There are very tiny SAW filters but I don't know whether they are
already commercially made:
http://www.ieee-uffc.org/archive/uffc/trans/Toc/abs/02/t0291269.htm

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:
http://crystal-oscillator.electronic.com.tw/ceramic-resonator/page17.htm

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
 
G

Guy Macon

Jan 1, 1970
0
Michael said:
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?

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.
 
M

Michael Noone

Jan 1, 1970
0
Hello Michael,

There are very tiny SAW filters but I don't know whether they are
already commercially made:
http://www.ieee-uffc.org/archive/uffc/trans/Toc/abs/02/t0291269.htm

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:
http://crystal-oscillator.electronic.com.tw/ceramic-resonator/page17.htm

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

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
 
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)

-Michael J. Noone

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.

http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1010,C1096,P7888

Jim
 
J

Joerg

Jan 1, 1970
0
Hello Michael,
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.

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
 
S

Spehro Pefhany

Jan 1, 1970
0
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

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
 
J

John Devereux

Jan 1, 1970
0
Michael Noone said:
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)


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

Roger Hamlett

Jan 1, 1970
0
John Devereux said:
Don't forget you can have double-sided surface mount.
Try:
http://www.cmac.com/mt/databook/oscillators/zarlink/e3179.pdf
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:
http://www.smi-xtal.com/pdf/52smx.pdf
The same manufacturer also does a version that is slightly shorter, and
wider (4mm*2.5mm).

Best Wishes
 
C

Chris Jones

Jan 1, 1970
0
Michael said:
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) [snip]
-Michael J. Noone

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

Example:

http://www.avxcorp.com/docs/Catalogs/ksx.pdf
 
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