Crystal Load Capacitance

? 17.5 pF

yout from the crystal to the SAM4 pin

to ground are needed.

t the oscillator and having tried it it seems to work fine

nternal load capacitor

I researched the issue of crystal selection with an Atmel MCU some years ag o and found not only issues with figuring out the required load capacitance , but just as important the crystal ESR. As a result of my conversations w ith the sales person the factory added an ESR spec to the data sheet, but t hey did it as a table for different frequencies with no info on how to inte rpolate the specified data. Still, that was a good start.

Regarding your problem, I can't quite follow all the data you provided. As suming you are interpreting the data correctly it does indeed seem that you need no additional capacitance. Just the same, I would recommend the addi tion of pads for adding capacitors, just in case.

Rick C.

- Tesla referral code

? 17.5 pF

layout from the crystal to the SAM4 pin

rs to ground are needed.

art the oscillator and having tried it it seems to work fine

internal load capacitor

ago and found not only issues with figuring out the required load capacitan ce, but just as important the crystal ESR. As a result of my conversations with the sales person the factory added an ESR spec to the data sheet, but they did it as a table for different frequencies with no info on how to in terpolate the specified data. Still, that was a good start.

Assuming you are interpreting the data correctly it does indeed seem that y ou need no additional capacitance. Just the same, I would recommend the ad dition of pads for adding capacitors, just in case.

rote:

:

?? 17.5 pF

e

k layout from the crystal to the SAM4 pin

tors to ground are needed.

start the oscillator and having tried it it seems to work fine

a internal load capacitor

s ago and found not only issues with figuring out the required load capacit ance, but just as important the crystal ESR. As a result of my conversatio ns with the sales person the factory added an ESR spec to the data sheet, b ut they did it as a table for different frequencies with no info on how to interpolate the specified data. Still, that was a good start.

Assuming you are interpreting the data correctly it does indeed seem that you need no additional capacitance. Just the same, I would recommend the addition of pads for adding capacitors, just in case.

What exactly did they agree with, that their data sheet is not clear and th ey would improve on it?

Rick C.

Tesla referral code -----

This discussion reminds me of when my home-built MC68HC11 oscillator wouldn't start. I'd built it according to the Pink Book, which showed two series capacitors as load - but the recommended value for each was the total series load expected. Or vice versa, I can't recall. I got an answer from the guy at Motorola who worked with the main author, who confirmed the error in the book. Anyhow I changed the capacitors and it fired up right away.

Clifford Heath.

s:

?? 17.5 pF

ce

ck layout from the crystal to the SAM4 pin

itors to ground are needed.

start the oscillator and having tried it it seems to work fine

n a internal load capacitor

ars ago and found not only issues with figuring out the required load capac itance, but just as important the crystal ESR. As a result of my conversat ions with the sales person the factory added an ESR spec to the data sheet, but they did it as a table for different frequencies with no info on how t o interpolate the specified data. Still, that was a good start.

d. Assuming you are interpreting the data correctly it does indeed seem th at you need no additional capacitance. Just the same, I would recommend th e addition of pads for adding capacitors, just in case.

d they would improve on it?

When you say "series capacitors" do you mean this?

|| |++| || o---||---||||---||---o || |++| ||

The caps should go to ground, in parallel with the crystal in essence but s eries with each other.

|++| o-+-----||||----+-o | |++| | | || || | +--||--+--||--+ || | || | --- V

Rick C.

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No

The series combination still presents a C load to the crystal, the fact that one node is earthed doesn't change that, because neither of the other two is earthed. I recall needing two 33pF caps (series load about 16pF), not two 18pF (series 9pF).

It was 23 years ago. The recommended crystal load was wrong anyhow, if you followed the Pink Book, and the oscillator wouldn't start. I can dig out the book and the facts if you care.

Clifford Heath.

te:

nts:

? 17.5 pF

ance

rack layout from the crystal to the SAM4 pin

acitors to ground are needed.

to start the oscillator and having tried it it seems to work fine

in a internal load capacitor

years ago and found not only issues with figuring out the required load cap acitance, but just as important the crystal ESR. As a result of my convers ations with the sales person the factory added an ESR spec to the data shee t, but they did it as a table for different frequencies with no info on how to interpolate the specified data. Still, that was a good start.

ded. Assuming you are interpreting the data correctly it does indeed seem that you need no additional capacitance. Just the same, I would recommend the addition of pads for adding capacitors, just in case.

and they would improve on it?

n t

ut series with each other.

I think we are saying the same thing.

I don't really need it as if I need an oscillator I usually use an oscillat or rather than a crystal. But it is an interesting subject if you are inte rested in sharing.

I spent a fairly large amount of time once researching crystals to try to g et to the derivation of the design issues. I found an HP paper that actual ly discussed the various crystal cuts in great detail. Not sure where it i s, but I likely could find it if I dig enough.

Rick C.

Tesla referral code ---+-

I looked, but couldn't find the Pink Book in the most likely places. I might have chucked it. The electronic version isn't easy to find online any more either. In any case, it wasn't very enlightening in terms of technical detail. It just had an incorrect schematic and text talking about crystal load capacitors. If I'd known then what I know now about crystals, I might not have believed it.

I still have my emails with the Motorola guy somewhere, but they've been archived so not directly searchable.

This?

Google is your friend(tm)

--
Best wishes, 
--Phil 
pomartel At Comcast(ignore_this) dot net

te:

rote:

ments:

? 17.5 pF

itance

track layout from the crystal to the SAM4 pin

apacitors to ground are needed.

h to start the oscillator and having tried it it seems to work fine

ed in a internal load capacitor

e years ago and found not only issues with figuring out the required load c apacitance, but just as important the crystal ESR. As a result of my conve rsations with the sales person the factory added an ESR spec to the data sh eet, but they did it as a table for different frequencies with no info on h ow to interpolate the specified data. Still, that was a good start.

vided. Assuming you are interpreting the data correctly it does indeed see m that you need no additional capacitance. Just the same, I would recommen d the addition of pads for adding capacitors, just in case.

r and they would improve on it?

d

as

an

it

but series with each other.

ad

ig

llator rather than a crystal. But it is an interesting subject if you are interested in sharing.

to get to the derivation of the design issues. I found an HP paper that ac tually discussed the various crystal cuts in great detail. Not sure where it is, but I likely could find it if I dig enough.

Yep, that's the one. Thanks.

Rick C.

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I think that was written by Rick Karlquist, designer of the HP 10811 series OCXOs. He's a former SED regular and is still quite active on the time-nuts list. He also did a super interesting direct frequency synthesizer based on cheap 455 kHz IF filters.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

A good book on crystal oscillators:

Crystal Oscillator Circuits, Robert J. Matthys John Wiley, 1983, ISBN 0-471-87401-9

Chapters on crystals, various oscillator circuits and plenty of practical examples. Not needed that often but good to have in the bookshelf. My copy came from abe books...

Chris

I've yet to see an analysis of digital oscillator circuits. I've seen one described but no real analysis of it. Performance is ok at lower frequencies (for undetermined values of "OK"), but no real measurements other than it was operable.

By "digital" I'm not talking about an inverter pressed into service as an amplifier. I mean a digital circuit mediated by software in the above case or it could be digital logic in other cases.

Rick C.

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You're hunting unicorns here ...

The digital frequency generation circuits, aided by software or not, are running behind some kind of clock. The clock is ultimately from an analog time/frequency reference part, e.g. a quartz crystal.

--

-TV

Microchip response. Incidentally Atmels development board has crystal with 22pf load capacitanc e, which does not meet the chip requirements of max 17pf

**** AUTOMATED MESSAGE - SEE RESPONSE METHODS BELOW ****

Below is a proposed resolution from Microchip Engineering Support Team for your Case 00352596.

Subject: Crystal

Reason: Hardware/Firmware Support

Product: ATSAM4SD32C

Problem Description:

I am using a ATSAM4SD32C processor. For the main clock crystal Atmel specifiey the following requirements: CLEXT Maximum External Capacitor on XIN and XOUT 17 pF Allowed Crystal Capacitance Load From crystal specification 12.5 ?

17.5 pF CLOAD Internal Equivalent Load Capacitance Integrated load capacitance (XIN and XOUT in series) 7.5min 9.5typical 10.5 pFmax

Calculation for Cload external

Where CPCB is the capacitance of the printed circuit board (PCB) track layo ut from the crystal to the SAM4 pin

If I use a 12pf load crystal it would appear that no external capacitors to ground are needed. Atmel give a max load capacitance of 17pf but no minimum. My pcb/pin tracks come in at about 2pf so I assume that is enough to start the oscillator and having tried it it seems to work fine Is this ok? I assume this is what they intended when they designed in a int ernal load capacitor Thanks

**** Proposed Resolution Begin ****

For the crystal capacitance used in this case(12 pF), with the typical load capacitance and a 2 pF CPCB, the CLEXT comes about 1 pF. Since the value of CLEXT is more or less negligible, this should work fine.

However, we do recommend to use a crystal load capacitance between 12.5 to

17.5 pF just to be on the safe side. **** Proposed Resolution End ******

I have Parzen's book of about the same vintage. A pretty good read.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

one described but no real analysis of it. Performance is ok at lower frequ encies (for undetermined values of "OK"), but no real measurements other th an it was operable.

an amplifier. I mean a digital circuit mediated by software in the above c ase or it could be digital logic in other cases.

You are assuming facts not in evidence... in other words, you are talking t hrough your hat. Drop all your misconceptions and think about it again. s oftware and logic do not need to be clocked.

If you were going to drive a crystal with a narrow pulse, what point in the cycle would it most beneficially be applied? Some of my thinking about th is was inspired by reading about the Shortt?Synchronome free pendul um clock. It uses a gravity arm to supply an impulse every thirty swings o f the main pendulum. I believe it is applied at the bottom of the swing. Whether that corresponds to the top of the crystal waveform or the midpoint depends on whether you consider position or velocity of the main pendulum.

I'm thinking the electrical analog would be the midpoint of the voltage sin ewave.

Rick C.

Tesla referral code ---++

So - you have the pendulum, which is equivalent to a resonator.

I like to quote Don Vonada, once the main designer at Digital Equipment Corp. 'All digital parts are made of analog components'.

The oscillators made of digital gates are using the analog delays in the gates a timing reference.

It is a completely different question, how the necessary energy feed to the timing reference is applied, like kicking the crystal.

--

-TV

n one described but no real analysis of it. Performance is ok at lower fre quencies (for undetermined values of "OK"), but no real measurements other than it was operable.

s an amplifier. I mean a digital circuit mediated by software in the above case or it could be digital logic in other cases.

ng through your hat. Drop all your misconceptions and think about it again . software and logic do not need to be clocked.

the cycle would it most beneficially be applied? Some of my thinking abou t this was inspired by reading about the Shortt?Synchronome free pe ndulum clock. It uses a gravity arm to supply an impulse every thirty swin gs of the main pendulum. I believe it is applied at the bottom of the swin g. Whether that corresponds to the top of the crystal waveform or the midp oint depends on whether you consider position or velocity of the main pendu lum.

sinewave.

There is no use of any timing delays of logic.

Very much like the Shortt clock. I suggest you read a bit about it. It is very interesting and educational.

In the short clock there are two pendulums to decouple the ancillary compon ents of the secondary pendulum from the free pendulum which actually contro ls the timing of the overall circuit. On detecting that the secondary pend ulum is at its mid-swing position, an electrical signal trips a relay to re lease a gravity arm which gives an even push to the free pendulum. Because the timing of the gravity arm is controlled by the free pendulum and not t he release relay the current can also be used to apply feedback to the seco ndary pendulum. Depending on whether the secondary pendulum is running too fast or too slow a spring is engaged to change the time constant for that one swing.

This complexity is not required in an electronic oscillator. The point is that the oscillator signal can be used to trigger a digital circuit to deli ver an impulse to the crystal to keep it running. I don't know much about the losses in a crystal circuit. I assume it can be self limiting in ampli tude, but this may work differently in a digital and analog circuit where t he amplifier loses gain when the signal reaches full swing. In a digital o scillator variation in amplitude corresponds to the energy stored in the cr ystal. I don't know this has any gain factors, but I suppose the Q of the crystal defines the losses and so the energy impulse required for a given l evel of output.

This is why I would like to find a book discussing such a circuit. I know these have been built, but I don't know if anyone has studied them.

Rick C.

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