Crystal Load Capacitance

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I am using a Atmel4SD32C 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 t
o 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  

Steve Jones

Re: Crystal Load Capacitance
On Monday, November 12, 2018 at 5:22:31 AM UTC-6, steve wrote:
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? 17.5 pF
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yout from the crystal to the SAM4 pin
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 to ground are needed.
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t the oscillator and having tried it it seems to work fine
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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.  

https://ts.la/richard11209 - Tesla referral code

Re: Crystal Load Capacitance
Thanks having contacted Microchip/Atmel they agreed!
On Monday, November 12, 2018 at 1:41:58 PM UTC, snipped-for-privacy@gmail.com wro
te:
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? 17.5 pF
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layout from the crystal to the SAM4 pin
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rs to ground are needed.
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art the oscillator and having tried it it seems to work fine
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 internal load capacitor
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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.  
Quoted text here. Click to load it
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.  
Quoted text here. Click to load it


Re: Crystal Load Capacitance
On Wednesday, November 14, 2018 at 9:33:35 AM UTC-6, steve wrote:
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rote:
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:  
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?? 17.5 pF
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e

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k layout from the crystal to the SAM4 pin
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tors to ground are needed.
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start the oscillator and having tried it it seems to work fine
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 a internal load capacitor
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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.  
Quoted text here. Click to load it
  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.  
Quoted text here. Click to load it

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 ----- https://ts.la/richard11209

Re: Crystal Load Capacitance
On 15/11/18 4:48 am, snipped-for-privacy@gmail.com wrote:
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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.

Re: Crystal Load Capacitance
On Wednesday, November 14, 2018 at 3:52:51 PM UTC-6, Clifford Heath wrote:
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m wrote:
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s:
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?? 17.5 pF
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ce

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ck layout from the crystal to the SAM4 pin
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itors to ground are needed.
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 start the oscillator and having tried it it seems to work fine
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n a internal load capacitor
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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.
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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.
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d they would improve on it?
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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.

  Tesla referral code ----+ https://ts.la/richard11209

Re: Crystal Load Capacitance
On 15/11/18 9:20 am, snipped-for-privacy@gmail.com wrote:
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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).

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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.

Re: Crystal Load Capacitance
On Wednesday, November 14, 2018 at 4:28:37 PM UTC-6, Clifford Heath wrote:
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te:
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com wrote:
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nts:
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? 17.5 pF
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ance

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rack layout from the crystal to the SAM4 pin
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acitors to ground are needed.
Quoted text here. Click to load it
to start the oscillator and having tried it it seems to work fine
Quoted text here. Click to load it
 in a internal load capacitor
Quoted text here. Click to load it
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.
Quoted text here. Click to load it
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.
Quoted text here. Click to load it
and they would improve on it?
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n
t
ut series with each other.
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I think we are saying the same thing.  


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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 ---+- https://ts.la/richard11209

Re: Crystal Load Capacitance
On 15/11/18 10:57 am, snipped-for-privacy@gmail.com wrote:
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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.

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Re: Crystal Load Capacitance
On 11/14/2018 18:57, snipped-for-privacy@gmail.com wrote:
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This? http://leapsecond.com/hpan/an200-2.pdf Google is your friend(tm)


--  
Best wishes,
--Phil
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Re: Crystal Load Capacitance
On Thursday, November 15, 2018 at 10:18:51 AM UTC-5, Phil Martel wrote:
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te:
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rote:
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l.com wrote:
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ments:
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 ? 17.5 pF
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itance

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 track layout from the crystal to the SAM4 pin
Quoted text here. Click to load it
apacitors to ground are needed.
Quoted text here. Click to load it
h to start the oscillator and having tried it it seems to work fine
Quoted text here. Click to load it
ed in a internal load capacitor
Quoted text here. Click to load it
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.
Quoted text here. Click to load it
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.
Quoted text here. Click to load it
r and they would improve on it?
Quoted text here. Click to load it
d
as
 an
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 it
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 but series with each other.
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ad
ig
llator rather than a crystal.  But it is an interesting subject if you are  
interested in sharing.
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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.
Quoted text here. Click to load it

Yep, that's the one.  Thanks.  

 Rick C.  

 Tesla referral code +-+ https://ts.la/richard11209

Re: Crystal Load Capacitance
On 11/15/18 2:04 PM, snipped-for-privacy@gmail.com wrote:
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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
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Re: Crystal Load Capacitance
On 11/15/18 19:38, Phil Hobbs wrote:
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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


Re: Crystal Load Capacitance
On Sunday, November 18, 2018 at 7:01:00 PM UTC-6, Chris wrote:
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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.  

  Tesla referral code - https://ts.la/richard11209

Re: Crystal Load Capacitance
On 19.11.18 05:30, snipped-for-privacy@gmail.com wrote:

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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

Re: Crystal Load Capacitance
On Monday, November 19, 2018 at 4:01:56 AM UTC-5, Tauno Voipio wrote:
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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.
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an amplifier.  I mean a digital circuit mediated by software in the above c
ase or it could be digital logic in other cases.
Quoted text here. Click to load it

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 ---++ https://ts.la/richard11209

Re: Crystal Load Capacitance
On 20.11.18 02:24, snipped-for-privacy@gmail.com wrote:
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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


Re: Crystal Load Capacitance
On Tuesday, November 20, 2018 at 4:21:23 AM UTC-5, Tauno Voipio wrote:
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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.
Quoted text here. Click to load it
s an amplifier.  I mean a digital circuit mediated by software in the above
 case or it could be digital logic in other cases.
Quoted text here. Click to load it
ng through your hat.  Drop all your misconceptions and think about it again
.  software and logic do not need to be clocked.
Quoted text here. Click to load it
 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.
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 sinewave.
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There is no use of any timing delays of logic.  


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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.  

  Tesla referral code --+-- https://ts.la/richard11209

Re: Crystal Load Capacitance
On 11/18/18 8:00 PM, Chris wrote:
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I have Parzen's book of about the same vintage.  A pretty good read.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Crystal Load Capacitance

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 ******


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