Forums

Crystal Load Capacitance

Started by steve November 12, 2018
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
CLEXT = 2 × (Ccrystal – CLOAD – CPCB).
Where CPCB is the capacitance of the printed circuit board (PCB) track layout 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 internal load capacitor
Thanks 

Steve Jones
On Monday, November 12, 2018 at 5:22:31 AM UTC-6, steve wrote:
> 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 > CLEXT = 2 × (Ccrystal – CLOAD – CPCB). > Where CPCB is the capacitance of the printed circuit board (PCB) track layout 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 internal load capacitor
I researched the issue of crystal selection with an Atmel MCU some years ago 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 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 interpolate the specified data. Still, that was a good start. Regarding your problem, I can't quite follow all the data you provided. 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. Rick C. https://ts.la/richard11209 - Tesla referral code
Thanks having contacted Microchip/Atmel they agreed!
On Monday, November 12, 2018 at 1:41:58 PM UTC, gnuarm.del...@gmail.com wrote:
> On Monday, November 12, 2018 at 5:22:31 AM UTC-6, steve wrote: > > 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 > > CLEXT = 2 × (Ccrystal – CLOAD – CPCB). > > Where CPCB is the capacitance of the printed circuit board (PCB) track layout 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 internal load capacitor > > I researched the issue of crystal selection with an Atmel MCU some years ago 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 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 interpolate the specified data. Still, that was a good start. > > Regarding your problem, I can't quite follow all the data you provided. 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. > > Rick C. > > https://ts.la/richard11209 - Tesla referral code
On Wednesday, November 14, 2018 at 9:33:35 AM UTC-6, steve wrote:
> Thanks having contacted Microchip/Atmel they agreed! > On Monday, November 12, 2018 at 1:41:58 PM UTC, gnuarm.del...@gmail.com wrote: > > On Monday, November 12, 2018 at 5:22:31 AM UTC-6, steve wrote: > > > 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 > > > CLEXT = 2 × (Ccrystal – CLOAD – CPCB). > > > Where CPCB is the capacitance of the printed circuit board (PCB) track layout 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 internal load capacitor > > > > I researched the issue of crystal selection with an Atmel MCU some years ago 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 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 interpolate the specified data. Still, that was a good start. > > > > Regarding your problem, I can't quite follow all the data you provided. 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. > > > > Rick C. > > > > https://ts.la/richard11209 - Tesla referral code
What exactly did they agree with, that their data sheet is not clear and they would improve on it? Rick C. Tesla referral code ----- https://ts.la/richard11209
On 15/11/18 4:48 am, gnuarm.deletethisbit@gmail.com wrote:
> On Wednesday, November 14, 2018 at 9:33:35 AM UTC-6, steve wrote: >> Thanks having contacted Microchip/Atmel they agreed! >> On Monday, November 12, 2018 at 1:41:58 PM UTC, gnuarm.del...@gmail.com wrote: >>> On Monday, November 12, 2018 at 5:22:31 AM UTC-6, steve wrote: >>>> 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 >>>> CLEXT = 2 × (Ccrystal – CLOAD – CPCB). >>>> Where CPCB is the capacitance of the printed circuit board (PCB) track layout 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 internal load capacitor >>> >>> I researched the issue of crystal selection with an Atmel MCU some years ago 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 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 interpolate the specified data. Still, that was a good start. >>> >>> Regarding your problem, I can't quite follow all the data you provided. 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. >>> >>> Rick C. >>> >>> https://ts.la/richard11209 - Tesla referral code > > What exactly did they agree with, that their data sheet is not clear and they would improve on it?
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.
On Wednesday, November 14, 2018 at 3:52:51 PM UTC-6, Clifford Heath wrote:
> On 15/11/18 4:48 am, gnuarm.deletethisbit@gmail.com wrote: > > On Wednesday, November 14, 2018 at 9:33:35 AM UTC-6, steve wrote: > >> Thanks having contacted Microchip/Atmel they agreed! > >> On Monday, November 12, 2018 at 1:41:58 PM UTC, gnuarm.del...@gmail.com wrote: > >>> On Monday, November 12, 2018 at 5:22:31 AM UTC-6, steve wrote: > >>>> 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 > >>>> CLEXT = 2 × (Ccrystal – CLOAD – CPCB). > >>>> Where CPCB is the capacitance of the printed circuit board (PCB) track layout 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 internal load capacitor > >>> > >>> I researched the issue of crystal selection with an Atmel MCU some years ago 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 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 interpolate the specified data. Still, that was a good start. > >>> > >>> Regarding your problem, I can't quite follow all the data you provided. 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. > >>> > >>> Rick C. > >>> > >>> https://ts.la/richard11209 - Tesla referral code > > > > What exactly did they agree with, that their data sheet is not clear and they would improve on it? > > 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.
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 series with each other. |++| o-+-----||||----+-o | |++| | | || || | +--||--+--||--+ || | || | --- V Rick C. Tesla referral code ----+ https://ts.la/richard11209
On 15/11/18 9:20 am, gnuarm.deletethisbit@gmail.com wrote:
> On Wednesday, November 14, 2018 at 3:52:51 PM UTC-6, Clifford Heath wrote: >> On 15/11/18 4:48 am, gnuarm.deletethisbit@gmail.com wrote: >>> On Wednesday, November 14, 2018 at 9:33:35 AM UTC-6, steve wrote: >>>> Thanks having contacted Microchip/Atmel they agreed! >>>> On Monday, November 12, 2018 at 1:41:58 PM UTC, gnuarm.del...@gmail.com wrote: >>>>> On Monday, November 12, 2018 at 5:22:31 AM UTC-6, steve wrote: >>>>>> 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 >>>>>> CLEXT = 2 × (Ccrystal – CLOAD – CPCB). >>>>>> Where CPCB is the capacitance of the printed circuit board (PCB) track layout 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 internal load capacitor >>>>> >>>>> I researched the issue of crystal selection with an Atmel MCU some years ago 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 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 interpolate the specified data. Still, that was a good start. >>>>> >>>>> Regarding your problem, I can't quite follow all the data you provided. 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. >>>>> >>>>> Rick C. >>>>> >>>>> https://ts.la/richard11209 - Tesla referral code >>> >>> What exactly did they agree with, that their data sheet is not clear and they would improve on it? >> >> 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. > > When you say "series capacitors" do you mean this? > > || |++| || > o---||---||||---||---o > || |++| ||
No
> The caps should go to ground, in parallel with the crystal in essence but series with each other.
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).
> |++| > o-+-----||||----+-o > | |++| | > | || || | > +--||--+--||--+ > || | || > | > --- > V > > Rick C. > > Tesla referral code ----+ https://ts.la/richard11209 >
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.
On Wednesday, November 14, 2018 at 4:28:37 PM UTC-6, Clifford Heath wrote:
> On 15/11/18 9:20 am, gnuarm.deletethisbit@gmail.com wrote: > > On Wednesday, November 14, 2018 at 3:52:51 PM UTC-6, Clifford Heath wrote: > >> On 15/11/18 4:48 am, gnuarm.deletethisbit@gmail.com wrote: > >>> On Wednesday, November 14, 2018 at 9:33:35 AM UTC-6, steve wrote: > >>>> Thanks having contacted Microchip/Atmel they agreed! > >>>> On Monday, November 12, 2018 at 1:41:58 PM UTC, gnuarm.del...@gmail.com wrote: > >>>>> On Monday, November 12, 2018 at 5:22:31 AM UTC-6, steve wrote: > >>>>>> 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 > >>>>>> CLEXT = 2 × (Ccrystal – CLOAD – CPCB). > >>>>>> Where CPCB is the capacitance of the printed circuit board (PCB) track layout 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 internal load capacitor > >>>>> > >>>>> I researched the issue of crystal selection with an Atmel MCU some years ago 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 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 interpolate the specified data. Still, that was a good start. > >>>>> > >>>>> Regarding your problem, I can't quite follow all the data you provided. 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. > >>>>> > >>>>> Rick C. > >>>>> > >>>>> https://ts.la/richard11209 - Tesla referral code > >>> > >>> What exactly did they agree with, that their data sheet is not clear and they would improve on it? > >> > >> 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. > > > > When you say "series capacitors" do you mean this? > > > > || |++| || > > o----||---||||---||---o > > || |++| || > > No > > > The caps should go to ground, in parallel with the crystal in essence but series with each other. > > 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).
I think we are saying the same thing.
> > |++| > > o--+-----||||----+-o > > | |++| | > > | || || | > > +--||--+--||--+ > > || | || > > | > > --- > > V > > > > Rick C. > > > > Tesla referral code ----+ https://ts.la/richard11209 > > > > 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.
I don't really need it as if I need an oscillator I usually use an oscillator rather than a crystal. But it is an interesting subject if you are interested in sharing. I spent a fairly large amount of time once researching crystals to try to get to the derivation of the design issues. I found an HP paper that actually discussed the various crystal cuts in great detail. Not sure where it is, but I likely could find it if I dig enough. Rick C. Tesla referral code ---+- https://ts.la/richard11209
On 15/11/18 10:57 am, gnuarm.deletethisbit@gmail.com wrote:
> On Wednesday, November 14, 2018 at 4:28:37 PM UTC-6, Clifford Heath wrote: >> On 15/11/18 9:20 am, gnuarm.deletethisbit@gmail.com wrote: >>> On Wednesday, November 14, 2018 at 3:52:51 PM UTC-6, Clifford Heath wrote: >>>> On 15/11/18 4:48 am, gnuarm.deletethisbit@gmail.com wrote: >>>>> On Wednesday, November 14, 2018 at 9:33:35 AM UTC-6, steve wrote: >>>>>> Thanks having contacted Microchip/Atmel they agreed! >>>>>> On Monday, November 12, 2018 at 1:41:58 PM UTC, gnuarm.del...@gmail.com wrote: >>>>>>> On Monday, November 12, 2018 at 5:22:31 AM UTC-6, steve wrote: >>>>>>>> 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 >>>>>>>> CLEXT = 2 × (Ccrystal – CLOAD – CPCB). >>>>>>>> Where CPCB is the capacitance of the printed circuit board (PCB) track layout 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 internal load capacitor >>>>>>> >>>>>>> I researched the issue of crystal selection with an Atmel MCU some years ago 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 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 interpolate the specified data. Still, that was a good start. >>>>>>> >>>>>>> Regarding your problem, I can't quite follow all the data you provided. 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. >>>>>>> >>>>>>> Rick C. >>>>>>> >>>>>>> https://ts.la/richard11209 - Tesla referral code >>>>> >>>>> What exactly did they agree with, that their data sheet is not clear and they would improve on it? >>>> >>>> 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. >>> >>> When you say "series capacitors" do you mean this? >>> >>> || |++| || >>> o----||---||||---||---o >>> || |++| || >> >> No >> >>> The caps should go to ground, in parallel with the crystal in essence but series with each other. >> >> 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). > > I think we are saying the same thing. > > >>> |++| >>> o--+-----||||----+-o >>> | |++| | >>> | || || | >>> +--||--+--||--+ >>> || | || >>> | >>> --- >>> V >>> >>> Rick C. >>> >>> Tesla referral code ----+ https://ts.la/richard11209 >>> >> >> 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. > > I don't really need it as if I need an oscillator I usually use an oscillator rather than a crystal. But it is an interesting subject if you are interested in sharing.
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.
> I spent a fairly large amount of time once researching crystals to try to get to the derivation of the design issues. I found an HP paper that actually discussed the various crystal cuts in great detail. Not sure where it is, but I likely could find it if I dig enough. >
On 11/14/2018 18:57, gnuarm.deletethisbit@gmail.com wrote:
> On Wednesday, November 14, 2018 at 4:28:37 PM UTC-6, Clifford Heath wrote: >> On 15/11/18 9:20 am, gnuarm.deletethisbit@gmail.com wrote: >>> On Wednesday, November 14, 2018 at 3:52:51 PM UTC-6, Clifford Heath wrote: >>>> On 15/11/18 4:48 am, gnuarm.deletethisbit@gmail.com wrote: >>>>> On Wednesday, November 14, 2018 at 9:33:35 AM UTC-6, steve wrote: >>>>>> Thanks having contacted Microchip/Atmel they agreed! >>>>>> On Monday, November 12, 2018 at 1:41:58 PM UTC, gnuarm.del...@gmail.com wrote: >>>>>>> On Monday, November 12, 2018 at 5:22:31 AM UTC-6, steve wrote: >>>>>>>> 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 >>>>>>>> CLEXT = 2 × (Ccrystal – CLOAD – CPCB). >>>>>>>> Where CPCB is the capacitance of the printed circuit board (PCB) track layout 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 internal load capacitor >>>>>>> >>>>>>> I researched the issue of crystal selection with an Atmel MCU some years ago 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 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 interpolate the specified data. Still, that was a good start. >>>>>>> >>>>>>> Regarding your problem, I can't quite follow all the data you provided. 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. >>>>>>> >>>>>>> Rick C. >>>>>>> >>>>>>> https://ts.la/richard11209 - Tesla referral code >>>>> >>>>> What exactly did they agree with, that their data sheet is not clear and they would improve on it? >>>> >>>> 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. >>> >>> When you say "series capacitors" do you mean this? >>> >>> || |++| || >>> o----||---||||---||---o >>> || |++| || >> >> No >> >>> The caps should go to ground, in parallel with the crystal in essence but series with each other. >> >> 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). > > I think we are saying the same thing. > > >>> |++| >>> o--+-----||||----+-o >>> | |++| | >>> | || || | >>> +--||--+--||--+ >>> || | || >>> | >>> --- >>> V >>> >>> Rick C. >>> >>> Tesla referral code ----+ https://ts.la/richard11209 >>> >> >> 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. > > I don't really need it as if I need an oscillator I usually use an oscillator rather than a crystal. But it is an interesting subject if you are interested in sharing. > > I spent a fairly large amount of time once researching crystals to try to get to the derivation of the design issues. I found an HP paper that actually discussed the various crystal cuts in great detail. Not sure where it is, but I likely could find it if I dig enough. > > > Rick C. > > Tesla referral code ---+- https://ts.la/richard11209 >
This? http://leapsecond.com/hpan/an200-2.pdf Google is your friend(tm) -- Best wishes, --Phil pomartel At Comcast(ignore_this) dot net