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 Post subject: SAMA5D31 Crystal Layout + Load Capacitance
PostPosted: Wed Jun 07, 2017 4:39 pm 
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Joined: Wed Jun 07, 2017 4:32 pm
Posts: 4
Hello,

I am looking for some guidance on the layout of the SAMA5D31 external oscillator. I have selected a 48MHz Crystal and I have a few questions.

- Should there be a ground plane beneath it and around it?
- Is a grounding ring a good idea tied to GND_PLL?
- I have estimated the parasitics as per below

    Cload = 2*(Cpara) + Cpackage + Cboard + Crouting
    Cpara - From CPU datasheet, I think this is internal to the chip, it is specified in the table as between 0.6-0.8pF, with 0.7pF typical. Let's take it as Cpara = 0.7pF.
    Cpackage - Due to package and bonding.. I think this also has to do with the CPU and its mounting to the board? Typical value of Cpackage = 0.75pF
    Crouting - Internal routing in the chip itself, typical value Crouting = 1.5pF
    Cboard - I think this is where our trace lengths come into play. As it stands our total trace length is now 16.137mm. I will explain this calculation out in further detail below.
    Cload = 4.347pF (Crystal is expecting 6pF, and CPU datasheet wants between 4-6pF).
    Cboard Calculation
    Trace Width - 0.00013m
    Height Above Return Path - Estimated at 0.0015748m
    Thickness - 0.00007112m (2oz Copper)
    Relative Permittivity (FR-4) = 4.4
    Trace Length - 0.016137m
    Calculator: https://technick.net/tools/impedance-ca ... icrostrip/
    Result - 43.21pF/m
    Cboard = 0.697pF

Thanks for your help,

Kai


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 Post subject: Re: SAMA5D31 Crystal Layout + Load Capacitance
PostPosted: Fri Jun 09, 2017 2:50 am 
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Joined: Wed Jun 07, 2017 4:32 pm
Posts: 4
Bump! Anyone able to provide some insight? Is any information missing?


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 Post subject: Re: SAMA5D31 Crystal Layout + Load Capacitance
PostPosted: Mon Jun 12, 2017 2:26 am 
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Joined: Thu Apr 19, 2007 10:15 pm
Posts: 1432
Location: USA
kairaspo wrote:
- Should there be a ground plane beneath it and around it?
- Is a grounding ring a good idea tied to GND_PLL?
Answers are in the application note SAMA5D3 Layout Recommendations.

Regards


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 Post subject: Re: SAMA5D31 Crystal Layout + Load Capacitance
PostPosted: Sat Jun 17, 2017 9:15 pm 
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Joined: Wed Jun 07, 2017 4:32 pm
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Hi there,

In the layout sheet it refers specifically to the XIN/XOUT pins for the 32kHz crystal. I am looking for layout recommendations for a higher frequency 48MHz Crystal on the XIN2/XOUT2 pins. Does anyone know where I can find this information? Or is this information generic to both?

Thanks,

Kai


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 Post subject: Re: SAMA5D31 Crystal Layout + Load Capacitance
PostPosted: Mon Jun 19, 2017 8:58 pm 
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Joined: Thu Apr 19, 2007 10:15 pm
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kairaspo wrote:
In the layout sheet it refers specifically to the XIN/XOUT pins for the 32kHz crystal.
To what "layout sheet" are you referring?

Regards


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 Post subject: Re: SAMA5D31 Crystal Layout + Load Capacitance
PostPosted: Wed Jun 21, 2017 9:01 pm 
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blue_z wrote:
kairaspo wrote:
In the layout sheet it refers specifically to the XIN/XOUT pins for the 32kHz crystal.
To what "layout sheet" are you referring?

Regards


I was referring to the layout recommendations you posted previously.

Thanks


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