SAMA5D31 Crystal Layout + Load Capacitance

Discussion around products based on ARM Cortex-A5 core.

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kairaspo
Posts: 5
Joined: Wed Jun 07, 2017 4:32 pm

SAMA5D31 Crystal Layout + Load Capacitance

Wed Jun 07, 2017 4:39 pm

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
kairaspo
Posts: 5
Joined: Wed Jun 07, 2017 4:32 pm

Re: SAMA5D31 Crystal Layout + Load Capacitance

Fri Jun 09, 2017 2:50 am

Bump! Anyone able to provide some insight? Is any information missing?
blue_z
Location: USA
Posts: 1500
Joined: Thu Apr 19, 2007 10:15 pm

Re: SAMA5D31 Crystal Layout + Load Capacitance

Mon Jun 12, 2017 2:26 am

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
kairaspo
Posts: 5
Joined: Wed Jun 07, 2017 4:32 pm

Re: SAMA5D31 Crystal Layout + Load Capacitance

Sat Jun 17, 2017 9:15 pm

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
blue_z
Location: USA
Posts: 1500
Joined: Thu Apr 19, 2007 10:15 pm

Re: SAMA5D31 Crystal Layout + Load Capacitance

Mon Jun 19, 2017 8:58 pm

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
kairaspo
Posts: 5
Joined: Wed Jun 07, 2017 4:32 pm

Re: SAMA5D31 Crystal Layout + Load Capacitance

Wed Jun 21, 2017 9:01 pm

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
blue_z
Location: USA
Posts: 1500
Joined: Thu Apr 19, 2007 10:15 pm

Re: SAMA5D31 Crystal Layout + Load Capacitance

Thu Jun 22, 2017 8:06 pm

Where in that app note do you think "it refers specifically to the XIN/XOUT pins for the 32kHz crystal"?

Regards
kairaspo
Posts: 5
Joined: Wed Jun 07, 2017 4:32 pm

Re: SAMA5D31 Crystal Layout + Load Capacitance

Mon Jun 26, 2017 5:04 pm

Hi, I am really just looking for a "technical" answer here. What is the reason for putting a ground plane beneath the crystal? Would it not just create board capacitance that would otherwise be eliminated without it?

The datasheet is not clear whether or not it applies to higher "MHz" crystals, it seems it only applies to the 32kHz crystal it mentions.

As well, does the grounding ring suggested in the layout recommendations actually work? I have seen mixed reviews in textbooks etc.

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