SAM E70 crystal / load cap selection problems

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Hi all,

I am using this 12 MHz crystal (X322512MSB4SI) as an input to the ATSAME70J21B (LQFP 64 pin variant). However, my program hangs on startup after switching to the oscillator. I am sure the crystal is the problem as everything is working fine with the internal RC oscillator.

I suspect the load capacitor selection or PCB layout is the issue. 

PCB and schematic are shown below. I would appreciate if anyone has some wisdom they can share on this matter.

 

Load capacitor 

Using Cl = (C1*C2)/(C1+C2)+Cp, with Cp = 5p and aiming for Cl = 20p I get C1=33pF, which is what I had my board made with. This did not work.

On further research, section 58.4.6 of the datasheet suggests a formula Cl = 2*(Ccrystal - Cmcu - Cpcb), from which I get 2*(20p - 9p - 5p) = 12pF.

I note the datasheet requires Ccrystal to be less than 17.5pF. However the V71 Xplained Ultra board is also using a 20pF crystal and I am running at 3.3V and room temperature so I think I should be getting something.
 

PCB layout 

I think this could also be a problem becuase I have also tried soldering on 15p, 18p, 22p and 33p caps at C4 and C55 to no avail. FWIW, the SAM E70 Xplained Ultra board uses 22p load caps and a 20p crystal.

The oscilloscope on XIN is showing nothing at all.

Schematic

PCB Layout

Oscillator Y1 is <1cm from the input pins

 

 

Last Edited: Tue. Aug 3, 2021 - 05:58 AM
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Hi,

 

what do you mean with " my program hangs on startup after switching to the oscillator" ? What is actually happening? Is there a hard fault? Or an infinity loop?

Maybe just the software for switching the clock source is wrong.

To check whether the crystal is working at all put you oscilloscope probe directly on crystal housing, leave the GND connector of the probe open - do you see any wave at your frequency?

Does your code always hang in the same way?

In most cases if the frequency is not stable the failure is not very reproduceable and looks a bit strange (once had a problem with an unstable crystal with the result of strange mcu restarts after 1-20 days)

 

Flo1991