Weird clock signal on dataread (AT91SAM9260)

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I tried posting this once before but I think it was rejected because
of inline images, i'll try posting it again with links to the images:

I have a 6 layered sam9260 board and I'm seeing something strange on
the 18Mhz clock.

This is a scan from the SAM9260-EK (reference) board, when there's any
data written to the dataflash, a probe on the crystal shows this
pattern. (notice the slightly raised ground, which then stabilizes and

here are a couple of scans from my custom board.
Similarly when writing to the dataflash, the clock goes through this
weird cycle. This ultimately results in an inability to communicate
with the dataflash.

this is a faster scan of the same clock, but with a capture of one of
the SPIO data lines on ch2.

Bypass capacitance was initially suspect since I don't have as many
10uF tantalums on my board as the EK does, but even a 220uF tantalum
across 3.3V didn't seem to solve this. There are enough (as many as
the EK has) 0.1uF bypass caps for both the micro and dataflash though.
Strangely a probe directly on the ground plane doesn't seem to show
any fluctuations during the cycle.
I am considering possible heat damage to the micro, and am having a
board made by the fab house instead of my toaster oven.

Anyone have any theories on why the clock might behave this way. And
the fact that the EK board does something similar but doesn't
fluctuate as much, but my custom board goes all crazy?

Re: Weird clock signal on dataread (AT91SAM9260)

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The clock signal you are looking at is really tiny -  my guess is that it
would be different if you did not have a scope probe attached. The odd
effects you are seeing on the clock may not be real.

Decoupling with tantalums won't do much good -  ESR much too high - try
ceramics you can get pretty high values in 0805 and 0603 sizes. If your
layout is bad they may not help.

How do you know it's the clock upsetting ability to talk to data flash.
Usually if your clock goes wrong the processor will crash or reset (many
have clock fail detection which may generate an exception).

Try running the data flash more slowly - does it do the same. Do you get the
same effect if you toggle other lines nearby.

I don't buy the heat damage theory -  just hunch !

Good luck.

Michael Kellett

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