Cross-talk square-wave?

David Bridgham dab at
Wed Mar 29 15:56:51 CDT 2017

> There are few things that come to mind here. The op seemed to indicate the lines are terminated. If they are not terminated in the characteristic impedance of the source and the transmission line, it is very unlikely he would be seeing nice square waves at either end. The reflections would distort the square wave. Given the reported squareness and that the op indicates terminated line, I do not think impedance mismatch is the issue here. 

There's certainly some ringing there but it's not the spike followed by
an exponential decay that I'd expect from an induced signal.  However,
maybe I just don't know what an induced signal can look like so that's
the question.

> I also agree that an induced current in an adjacent line would not be square. So I agree with the op's thoughts that this signal is getting on this line in some other fashion, I don't believe this is an issue of cross talk. However, some pictures of some waveforms would be interesting to see

Ask and receive.  See:

The bottom signal is the one in question and the top signal is the clock
I'm sending to the SD card (which isn't plugged in at this time).  I
wanted to see if it was the clock single I was seeing coupled here and
it's obviously not, though you can clearly see the clock inducing some
noise in the CS signal. 

The other thing I'm seeing from this trace that I hadn't really noticed
before is that 0 is not 0.  The 0 for the bottom trace is where the 2 is
on the left side.  This line is suppose to be going from the 270k
resistor to ground on one side across the ribbon cable to an FPGA pin
which is set to high-impedance on the other.  Clearly something else is
going on here.

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