Cross-talk square-wave?

David Bridgham dab at
Thu Mar 30 22:07:39 CDT 2017

> It's not clear C-coupling is what's going on here (the wave shape looks pretty sharp for what I understand of the circuit/layout).
> Notably though, C-coupling would remove any DC bias, but David's screen shot indicates a DC bias on the line.
> Is this line currently connected to the FPGA, or is it just the wire and R?
> Perhaps the bias is coming from the FPGA, with C-coupling of the wave via the wire.
> Or perhaps it's all crosstalk from within the FPGA, 'visible' because of the high load R.

Yes, the wire is connected to the FPGA at one end.  That FPGA I/O pin is
*supposed* to be configured for high-Z but that's the only place I can
see the DC bias coming from.

> If the wire and FPGA pin are connected, separate them (reduce the wire circuit to just the wire and R to GND): see whether the DC bias and/or the square wave disappear.

Because of the way it's built, I'm not seeing a reasonable way to
disconnected the FPGA while leaving the rest intact.  However, I did
move the signal to another wire in the ribbon cable and the problem is
basically gone.  This lets me move on with other issues but am still a
bit puzzled with this one.

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