dab at froghouse.org
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.
More information about the cctalk