Electronic – Avoiding crosstalk between makeshift wires

crosstalkfpgasignal integrity

My setup is as follows. I have 6 foot long 22-gauge wires connecting pin headers of an audio codec eval board and an FPGA daughterboard. I am sending an 8KHz and 128KHz clock signals and a data signal that changes with the fast clock. At present all I've done is loopback the signals from the audio board into the FPGA back to the audio board. I can do this same loopback bypassing the FPGA and the signal integrity is fine. The problem is when I route through the FPGA. At the output pins at every fast clock transition there is significant noise on the slow clock, so much so that it sometimes changes logic level. How do I isolate these from each other effectively? Thes pins on the daughterboard are currently right next to each other. Would moving the pin assignments make a difference or would it not matter because the wires will still be next to each other?

Thanks for any advice!

Best Answer

Some things I'd try:

  • Use the slowest drive you can get away with. At these clock rates, you should be able to use very slow drivers. You don't mention a specific FPGA type, but if it were Xilinx, I'd be using the "quiet" drivers, or "slow" if not available, and a 4mA drive current. See how it looks on the scope, increase the drive current if necessary.
  • Twist a ground wire with each of your signals, or at least with the clock signals. Or if using ribbon cable, put a ground on either side of both the clocks. Keep the clocks well away from each other. In the extreme, use coax for the clocks (but be careful to use a very short connection to ground for the shield, otherwise you stand a good chance of not gaining anything)

You have to think about how the current is flowing - both "out" and "back" - any time the current of one signal mixes with another (say using a single ground return wire), you will have potential for problems.

Another suggestion - in order to understand what's going on so you can predict these problems next time, read (and inwardly digest :) a book on signal integrity

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