What should be taken care of when two FPGA broad connected together?

If the two grounds are different, and you don't connect the grounds together, you'll damage the FPGAs.

Even if they are at the same potential, and you don't connect the grounds together, interface signals between them will likely not work.

Connect the grounds.

Andy

jasonL,

if you do not connect the two grounds, then you will almost certqainly damage the two devices, or, nothing will happen (as they have no common reference).

If the two grounds can not be connected without causing a current to flow, there is something seriously wrong with you building wiring!

It could kill you, in fact.

Connect the two grounds together with a DVM in between, or current scale. There should be 0 AC current,and 0 DC current flowing between the boards (before any signals are switching).

If this is not the case, consult an electrician (as electronics engineer is not needed, but someone who understands how wall sockets are wired, is required).

Austin

Your two boards are powered by power supplies. Those supplies should be isolated. If the two grounds were connected together, the only imperfection in the ground should be due to imperfections in the isolation creating very little (if any) current flow. If they're isolated, the voltage between the two might be measered by a DVM to be a very bad number! But if you attached a 10k resistor between the two grounds, you would find the voltage difference to be negligible.

For single ended electrical signals there must be a common ground reference for the transmitter and receiver. When a signal transitions, current flows in both the wire that's transitioning and the ground that provides the current return path. If the loop for the signal current and return current is large, ugly things will happen (crosstalk, electromagnetic interference, general signal integrity issues).

For differential signals, the signal current has no (intended) return current through the ground plane. The differential signal should balance the currents so the switching point isn't affected by grounding issues. The differential receiver *does* still need to have the signal in its acceptable common mode range. To this end, it's better to have the common ground between the two boards powered by isolated power supplies.

There are cases when the grounds cannot be the same. For these circumstances, simple electrical connections won't do. Isolation through transformers or opto-isolators are required to get the communication going.

If you're working with development boards, the supplies should be isolated. Check the voltage with a 10k resistor between them if you're worried. If you see a little AC voltage, consider what the current flow is and whether you're comfortable with those (sub)milliamps running between boards.

It's not rocket science, but there's nothing more unnerving than seeing sparks fly and losing a $1k investment. You're good to be cautious but the situation is probably fine.

- John_H

Thanks to all of you. Your reply help me a lot. I am sure what to do now.