Re: What should be taken care of when two FPGA broad connected together?
- From: "John_H" <newsgroup@xxxxxxxxxxxxxxxx>
- Date: Wed, 6 Jun 2007 14:44:46 -0700
"jasonL" <junsong.liao@xxxxxxxxx> wrote in message
news:1181160552.039333.85840@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have two FPGA development boards, using different power source. when
I connected their IO pins, should I connect the GND of board too?
I hesitate to do that because I am not sure if these two GNDs are
sharing a common GND. If they do not have a common GND, will it
generate a current between GNDs when I connect them and damage the
boards?
Any other things I should take care of when I connect two FPGA board
together?
Thanks
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
.
- References:
- Prev by Date: Re: Virtex4 CLKX2 DCM Jitter
- Next by Date: Re: Virtex4 CLKX2 DCM Jitter
- Previous by thread: Re: What should be taken care of when two FPGA broad connected together?
- Next by thread: Re: What should be taken care of when two FPGA broad connected together?
- Index(es):
Relevant Pages
|
