Xilinix Spartan-3 Board, problem

I've noticed that the power connection that it comes with has no facility for an earth connection.

I will need to interface the board's outputs to external circuitry (a 8051 board I made, along with a +12V Stepper motor circuit) which uses a different power supply that's earthed... What should I do?

Connecting each ground to each other out of the question? Your input is appreciated!

-abionnnn

Hi abionnnn,
I don't think you should ever think of connecting the earth from the power into your sensitive electronic equipment.

when oyu have different power sources, such as this case, if your signals are isolated by a transformer, you need to have a comon ground, no matter how difficult it is. If you don't you may end up with very unpleasent surprises that will destroy your most valuable components.

You can have one of the options: Use opto-isolated bus between the 2 boards (expensive and bulky). Or connect the 2 grounds with one single THICK wire to make sure you have a solid common ground.

the earth on the power supply is not an importan issue for most of the applications. Specially in small signal devices. Earth connector is important if you have high-voltage (more than 60V) and/or hich current applications to prevent electricution of your users. for 5 and 12 volt logic, you definitely do not need an earth.

BR,
/Farhad

Dear farhada,

Thanks for your reply. I thought something like this would be an issue. I actually did use a few 4N25 opto's but as you said, this would be an expensive and bulky situation, especially when if I use the full potential of all the FPGA's I/O.

My main concern with the earth is I don't know what is inside the transformer (though it says it's a switch mode) , but it only has the active and neutral connections which means it's floating with respect to earth. So I suppose it's ok right?

Is there any other issues than the resistance of the ground connection? If not I think this is the only plausible solution.

Thanks again,

-abionnnn

Hi,
you don't need to worry about the earth at all. Your main concern should be the stabilityof your ground level when you have multiple power wources.

The key issue in your case is to keep the signal levels in the same level. By having a good ground signal, you make sure that the 5 volt on the output of your FPGA is the same as 5 V on the input side, since they both have the same reference point.

If your application is an industrial application, I would recomend to use a transzorb on all the output signals the value of the transzorb should be at leasr 2 times the maximum voltage. If you do so, add a small 0.1-5 ohm resistor to your output signals that will work as fuses for your device in case you get a nasty big spike, the transzorb will be activated and the resistor will burn, but the rest of your devices wil be OK (I used to work with 3000 volt control of devices using Xilinx XC4005 FPGAs).

Hope you find the information helpfull. May I ask if this is a school project, a hubby project or for a device?

BR,
/Farhad

Thanks for the transzorber idea, I was just going to use diodes on the h-bridge to protect against back-action but perhaps this is not enough.

Hope you find the information helpfull. May I ask if this is a school project, a hubby project or for a device?

Click to expand...

Well, it's mainly so I can learn FPGA's ahead of my University course, but where I work we may need a specialised stepper motor controller (it needs to turn a stiff Knob in a cryogenic enviroment with some precision) so I thought this simple project would be a double edged sword. I think an FPGA would a bit of an overkill since a µC can do the same job cheaper (and with more convinient voltages...), so a device is out of the question.

-abionnnn