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Mains pick-up on I/O pins

Hello,

I am trying to program an RC-100 demo board, which contains a Spartan-II chip. The board is supposed to send three logic outputs to external devices through I/O pins provided on an expansion header. I am pretty sure I've coded the program correctly (only to the extent that it works as intended in simulation), but when I probe the I/O pins with a scope all I'm getting is what I assume is mains pick-up (50 Hz ~5Vpk-pk).

My probes have been calibrated; the probe and scope have adequate bandwidth (greater than 5x the signal bandwidth), and I get the same result whether I use the 'Auto-scale' function or manually set the 'Volts' and 'Time' division scales to the capture the expected waveforms. In the event that the source of the problem isn't my code, does anyone have any suggestions as to what I might be doing wrong?

Thanks, mees

Reply to
m_oylulan
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Try writing some very simple code (a few lines of VHDL) that just drives an output high or low, and see if that works.

Leon

Reply to
Leon

It sounds to me like you're not getting a good ground return to your scope. If the board is powered by a "wall wart" supply it is most likely isolated from third-wire ground. If you have a ground clip attached to the board, check to see that you're not also seeing the 50 Hz when you probe the ground clip. This could indicate a bad connection to the probe ground return.

Good Luck, Gabor

m snipped-for-privacy@hotmail.com wrote:

Reply to
Gabor

Did you actually set constraints to put the outputs of your top level module on the specific IO pins you want?

--
Ben Jackson

http://www.ben.com/
Reply to
Ben Jackson

Thanks for all the advice above. Gabor was right about the grounding issue, although while now I am not picking up the mains singal, I'm still not seeing the signals I need (scope doesn't pick up anything but a noisy signal).

I've tried Leon's suggestion of just writing a simple program sending out a square wave and still no luck (although the code is written in Handel-C not VHDL... I don't know the VHDL language, and thought Handel-C would be easier to get a quicker design since C was more familiar).

Finally, in answer to Ben's question, yes, I had assigned the data to specific pins and the scope doesn't see anything (even with proper grounding).

Mees

Reply to
m_oylulan

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