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Old 05-30-2006, 11:45 AM
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Default 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

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Old 05-30-2006, 01:10 PM
Leon
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Default Re: Mains pick-up on I/O pins

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

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Old 05-30-2006, 02:03 PM
Gabor
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Default Re: Mains pick-up on I/O pins

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

[email protected] wrote:
> 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


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  #4 (permalink)  
Old 05-30-2006, 10:57 PM
Ben Jackson
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Default Re: Mains pick-up on I/O pins

On 2006-05-30, [email protected] <[email protected]> wrote:
> 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.


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

--
Ben Jackson
<[email protected]>
http://www.ben.com/
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  #5 (permalink)  
Old 06-01-2006, 12:53 PM
Guest
 
Posts: n/a
Default Re: Mains pick-up on I/O pins


[email protected] wrote:
> 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


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

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