Electronics > FPGA

Lattice iCE 40 FPGA distorted clock signal

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Hi all,
I am generating SPI signals with a  Lattice iCE 40 FPGA Eval Board. My clock (36 MHz) and chip select signals look very  distorted as in the attached picture.
If you look carefully the clock signal goes below ground  :-//
Is this ok? What could be the cause? Has anyone experienced something similar?

Perhaps first of all you need to look at your probing technique? What are you using as a ground lead and how?

Completely normal, if you're not using a probe with a sufficiently short ground spike. If you're using the standard croc clip lead, which adds a few inches' worth of inductance to your ground connection, then that's exactly the sort of poor signal integrity and crosstalk you can expect.

With your probe's accessory kit there should be a short ground spike made of spring steel. Remove the croc clip lead, install this on your probe tip, and try again with the tip of the spike touching a ground point somewhere within, say, 10mm of the signal you're looking to probe.

Yep. Improper grounding. Apart from the distortion, you'll notice the significant crosstalk.

Although the most likely culprit here is grounding, also check your probes' bandwidth. A 36 MHz digital clock signal requires significant bandwidth to be displayed appropriately.

As a rule of thumb, I use the following:
- The typical rise time (s) with a bandwidth of BW (Hz) is ~ 0.35/BW;
- I consider a decent rise time for a digital signal to be max. 1/10 of the pulse width; for a 50% duty cycle clock, that would be 1/20 of its period;

From the above we can infer: BW >= 7.Fclock

So, for a 36 MHz digital clock, that would require a bandwidth >= 252 MHz

Sure you can acquire such signal with a lower bandwidth, but then it won't look anything close to a square wave.

Hi, thanks all. Yes I am using a standard lab oscilloscope probe, with the small hook tip for the signal and crock for the ground.. but I don't think I have the small tip you suggested for the ground. Would a short thin wire do the job? However I'll try and let you guys know


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