Hi,
I'm setting up a statistical eye pattern scanner for use on a custom board, but I'm testing it on KC705. It's based upon XAPP743, but several adaptations were made to work in the target environment. In particular:
- The code, that controls the scanning through DRP, runs on a Linux host rather than on an on-chip Microblaze processor. This required a replacement of the wrappers for read/write to the DRP and some other changes in the main-level control of the scan. However the es_simple_eye_acq() function, which does the actual work, remained intact.
- The result data is displayed with octave (a free MATLAB clone) using the “cool” colormap rather than Xilinx' IBERT plot viewer.
The tested channel ran at 10 Gbps through the SMA ports of the KC705 board (40 bits with 8b/10b enabled, data randomized with a scrambler, DFE enabled). Two RG-316 cables of 6” each were used to loop back TXN to RXN and TXP to RXP.
The channel ran practically error free (no errors detected for several minutes).
After running an eye scan with max_prescale=9, both horizontal and vertical increments at unity (took 30 minutes or so), this is what I got:
The turquoise segment in the middle is the no-error zone. Purple marks the high-error region, and intermediate colors show a logarithmic transition between the two regions.
I would be happy with this eye scan, had it not been for it's vertical asymmetry: I can't see any reason why the no-error zone is pushed towards a positive voltage when sampling earlier in time.
The cables are not to blame. I've also run a similar test with RG-316 cables of 36” (!) and got very similar results. The equalizer does a good job.
This is not a matter of differences in cable lengths either: When disconnecting either of the cables (running only through an P or N channel), the turquoise region gets smaller and rounder, but with the same downwards slope.
So I said OK, maybe it's because I'm running at 10 Gbps through lousy cables. I reduced the rate to 1 Gbps, with the same parameters, and changed max_prescale=3 to speed up the scan.
This is what I got:
Looks somewhat better: There's still a slight vertical asymmetry, but the really weird thing is around horizontal shift zero: There are errors. And it gets weirder: These errors appear only at horizontal positions from 1 to 6 with an error rate of 2*10^(-5) at position 1 and going towards zero at 6. What makes this even more weird, is that no errors were measured at the respective negative horizontal shifts or at zero. Let's keep in mind that the scan starts at horizontal shift 0, and then jumps to -1, 1, -2, 2 etc. So the errors are consistent with the horizontal positions. Needless to say, I ran this twice and got very similar results.
So -- has anyone an idea on what's going on here? Have I made a silly mistake?
Thanks in advance,
Eli
