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CDai0's avatar
CDai0
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6 years ago

Transceiver toolkit reference design: why is BER never 1 or close to 1

I have questions on the reference design for transceiver toolkit on cyclone 10 dev kit provided here: https://fpgacloud.intel.com/devstore/platform/17.1.1/Pro/cyclone-10-gx-xcvr-toolkit-reference-des...
Deshi_Intel's avatar
Deshi_Intel
Icon for Regular Contributor rankRegular Contributor
6 years ago
Hi Chenxi, It's good to know that you figured the polarity swap issue. For the toolkit colour code issue - My test case is different from your. I still have the loopback cable connected. I am merely playing with different PRBS test pattern to generate the data error. TX sending PRBS31 but RX is expecting PRBS7. - But typically what I noticed is even for the loopback disconnected scenario, the toolkit should still switch between green and yellow colour. - What happen is the CDR will loose lock (yellow), then attempt to lock back (green), then failed and loose lock again (yellow). The whole process will keep on repeat itself. - In short, switching between green and yellow is fine but permanently stay green in CDR loose lock condition is weird. Hopefully you are not seeing that. Thanks. Regards, dlim
CDai0's avatar
CDai0
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6 years ago

Hi Dlim,

Thanks for your response. Some urgent tasks came up, and I have been a bit busy lately, but now I will begin to pick this up again. In my tests, most of the time, the indicator is correct, meaning when I disconnect loopback, it will alternate between green and yellow. However, in some cases it will stay green always, and not turn to yellow, when loopback is disconnected after running the transceiver toolkit. It seems to depend on the cable length used and PRBS pattern selected. I have not yet narrowed down exactlywhen it occurs, but it seems to happen more for smaller PRBS (e.g. PRBS7)and shorter cables.

In cases where this happens, the indicator will be yellow , and correct if the transceiver toolkit is started without a loopback, and will turn green when loopback connector is hot-plugged in. Then, when I disconnect the loopback, the toolkit still shows the CDR lock always green. When this happens seem to depend on which PRBS setting I choose and which length cable I use. Also, i believe that with the reversed differential pair polarity would always behave like this, though I would need to double check that.

For my earlier question of why BER was stuck at 1.55E-2, I am still a bit confused by the explanation that it is BER counter limit, because if I keep running transceiver toolkit, the number of error count still goes up, so that would imply counter didn't reach a limit. The BER result also seems to agree with error counter, are they separately counted, or calculated from each other. Is there any documentation of the transceiver toolkit error counter limits?

Thanks

Chenxi Dai