Design Con 2015

PCIe 3.0 Equalization

-January 24, 2013

In a live webinar today, Synopsys talked about the problems associated with getting 8Gbps out of the channel for a PCE express 3.0 implementation. It is a problem being faced by many interfaces these days and bandwidth is perhaps a greater limiting factor than processing power. Consider one example that they looked at for a typical communications link as shown below.

If were to try and run at 8Gbps without doing anything special, we would see something like this.

As you can see there is no eye in this diagram or anything that even looks vaguely like an eye. This is why several forms of equalization are necessary including Tx equalization, Rx equalization and equalization training. David Rennie, senior analog design engineer for mixed-signal interface IP at Synopsys, took us through the technologies. While a two tap fixed coefficient de-emphasis filer was suitable for PCIe 2.0, much more sophisticated schemes are necessary for 3.0. After applying Tx equalization, the waveform looks better, but there is still no clearly defined eye and thus there would be a significant error rate.

Next, David took us through receiver equalization and looked at the differences between continuous time linear equalizers (CTLE) and decision feedback equalizers (DFE) and how they can work together. After this has been applied we can start to see an eye forming and the hope that this may actually work. At this point I will admit that it got way beyond my knowledge but David showed how a 5-tap DFE would result in the eye opening.

However, David said that this is still not enough and that adaptive equalization/link training is required. In a series of slides it was shown how with training, the eye gradually opens and the circuitry necessary to cover sufficient PVT variations and enough resolution to properly equalize the channel.

For someone who deals with these kinds of issues every day, I am sure this will make a lot more sense than it does to me. The event was well attended and luckily for you, it is now available as an on-demand webinar and with a completely downloadable set of slides, so I would strongly encourage you to take a look.


Brian Bailey – keeping you covered

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