SI Methodology for Multi-Gigabit Serial Link Interfaces (7 of 8)

Backchannel Training

Another capability related to equalization adaptation is backchannel training. Many high speed serial link protocols enable the SerDes receiver to evaluate the signal quality of training patterns sent by the transmitter, decide if it wants more or less equalization from the transmitter, communicate that request back to the transmitter, then receive another training pattern for evaluation. This process is repeated multiple times until the receiver is satisfied with the transmitter settings, then the actual data payload is transmitted with those preferred settings.

Figure 13 – Backchannel training

While the current IBIS standard does not support backchannel capability yet, there is a pending update to support this in IBIS with BIRD (Buffer Issue Resolution Document) 147, which will be incorporated into the next version of the IBIS specification.

Consider the following PCI Express Gen 4 example with and without the utilization of backchannel training.

Figure 14 – Initial channel simulation results

The initial result (in red) is shown without backchannel enabled. In this case, the transmitter’s AMI model self-optimizes its FFE tap coefficients based on the characteristics of the channel, while the receiver’s AMI model adaptation is done real-time, throughout the channel simulation. The second result (in green) is with backchannel training enabled, and clearly produces a more open eye. The interesting item to note is that if you look at the difference between the FFE tap coefficients used in both cases, you will see that the FFE coefficients have been turned down in the backchannel case. For example, this is how the pre-cursor tap coefficient adapted during the back-channel training:

Figure 15 – FFE adaptation during backchannel training

Here you can see that the pre-cursor tap coefficient starts out initially at an absolute value of almost 0.16, and then over the backchannel training process, gets turned down to the 0.14 range, based on the receiver’s discretion. This enables the receiver’s more advanced equalization functionality to do more of the “heavy lifting” and ultimately produce a better overall result. This shows the importance of enabling the backchannel communication in the channel simulation process, and developing AMI models that closely emulate the real-world behavior of the SerDes devices in actual hardware.

For more information on Backchannel Modeling, see our paper "Backchannel Modeling and Simulation Using Recent Enhancements to the IBIS Standard" presented at DesignCon 2018.

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