Fundamentals of SerDes Systems

Modern high-speed electronic systems are characterized by increased data speed integrated circuits (ICs). The input/output performance remains the bottleneck that limits the overall performance of a high-speed system. Serial data transfer is the most efficient way of communicating large data quickly between computer chips on printed circuit boards through copper cables and through short, medium, and long length fiber optics.

Thus, many systems now aggregate and serialize multiple input/ output (I/O) signals for transmission across fiber and copper cables and PCBs at a higher data rate, recovering and de-serializing the individual signals on the receiving end. These SerDes (Serializer/De-Serializer) implementations employ additional silicon real estate to perform sophisticated equalization required for reliable signal transmission at very high data speeds. This approach helps maximize throughput at the system level.

SerDes design is a complex, iterative process that typically starts with a baseline SerDes system that demonstrates the feasibility of a design approach. This system also establishes budgets for the different parts of the serial channel and associated transmitter (TX) and receiver (RX) equalization circuitry. The data that describes the desired behavior of each of the equalization filters in both the transmitter and the receiver is then back-annotated in the behavioral models with the correlation with simulations or measurements. The final step is to implement the training algorithms and control loops that will be executed by the chip during startup and from time to time when the channel needs to be retrained.

The SerDes system is then compiled into IBIS-AMI (Input/Output Buffer Information Specifications — Algorithmic Model Interface) models.

There are six sections of a SerDes system:

  • TX equalization — This becomes the IBIS-AMI dll for the transmitter.

  • TX AnalogOut — This becomes the analog model of the transmitter. It is part of the IBIS model for TX, and is typically represented by the I-V and V-T characteristics curves in the .ibs file.

  • Channel — This becomes the model of the physical channel, including the TX and RX package models.

  • RX AnalogOut — This becomes the analog model of the receiver. It is part of the IBIS model for RX, and is typically represented by the I-V and V-T characteristics curves in the .ibs file.

  • RX equalization — This becomes the IBIS-AMI dll for the receiver.

  • Training algorithms and control loops — These become the on-chip microcode that is executed inside of the chip during startup and when the channel needs to be retrained.