DDR3/4 fly-by topology is similar to daisy chain or multi-drop topology, but it includes very short stubs to each memory device in the chain to reduce the reflections. The advantage of fly-by topology is that it supports higher-frequency operation and improves signal integrity and timing on heavily loaded signals. If you are employing high-frequency DDR4, then the bandwidth of the channel needs to be as high as possible. However, with today’s extremely fast edge rates, the sequencing of the stubs and the end termination, and the associate load, can make a measurable difference in signal quality. In this month’s column I will look at how best to route DDR3/4 fly-by topology.
Reflections occur whenever the impedance of the transmission line changes along its length. This can be caused by unmatched drivers/loads, layer transitions, different dielectric materials, stubs, vias, connectors, terminations and IC packages. By understanding the causes of these reflections and eliminating the source of the mismatch, a design can be engineered with reliable performance. For perfect transfer of energy and to eliminate reflections, the impedance of the source must equal the impedance of the trace, as well as the impedance of the load.
As signal rise times increase, consideration should be given to the propagation time and reflections of a routed trace. If the propagation time and reflection from source to load are longer than the edge transition time, an electrically long trace will exist. If the transmission line is short, reflections still occur but will be overwhelmed by the rising or falling edge and may not pose a problem. But even if the trace is short, termination may still be required if the load is capacitive or highly inductive to prevent ringing.
Series termination is an excellent strategy for point-to-point routes, one load per net. Whereas parallel (end termination), is preferred for busses with a number of loads in a multi-drop topology. For DDR3/4 layouts, a series termination is generally not required for on-board memory devices. However, if your design has plug-in memory then the data and data mask signal length may be excessive and require a series termination.
To read this entire column, which appeared in the June 2018 issue of Design007 Magazine, click here.