Using Data Bus Inversion to Mitigate Simultaneously Switching Noise


Reading time ( words)

This is the third post in a three part series that examines the problem of SSN and explores methods of reducing SSN in your designs.

DBI is an optional feature in DDR4.  If DBI is enabled, then when the driver (controller during a write or DRAM during a read) is sending out data on a lane, it counts the number of “0” (logic low) bits.  If the number of bits driving “0” in the lane is five or more, then the entire byte is inverted, and a ninth bit indicating DBI is asserted low.  This ensures that out of the 8 DQ bits and the 9th DBI bit, at least five bits are “1” during any given transaction.  This also ensures that out of the entire data lane, the maximum total number of signals transitioning is either five 1’s to 9 1’s or vice-versa.  There can never be a situation where all bits go from 0 to 1 or from 1 to 0.

So, if we run the same data bus with data patterns which would be the output of the DBI logic, we get the waveform for DQ0 in Figure 1.

Figure-1_DBI-processed-bit-patterns-with-improved-PDN-520x288.png

Figure 1: DBI processed bit patterns with improved PDN

The eye-height for DQ0 in this case is over 315mV, which surpasses all the other conditions.  Now, since DBI is data dependent, the benefits of DBI may vary and need to be analyzed before implementation.

Thank you for following our blog series on SSN—we hope you find this information valuable and share your thoughts in the comments. With a good design of the PDN, and possibly selecting the DBI feature in DDR4, SSN shouldn’t be a bother in your design. If you’d like to learn more about SSN and similar challenges, check out our white paper “DDR4 Board Design and Signal Integrity Challenges,” which was recently nominated for the DesignCon Best Paper Award.

Share

Print


Suggested Items

Just Ask Happy: Calculating Trace Temps in a Vacuum

07/07/2020 | I-Connect007 Editorial Team
We asked for you to send in your questions for Happy Holden, and you took us up on it! The questions you've posed run the gamut, covering technology, the worldwide fab market, and everything in between. Enjoy.

Just Ask Happy: PCB Design For EE Undergrads?

07/02/2020 | I-Connect007 Editorial Team
We asked for you to send in your questions for Happy Holden, and you took us up on it! The questions you've posed run the gamut, covering technology, the worldwide fab market, and everything in between. Enjoy.

Book Excerpt: Signal Integrity by Example, Chapter 3

06/24/2020 | I-Connect007 Editorial Team
The following is an excerpt from The Printed Circuit Designer's Guide to... Signal Integrity by Example, written by Fadi Deek of Mentor, a Siemens Business. Deek explores how to reach effective design solutions and make strong engineering tradeoffs through analysis techniques, best design principles, and software tools to achieve accurate simulations and measurements.



Copyright © 2020 I-Connect007. All rights reserved.