Author: John Nemec Company: California Micro Devices Corp. Date Published: 9/10/1996
Surface Mount International
Abstract: As microprocessors, memories, and other components increase in speed, the printed circuit board connections which allow these components to communicate with one another begin to behave like transmission lines. These transmission line characteristics were always present, but as edge rates increase and the transmission rates increase, the effective line lengths become longer and the transmission line effects become especially important -- important to the degree that if they are not addressed, the system will not work. Reflections in the transmission line cause distortions of the signal at the receiving end of the line. These distortions can cause false triggering in clock lines, can cause erroneous data on data, address and control lines, and contribute significantly to clock and signal jitter. The phenomenon that causes this is reflections at various points in the transmission line. For digital signals, this can be modeled as an effective rise time which is longer than the rise time of the signal that is launched by the driver. There are several very well done explanations of this. Motorola has several application notes which treat reflections very well with clear explanations of what is happening along with graphical techniques for working out the resulting signals. The Cypress Semiconductor Applications Handbook has a good explanation of the results especially the effective rise time. This application note starts from these points and discusses the practical aspects of termination. In today’s equipment, meeting the ideal theoretical termination is seldom practical or for other reasons not really desirable. This note will discuss practical terminations and how they are applied.