MS-538| Application Note

Description and Family Characteristics

March 1993 Revised November 1999

Description and Family Characteristics
ABT Logic for TTL Backplanes
An application's required backplane speed, number of loads (cards), and physical layer are all considerations when selecting the optimal backplane driver IC. Differential and/or reduced swing technologies such as ECL and BTL provide the best noise immunity and are recommended when system performance is at a premium. For most applications, however, TTL-based solutions provide sufficient performance while adding the advantages of lower cost and wider availability. As shown in Figure 1, Fairchild Semiconductor's ABT and LVT BiCMOS technologies are best suited for the most demanding TTL backplane environments, e.g., those with the highest data rates and largest number of loads. ABT and LVT also feature Power-Up 3STATE capability--a requirement for live insertion. One difficulty in predicting backplane performance is that most buffer ICs specify propagation delay with a single output switching and a moderate 50 pF load. Conversely, design calculations require the worst case or actual environment which is multiple
outputs switching and often a much heavier load. To aid the system designer, most of Fairchild's ABT devices offer extended specifications which include guaranteed AC limits with all outputs switching and driving a 250 pF load. Fairchild's extended specifications also include guaranteed worst-case noise performance and skew. Noise generation is frequently associated with ground bounce, but also includes VCC droop and dynamic threshold, or shifting of input thresholds due to noise. ABT has the lowest noise generation of any high-speed Logic family. "Skew" refers to the difference in propagation delays between respective paths. Several factors contribute to skew, including the driver IC. Fairchild's ABT logic offers guaranteed skew specifications for three conditions: pin to pin, duty cycle variation, and device to device. Live insertion is another frequent requirement in any system having high up-time. Input circuitry inherent in Fairchild's ABT and LVT logic devices protects the device during live insertion
while helping to minimize backplane glitch. Please refer to the Applications section of this databook for a detailed discussion of ABT and Live Insertion. Maximum performance from a backplane is achieved with thorough understanding of the application's environment, including its buffer ICs. Fairchild's extended ABT specifications help provide that knowledge. For details, please refer to the Rating, Specifications and Waveforms section of this databook.

ABT and Computing
Continually higher clock rates combined with tighter noise and EMI constraints have fueled the need for super-fast, yet quiet, Logic. ABT fills this need, coupling the industry's fastest propagation delays with the lowest noise generation of any Advanced Logic family. ABT's BiCMOS process also consumes low power in both static and dynamic conditions. See Sections 2 and 4 of this databook for detailed discussions of power dissipation and ground bounce.

FIGURE 1. Approximate Backplane Technology Positioning

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1999 Fairchild Semiconductor Corporation

MS011558

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