A differential CMOS Logic family that is well suited to automated logic minimization and placement and routing techniques, yet has comparable performance to conventional CMOS, will be described. A CMOS circuit using 10,880 NMOS differential pairs has been developed using this approach.

Cascode voltage switch logic: A differential CMOS logic family

Describes a new dynamic CMOS technique which is fully racefree, yet has high logic flexibility. The circuits operate racefree from two clocks /spl phi/ and /spl phi/~ regardless of their overlap time. In contrast to the critical clock skew specification in the conventional CMOS pipelined circuits, the proposed technique imposes no restriction to the amount of clock skew. The main building blocks of the NORA technique are dynamic CMOS and C/SUP 2/MOS logic functions. Static CMOS functions can also be employed. Logic composition rules to mix dynamic CMOS, C/SUP 2/MOS, and conventional CMOS will be presented. Different from Domino technique, logic inversion is also provided. This means higher logic flexibility and less transistors for the same function. The effects of charge redistribution, noise margin, and leakage in the dynamic CMOS blocks are also analyzed. Experimental results show the feasibility of the principles discussed.

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NORA: a racefree dynamic CMOS technique for pipelined logic structures

The pass-transistor based cell library and synthesis tool are constructed, for the first time, to clarify the potential of top-down pass-transistor logic. The entire scheme is called LEAP (Lean Integration with Pass-Transistors). The feature of a pass-transistor based cell is its multiplexer function and the open-drain structure. This cell has the flexibility of transistor level circuit design and compatibility with conventional cell based design. An extremely simple cell library with only seven cells combined with a synthesis tool called "circuit inventor" is compared with the conventional CMOS library that has over 60 cells combined with the state-of-the-art logic synthesis. The results show that the area, delay, and power dissipation are improved by LEAP and that the value-cost ratio is improved by a factor of three. This demonstrates that LEAP has the potential to achieve a quantum leap in value of LSI's while reducing the cost. Key issues which have to be cleared before pass transistor logic is used as the generic logic scheme replacing CMOS are also discussed.

Top-down pass-transistor logic design

Run-length coding (RLC) and variable-length coding (VLC) are widely used techniques for lossless data compression. A high-speed entropy coding system using these two techniques is considered for digital high definition television (HDTV) applications. Traditionally, VLC decoding is implemented through a tree-searching algorithm as the input bits are received serially. For HDTV applications, it is very difficult to implement a real-time VLC decoder of this kind due to the very high data rate required. A parallel structured VLC decoder which decodes each codeword in one clock cycle regardless of its length is introduced. The required clock rate of the decoder is thus lower, and parallel processing architectures become easy to adopt in the entropy coding system. The parallel entropy coder and decoder are designed for implementation in two experimental prototype chips which are designed to encode and decode more than 52 million samples/s. Some related system issues, such as the synchronization of variable-length codewords and error concealment, are also discussed. >

An entropy coding system for digital HDTV applications

As cycle time of chips shrinks and die size grows, clock skew measured as a fraction of the cycle time is increasing. Traditional domino circuits shown are especially sensitive because skew must be budgeted in both half-cycles. The problem with such domino pipelines is that evaluation starts when the clock connected to the first gate in the half-cycle rises but the output needs to be valid before the clock on the output latch falls. In the worst case, the evaluate clock is late and the latch clock is early, decreasing time for logic. Many designers realize that some of the overhead can be reduced by using differential domino (also called dual rail) designs. An SR latch or pipeline latch at the end of dual-rail circuits lessens sensitivity to the falling edge. Self-timed techniques eliminate clocks and clock skew, but raise new issues of control overhead, timing assumption verification, and testability. The methodology reported here boosts operating frequency by tolerating clock skew, eliminating latches from the critical path, and better balancing logic between phases of the pipeline.

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Skew-tolerant domino circuits

Characteristics of various CMOS and NMOS circuit techniques are described, along with the shortcomings of each. Then a new circuit type, the CMOS domino circuit is described. This involves the connection of dynamic CMOS gates in such a way that a single clock edge can be used to turn on all gates in the circuit at once. As a result, complex clocking schemes are not needed and the full inherent speed of the dynamic gate can be utilized. The circuit is most valuable where gates are complex and have high fan-out such as in arithmetic units. Examples are shown of the use of domino circuits in an 8-bit ALU, where simulations indicate a speed advantage of 1.5 to 2 over traditional circuits, and in a 32-bit ALU where a worst case add in 124 ns was projected and a time less than 100 ns was achieved.

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C.M. Lee

Papers

High-speed compact circuits with CMOS