Nikhil Jayakumar

A metal and via maskset programmable VLSI design methodology using PLAs

In recent times there has been a substantial increase in the cost and complexity of fabricating a VLSI chip. The lithography masks themselves can cost between /spl epsi/ and /spl ges/. It is conjectured that due to these increasing costs, the number of ASIC starts in the last few years has declined. We address this problem by using an array of dynamic PLAs which require only metal and via mask customization in order to implement a new design. This would allow several similar-sized designs to share the same base set of masks (right up to the metal layers) and only have different metal and via masks. We have implemented our methodology for both combinational and sequential designs, and demonstrate that our approach strikes a reasonable compromise between ASIC and field programmable design methodologies in terms of placed-and-routed area and delay. Our method has a 2.89/spl times/ (3.58/spl times/) delay overhead and a 4.96/spl times/ (3.44/spl times/) area overhead compared to standard cells for combinational (sequential) designs.

A Fast Hardware Approach for Approximate, Efficient Logarithm and Antilogarithm Computations

The realization of functions such as log() and antilog() in hardware is of considerable relevance, due to their importance in several computing applications. In this paper, we present an approach to compute log() and antilog() in hardware. Our approach is based on a table lookup, followed by an interpolation step. The interpolation step is implemented in combinational logic, in a field-programmable gate array (FPGA), resulting in an area-efficient, fast design. The novelty of our approach lies in the fact that we perform interpolation efficiently, without the need to perform multiplication or division, and our method performs both the log() and antilog() operation using the same hardware architecture. We compare our work with existing methods, and show that our approach results in significantly lower memory resource utilization, for the same approximation errors. Also our method scales very well with an increase in the required accuracy, compared to existing techniques.

Practical techniques to reduce skew and its variations in buffered clock networks

Clock skew is becoming increasingly difficult to control due to variations. Link based non-tree clock distribution is a cost-effective technique for reducing clock skew variations. However, previous works based on this technique were limited to unbuffered clock networks and neglected spatial correlations in the experimental validation. In this work, we overcome these shortcomings and make the link based non-tree approach feasible for realistic designs. The short circuit risk and multi-driver delay issues in buffered non-tree clock networks are investigated. Our approach is validated with SPICE based Monte Carlo simulations, considering spatial correlations among variations. The experimental results show that our approach can reduce the maximal skew by 47%, improve the skew yield from 15% to 73% on average with a decrease on the total wire and buffer capacitance.

A design approach for radiation-hard digital electronics

In this paper, we present a novel circuit design approach for radiation hardened digital electronics. Our approach is based on the use of shadow gates, whose task it is to protect the primary gate in case it is struck by a heavy cosmic ion. We locally duplicate the gate to be protected, and connect a pair of transistors (or diodes) between the outputs of the original and shadow gates. These transistors turn on when the voltages of the two gates deviate during a radiation strike. Our experiments show that at the level of a single gate, our circuit structure has a delay overhead of about 4% on average, and an area overhead of over 100%. At the circuit level, however, we do not need to protect all gates. We present a methodology to selectively protect specific gates of the circuit in a manner that guarantees radiation tolerance for the entire circuit. With this methodology, we demonstrate that at the circuit level, the delay overhead is about 4% and the placed-and-routed area overhead is 30%, compared to an unprotected circuit (for delay mapped designs)

Do's and don'ts of CTL state coverage estimation

Coverage estimation for model checking quantifies the completeness of a set of properties. We present an improved version of the algorithm of Hoskote et al. [7] that applies to a larger subset of CTL; we prove properties of the algorithm and apply it to three case studies. From these case studies we derive recommendations for an effective use of coverage estimation.

A variation-tolerant sub-threshold design approach

Due to their extreme low power consumption, subthreshold design approaches are appealing for a widening class of applications which demand low power consumption and can tolerate larger circuit delays. However, subthreshold circuits are extremely sensitive to variations in supply, temperature and processing factors. In this paper, we present a subthreshold design methodology which dynamically self-adjusts for inter and intra-die process, supply voltage and temperature (PVT) variations. This adjustment is achieved by performing bulk voltage adjustments in a closed-loop fashion, using a charge pump and a phase-detector.

Circuit-Level Design Approaches for Radiation-Hard Digital Electronics

In this paper, we present a novel circuit design approach for radiation hardened digital electronics. Our approach is based on the use of shadow gates, whose task it is to protect the primary gate in case it is struck by a heavy cosmic ion. We locally duplicate the gate to be protected, and connect a pair of diode-connected transistors (or diodes) between the outputs of the original and shadow gates. These transistors turn on when the voltages of the two gates deviate during a radiation strike. Our experiments show that at the level of a single gate, our circuit structure has a delay overhead about 1.76% on average, and an area overhead of 277%. At the circuit level, however, we do not need to protect all gates. We present a methodology to selectively protect specific gates of the circuit in a manner that guarantees radiation tolerance for the entire circuit. With this methodology, we demonstrate that at the circuit level, the average delay overhead is about 3% and the average placed-and-routed area overhead is 28%, compared to an unprotected circuit (for delay mapped designs). We also propose an improved circuit protection algorithm to reduce the area overhead associated with our approach. With this approach for circuit protection, the area and delay overheads are further lowered.

High-throughput VLSI implementations of iterative decoders and related code construction problems

We describe an efficient, fully-parallel Network of Programmable Logic Array (NPLA)-based realization of iterative decoders for structured LDPC codes. The LDPC codes are developed in tandem with the underlying VLSI implementation technique, without compromising chip design constraints. Two classes of codes are considered: one, based on combinatorial objects derived from difference sets and generalizations of non-averaging sequences, and another, based on progressive edge-growth techniques. The proposed implementation reduces routing congestion, a major issue not addressed in prior work. The operating power, delay and chip-size of the circuits are estimated, indicating that the proposed method significantly outperforms presently used standard-cell based architectures. The described LDPC designs can be modified to accommodate widely different requirements, such as those arising in recording systems, as well as wireless and optical data transmission devices.

An algorithm to minimize leakage through simultaneous input vector control and circuit modification

Leakage power currently comprises a large fraction of the total power consumption of an IC. Techniques to minimize leakage have been researched widely. In this paper, the authors present an approach which minimizes leakage by simultaneously modifying the circuit while deriving the input vector that minimizes leakage. In this approach, the authors selectively modify a gate so that its output (in sleep mode) is in a state which helps minimize the leakage of other gates in its transitive fanout. Gate replacement is performed in a slack-aware manner, to minimize the resulting delay penalty

A PLA based asynchronous micropipelining approach for subthreshold circuit design

Power consumption is a dominant issue in contemporary circuit design. Sub-threshold circuit design is an appealing means to dramatically reduce this power consumption. However, sub-threshold designs suffer from the drawback of being significantly slower than traditional designs. To reduce the speed gap between sub-threshold and traditional designs, we propose a sub-threshold circuit design approach based on asynchronous micropipelining of a levelized network of PLAs. We describe the handshaking protocol, circuit design and logic synthesis issues in this context. Our preliminary results demonstrate that by using our approach, a design can be sped up by about 7times, with an area penalty of 47%. Further, our approach yields an energy improvement of about 4times, compared to a traditional network of PLA design. Our approach is quite general, and can be applied to traditional circuits as well