Global versus Local Weak-Indication Self-Timed Function Blocks - A Comparative Analysis

📝 Abstract

This paper analyzes the merits and demerits of global weak-indication self-timed function blocks versus local weak-indication self-timed function blocks, implemented using a delay-insensitive data code and adhering to 4-phase return-to-zero handshaking. A self-timed ripple carry adder is considered as an example function block for the analysis. The analysis shows that while global weak-indication could help in optimizing the power, latency and area parameters, local weak-indication facilitates the optimum performance in terms of realizing the data-dependent cycle time that is characteristic of a weak-indication self-timed design.

💡 Analysis

This paper analyzes the merits and demerits of global weak-indication self-timed function blocks versus local weak-indication self-timed function blocks, implemented using a delay-insensitive data code and adhering to 4-phase return-to-zero handshaking. A self-timed ripple carry adder is considered as an example function block for the analysis. The analysis shows that while global weak-indication could help in optimizing the power, latency and area parameters, local weak-indication facilitates the optimum performance in terms of realizing the data-dependent cycle time that is characteristic of a weak-indication self-timed design.

📄 Content

Global versus Local Weak-Indication Self-Timed Function Blocks
– A Comparative Analysis

P. BALASUBRAMANIAN*, N.E. MASTORAKIS§¶

• School of Computer Engineering Nanyang Technological University 50 Nanyang Avenue SINGAPORE 639798 Email: balasubramanian@ntu.edu.sg § Department of Computer Science Military Institutes of University Education Hellenic Naval Academy Piraeus 18539, GREECE Email: mastor@hna.gr ¶ Department of Industrial Engineering Technical University of Sofia Sofia 1000, Boulevard Kliment Ohridski 8 BULGARIA Email: mastor@tu-sofia.bg

Abstract: - This paper analyzes the merits and demerits of global weak-indication self-timed function blocks versus local weak-indication self-timed function blocks, implemented using a delay-insensitive data code and adhering to 4-phase return-to-zero handshaking. A self-timed ripple carry adder is considered as an example function block for the analysis. The analysis shows that while global weak-indication could help in optimizing the power, latency and area parameters, local weak-indication facilitates the optimum performance in terms of realizing the data-dependent cycle time that is characteristic of a weak-indication self-timed design.

Key-Words: - Self-timed design, Function block, Indication, Ripple carry adder, CMOS, Standard cells

1 Introduction
The International Technology Roadmap for Semiconductors (ITRS) [1] has identified design for reliability and resilience as one of the futuristic grand challenges for design technology in its 2011 edition. The percentage of design reuse in system- on-chip designs which was estimated to be 46% in the 2009 ITRS edition is expected to become 96% by the year 2024. Further, the proportion of design blocks reuse with respect to glue logic is expected to reach 60% in the year 2024. Moreover, parameter uncertainty as a percentage effect on sign-off delay is expected to increase from a current level of 18% to 32% by 2024. In this backdrop, self-timed design, which is a robust flavor of asynchronous design, is acclaimed to be a strong contender or an inevitable counterpart for digital circuit/system designs in the nanoelectronics regime. This is because self-timed designs are inherently modular (i.e. permits design reuse) [2], are self-checking [3], exhibit superior EMI compatibility [4], are noise-tolerant [5], have the ability to cope with parametric uncertainty, supply voltage, threshold voltage, and temperature variations [6] [7], consume power only when and where active [8], and are able to guarantee greater security and robustness against hostile attacks compared to synchronous designs in the case of sensitive industrial applications [9] [10]. Taking cognizance of these facts, the ITRS design report has projected a growing requirement for asynchronous signaling in the nanoelectronics era and also emphasizes the continuous development of asynchronous circuit and system design tools over the foreseeable future.
In this paper, we focus on analyzing the merits and demerits of global weak-indication self-timed function blocks versus local weak-indication self- timed function blocks by building upon a prior work [11], which reported that global weak-indication is preferable compared to local weak-indication for realizing a cascade of function blocks from power, latency and area perspectives. Whilst confirming Recent Advances in Circuits, Systems, Signal Processing and Communications ISBN: 978-1-61804-366-5 86 that the observations reported in the previous work [11] are correct, we additionally show that local weak-indication is actually preferable from the point of view of cycle time than global weak-indication. In other words, we clarify that global weak- indication could reduce the throughput aspect of a weak-indication self-timed design, and with respect to realizing the true timing advantage inherent in a weak-indication self-timed design, the local weak- indication design method is indeed preferable.
In the remainder of this paper, Section 2 presents the fundamental concepts of self-timed design with some illustrations. Section 3 briefly discusses the local and global weak-indication self-timed system architectures by considering a 32-bit ripple carry adder (RCA) as an example function block. This is followed by the simulation results obtained for two local and global weak-indication self-timed 32-bit RCAs. Subsequently, the theoretical evaluation of cycle times for the two RCAs corresponding to local and global weak-indication is presented. Finally, the conclusions are derived in Section 4.

2 Preliminaries and Background
A self-timed (i.e. asynchronous) function block is the combinational logic equivalent of a synchronous digital system [12] [13]. Self-timed function blocks represent a robust flavor of asynchronous function blocks and are constructed using delay-inse

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