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2012 : Asynchronous Logic in Real-Time Systems

Author(s)
Markus Ferringer
Abstract
While asynchronous logic has many potential advantages compared to traditional synchronous designs, one of the major drawbacks is its unpredictability with respect to temporal behavior. Having no high-precision oscillator, a self-timed circuit's execution speed is heavily dependent on temperature and supply voltage. Small fluctuations of these parameters already result in noticeable changes of the design's throughput and performance. Without further provisions this jitter makes the use of asynchronous logic hardly feasible for real-time applications. In this work, which is part of project ARTS2 (Asynchronous Logic in Real-Time Systems), we investigate the temporal characteristics of self-timed circuits regarding their usage in real-time systems, especially the Time-Triggered Protocol. We propose a timing model capable of dealing with deterministic as well as probabilistic timings caused -- besides others -- by PVT (process, voltage, temperature) variations, and elaborate self-adapting circuits which shall derive a suitable notion of time for an asynchronous TTP controller. Out of the proposed variants we find the simple LFSR (linear feedback shift register) implementation with rate correction most promising for our purposes. We further introduce and analyze the jitter compensation concept, which is a three-fold mechanism to keep the asynchronous circuit's notion of time tightly synchronized to the remaining communication participants. To demonstrate the robustness of our solution, we perform di erent tests and investigate their impact on jitter and frequency stability. These tests include, e.g., varying operating temperature, changing core supply voltage, and process variations among several devices of the same type. The experiments in combination with the theoretical analysis reveal some interesting insights for the temporal behavior of self-timed circuits: Even though the used design style is strongly indicating, considerable data-dependent jitter effects can be identi ed. It also turns out that process variations signi cantly influence the jitter characteristics and performance of asynchronous circuits. Nevertheless, the proposed self-adaptive time reference generation circuit is capable of tolerating di erent temporal conditions. Measurements with the fully functional asynchronous TTP controller reveal that it is indeed possible to use asynchronous logic in real-time systems. However, there are some major limitations (especially for actively sending messages in a time-triggered system) that must be considered.
Bibtex
@phdthesis{ ferringer:2012,
  author =      "Markus Ferringer",
  title =       "Asynchronous Logic in Real-Time Systems",
  address =     "Treitlstr. 3/3/182-1, 1040 Vienna, Austria",
  school =      "Technische Universit{\"a}t Wien, Institut f{\"u}r Technische Informatik",
  year =        "2012"
}
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