Coming clean with fuel cells

Fuel cells, mostly phosphoric acid, have been shown to operate well on renewable biogas fuels, such as anaerobic digester gas (ADC) produced at wastewater treatment plants as well as landfill gas (LFG) and gas produced in beer breweries. This paper discusses an innovative emission-offset project that utilizes anaerobic digester gas-powered fuel cells to produce electricity in New York. The use of fuel cells at wastewater treatment plants is also discussed.     

Guest Editors' Introduction: GALS Design and Validation

Globally asynchronous, locally synchronous (GALS) design has grown in popularity in both academia and industry. Breaking the synchrony assumption in digital design is often unsettling for designers, and to alleviate the difficulty, researchers in EDA have been proposing various GALS-based solutions. However, the tools, verification techniques, and testing methodologies for asynchronous designs are not as widespread as for synchronous digital design, leading to the hitherto limited usage of GALS design approaches. This special issue introduces some of the basic issues of GALS design and validation in the hardware domain. The hope is that this special issue will generate more interest by researchers and industry practitioners in creating design tools, techniques, and validation methodologies for GALS design.     

Logic decomposition of speed-independent circuits

Logic decomposition is a well-known problem in logic synthesis, but it poses new challenges when targeted to speed-independent circuits. The decomposition of a gate into smaller gates must preserve not only the functional correctness of a circuit but also speed independence, i.e., hazard freedom under unbounded gate delays. This paper presents a new method for logic decomposition of speed-independent circuits that solves the problem in two major steps: (1) logic decomposition of complex gates and (2) insertion of new signals that preserve hazard freedom. The method is shown to be more general than previous approaches and its effectiveness is evaluated by experiments on a set of benchmarks     

On the Performance Evaluation of Multi-Guarded Marked Graphs with Single-Server Semantics

In discrete event systems, a given task can start executing when all the required input data are available. The required input data for a given task may change along the evolution of the system. A way of modeling this changing requirement is through multi-guarded tasks. This paper studies the performance evaluation of the class of marked graphs extended with multi-guarded transitions (or tasks). Although the throughput of such systems can be computed through Markov chain analysis, two alternative methods are proposed to avoid the state explosion problem. The first one obtains throughput bounds in polynomial time through linear programming. The second one yields a small subsystem that estimates the throughput of the whole system.     

Analysis and identification of speed-independent circuits on an event model

The object of this article is the analysis of asynchronous circuits for speed independence or delay insensitivity. The circuits are specified as a netlist of logic functions describing the components. The analysis is based on a derivation of an event specification of the circuit behavior in a form of a signal graph. Signal graphs can be viewed either as a formalization of timing diagrams, or as a signal interpreted version of marked graphs (a subclass of Petri nets). The main advantage of this method is that a state explosion is avoided. A restoration of an event specification of a circuit also helps to solve the behavior identification problem, i.e., to compare the obtained specification with the desired specification. We illustrate the method by means of some examples.     

Deadlock prevention using Petri nets and their unfoldings

Unfoldings of Petri nets (PN) provide a method for the analysis of concurrent systems without restoring the state space of a system. This allows one to overcome the state explosion problem. Many properties of the initial PN (boundedness, safety, persistency and hazards) can be checked by constructing the unfolding. A deadlock prevention procedure first detects deadlocks using an unfolding. Then, the first method reduces the unfolding to a set of deadlock-free subunfoldings that cover all live behaviours. The second method uses a direct transformation at the level of the original PN. The methods are implemented as subroutines in the Berkeley program SIS. Although the deadlock detection problem is known to be NP-complete, experimental results show that for highly parallel specifications deadlock prevention by unfoldings is typically more efficient than deadlock prevention based on symbolic BDD (binary decision diagrams) traversal of the corresponding reachability graph.