Cecilia Metra

Latch Susceptibility to Transient Faults and New Hardening Approach

In this paper, we analyze the conditions making transient faults (TFs) affecting the nodes of conventional latch structures generate output soft errors (SEs). We investigate the susceptibility to TFs of all latch nodes and identify the most critical one(s). We show that, for standard latches using back-to-back inverters for their positive feedback, the internal nodes within their feedback path are the most critical. Such nodes will be hereafter referred to as internal feedback nodes. Based on this analysis, we first propose a low-cost hardened latch that, compared to alternative hardened solutions, is able to completely filter out TFs affecting its internal feedback nodes while presenting a lower susceptibility to TFs on the other internal nodes. This is achieved at the cost of a reduced robustness to TFs affecting the output node. To overcome this possible limitation (especially for systems for high-reliability applications), we propose another version of our latch that, at the cost of a small area and power consumption increase compared to our first solution, also improves the robustness of the output node, which can be higher than that of alternative hardened solutions. Additionally, both proposed latches present a comparable or higher robustness of the input node than alternative solutions and provide a lower or comparable power-delay product and area overhead than classical implementations and alternative hardened solutions.

A model for transient fault propagation in combinatorial logic

Transient faults (TFs) are increasingly affecting micro-electronic devices as their size decreases. During the design phase, the robustness of circuits for high reliability applications with respect to this kind of faults is generally validated through simulations. However, traditional HSPICE like simulators are too slow for the task of simulating the effects of TFs on large circuits. In this paper, we present a novel mathematical model to accurately estimate the possible propagation of transient fault-due glitches through a CMOS combinational circuit, which is suitable to be used into a new simulation tool able to provide good accuracy, while significantly speeding up simulations, with respect to HPSICE. In particular, our model allows approximately 90% accuracy with respect to HSPICE simulations.

Exploiting ECC redundancy to minimize crosstalk impact

Signal integrity in high-speed bus designs is put at risk by crosstalk-related-bus delays. This article provides a comprehensive study of the usefulness of error-correcting code (ECC) redundancy in reducing such delays. It shows that dual Rail codes perform better at this task than Hamming codes. We analyze the impact of different ECCs on crosstalk-induced-bus delays (CIBD). We investigate the possibility of exploiting the information redundancy previously necessary to limit CIBD, thus reducing the consequent risk of a systems incorrect operation.

Multiple transient faults in logic: an issue for next generation ICs?

In this paper, we first evaluate whether or not a multiple transient fault (multiple TF) generated by the hit of a single cosmic ray neutron can give rise to a bidirectional error at the circuit output (that is an error in which all erroneous bits are 1s rather than 0s, or vice versa, within the same word, but not both). By means of electrical level simulations, we show that this can be the case. Then, we present a software tool that we have developed in order to evaluate the likelihood of occurrence of such bidirectional errors for very deep submicron (VDSM) ICs. The application of this tool to benchmark circuits has proven that such a probability can not be neglected for several benchmark circuits. Finally, we evaluate the behavior of conventional self-checking circuits (generally designed accounting only for single TFs) with respect to such events. We show that the modifications generally introduced to their functional blocks in order to avoid output bidirectional errors due to single TFs (as required when an AUED code is implemented) can significantly reduce (up to the 40%) also the probability to have bidirectional errors because of multiple TFs.

Modeling Crosstalk Effects in CNT Bus Architectures

Carbon nanotubes (CNTs) have been widely proposed as interconnect fabric for nano and very deep submicron (silicon-based) technologies due to their robustness to electromigration. In this paper, issues associated with crosstalk among bus lines implemented by CNTs are investigated in detail. CNT-based interconnects are modeled and the effects of crosstalk on performance and correct operation are evaluated by simulation. Existing models are modified to account for geometries in bus architectures made of parallel single-walled nanotubes and a single multiwalled nanotube. New RLC equivalent circuits are proposed for these bus architectures. A novel bus architecture with low crosstalk features is also proposed. This bus architecture is made of dual-walled nanotubes arranged in parallel. In this architecture, the crosstalk-induced delay and corresponding uncertainty (as well as crosstalk-induced peak voltage) are significantly reduced; a modest area penalty is incurred. Reductions up to 59% for the crosstalk-induced delay and up to 81% for the crosstalk-induced peak voltage are reported. These results confirm that the proposed bus arrangement noticeably improves performance and provides reliable operation

Novel transient fault hardened static latch

University of Bologna

Self-checking detection and diagnosis of transient, delay, and crosstalk faults affecting bus lines

We present a self-checking detection and diagnosis scheme for transient, delay, and crosstalk faults affecting bus lines of synchronous systems. Faults that are likely to result in the connected logic sampling incorrect bus data are on-line detected. The position of the affected line(s) within the considered bus is identified and properly encoded. The proposed scheme is self-checking with respect to a realistic set of possible internal faults, including node stuck-ats, transistor stuck-ons, transistor stuck-opens, resistive bridgings, transient faults, delays and crosstalks.

Reversible Gates and Testability of One Dimensional Arrays of Molecular QCA

An extensive literature exists on the mathematical characterization of reversible logic. However, the possible technological basis of this computing paradigm still remains unsolved. In this paper, quantum-dot cellular automata (QCA) is investigated for testable implementations of reversible logic. Two new reversible gates (referred to as QCA1 and QCA2) are proposed. These gates are compared (in terms of delay, area and logic synthesis) with other reversible gates (such as Toffoli and Fredkin) for QCA implementation. Due to the expected high error rates in nano-scale manufacturing, testing of nano devices, including QCA, has received considerable attention. The focus of this paper is on the testability of a one-dimensional array made of QCA reversible gates, because the bijective nature of reversible gates significantly facilitates testing of arrays. The investigation of testability relies on a fault model for molecular QCA that is based on a single missing/additional cell assumption. It is shown that C-testability of a 1D reversible QCA gate array can be guaranteed for single fault. Theory and circuit examples show that error masking can occur when multiple faults are considered.

Configurable Error Control Scheme for NoC Signal Integrity

In this paper we propose a novel error control scheme to cope with errors affecting the communication links of a NoC. Our scheme can be configured in correction mode, detection mode, and mixed mode, depending on the particular application, thus allowing to meet different quality of service (QoS) levels in terms of error control. For each configuration mode, we propose different error control policies and we consider SEC hamming codes SEC/DED Hsiao codes, and symbol error correcting codes. We evaluate advantages and drawbacks of each approach, in terms of signal integrity, area overhead and impact on performance.

New ECC for crosstalk impact minimization

Signal integrity in high-speed bus designs is put at risk by crosstalk-related bus delays. This article provides a comprehensive study of the usefulness of error correcting code (ECC) redundancy in reducing such delays. It shows that Dual Rail codes perform better at this task than Hamming codes. We describe the modification of DR code that offers some distinct advantages.