In a field programmable gate array (FPGA) allowing dynamic reconfiguration in time multiplexing fashion, duplicate copies are configured in a time multiplexing manner which are functionally identical to a primary circuit specified for a predetermined FPGA's application. The primary and duplicate circuits are interrogated by a voting circuit which determines the existence of a faulted circuit in order to eliminate the faulted circuit from the operation of the FPGA. In this manner, without physical addition of redundant circuits, fault tolerancy for the FPGA is provided to minimize the cost, weight, volume, heat and energy associated issues of conventional redundance techniques.

Method for creating circuit redundancy in programmable logic devices

Logic emulation enables designers to functionally verify complex integrated circuits prior to chip fabrication. However, traditional FPGA-based logic emulators have poor inter-chip communication bandwidth, commonly limiting gate utilization to less than 20%. Global routing contention mandates the use of expensive crossbar and PC-board technology in a system of otherwise low-cost commodity parts. Even with crossbar technology, current emulators only use a fraction of potential communication bandwidth because they dedicate each FPGA pin (physical wire) to a single emulated signal (logical wire). Virtual wires overcome pin limitations by intelligently multiplexing each physical wire among multiple logical wires, and pipelining these connections at the maximum clocking frequency of the FPGA. The resulting increase in bandwidth allows effective use of low-dimension direct interconnect. The size of the FPGA array can be decreased as well, resulting in low-cost logic emulation. This paper covers major contributions of the MIT Virtual Wires project. In the context of a complete emulation system, we analyze phase-based static scheduling and routing algorithms, present virtual wires synthesis methodologies, and overview an operational prototype with 20 K-gate boards. Results, including in-circuit emulation of a SPARC microprocessor, indicate that virtual wires eliminate the need for expensive crossbar technology while increasing FPGA utilization beyond 45%. Theoretical analysis predicts that virtual wires emulation scales with FPGA size and average routing distance, while traditional emulation does not.

/pdf/logic-emulation-with-virtual-wires-3zowsadhie.pdf

Logic emulation with virtual wires

A switch module M with W terminals on each side is said to be universal if every set of nets satisfying the dimensional constraint (i.e., the number of nets on each side of M is at most W) is simultaneously rout able through M. In this article, we present a class of universal switch modules. Each of our switch modules has 6Wswitches and switch-module flexibility three (i.e, Fs=3). We prove that no switch module with less than 6W switches can be universal. We also compare our switch modules with those used in the Xilinx XC4000 family FPGAs and the antisymmetric switch modules (with FS=3) suggested by Rose and Brown [1991]. Although these two kinds of switch modules also have FS=3 and 6W switches, we show that they are not universal. Based on combinatorial counting techniques, we show that each of our universal switch modules can accommodate up to 25% more routing instances, compared with the XC4000-type switch module of the same size. Experimental results demonstrate that our universal switch modules improve routability at the chip level. Finally, our work also provides a theoretical insight into the important observation by Rose and Brown [1991] (based on extensive experiments) that FS=3 is often sufficient to provide high routability.

/pdf/universal-switch-modules-for-fpga-design-1pjp1nlw26.pdf

Universal switch modules for FPGA design

A report is presented on procedures investigated to determine flip-flops to be scanned in partial-scan designs for sequential circuits. The main idea pursued is to derive a minimal feedback vertex set of the so-called S-graphs. Results of applying optimal and heuristic procedures on a set of benchmark circuits indicate that heuristic methods give fast and near minimal solutions.<<ETX>>

Determining scan flip-flops in partial-scan designs

SEU mitigation, detection, and correction techniques are disclosed. Mitigation techniques include: triple redundancy of a logic path extended the length of the FPGA; triple logic module and feedback redundancy provides redundant voter circuits at redundant logic outputs and voter circuits in feedback loops; enhanced triple device redundancy using three FPGAs is introduced to provide nine instances of the user's logic; critical redundant outputs are wire-ANDed together; redundant dual port RAMs, with one port dedicated to refreshing data; and redundant clock delay locked loops (DLL) are monitored and reset if each DLL does not remain in phase with the majority of the DLLs. Detection techniques include: configuration memory readback wherein a checksum is verified; separate FPGAs perform readbacks of configuration memory of a neighbor FPGA; and an FPGA performs a self-readback of its configuration memory array. Correction techniques include reconfiguration of partial configuration data and “scrubbing” based on anticipated SEUs.

Techniques for mitigating, detecting and correcting single event upset effects in systems using SRAM-based field programmable gate arrays

In the process of VLSI design, specifications are often changed. It is desirable that such changes will not lead to a very different design so that a large part of engineering effort can be preserved. We consider synthesis algorithms for handling such engineering changes. Given a synthesized network, our algorithm modifies it minimally to realize a new specification.

Logic Synthesis for Engineering Change

Conventional scan design imposes considerable area and delay overhead by using larger scan flip-flops and additional scan wires without utilizing the functionality of the combinational logic. We propose a novel low-overhead scan design methodology, called cost-free scan, which exploits the controllability of primary inputs to establish scan paths through the combinational logic. The methodology aims at reducing scan overhead by (1) analyzing the circuit to determine all the cost-free scan flip-flops, and (2) selecting the best primary input vector to establish the maximum number of cost-free scan flip-flops on the scan chain. Significant reduction in the scan overhead is achieved on ISCAS89 benchmarks, where in full scan environment, as many as 89% of the total flip-flops are found cost-free scannable, while in partial scan environment, reduction can be as high as 97% in scan flip-flops needed to break sequential loops.

Cost-free scan: a low-overhead scan path design methodology

Traditionally, the circuit partitioning problem is done by first modeling a circuit as a graph and then partitioning is performed on the modeling graph. Using the concept of alternative wires, we propose an efficient method that is able to preserve a local optimal solution in the graph domain while a different graph, representing the same circuit, is generated. When a conventional graph partitioning technique reaches a local optimal solution, our proposed technique generates a different graph that is logically equivalent to the original circuit, and that has equal or better partitioning solution. Faced with a different graph which is newly generated, together with a currently good partitioning solution, a conventional graph partitioning technique may then escape from the local optimum and continue searching for better solutions in a different graph domain. The proposed technique can be combined with almost any graph partitioner. Experiments show encouraging results.

Circuit partitioning with logic perturbation

In this paper the problem of selecting an appropriate programmable cell structure for FPGA architecture design is addressed. The cells studied here can be configured to the desired functionality by applying input permutation, negation, bridging or constant assignment, or output negation. A general methodology to determine logic description of such cells, which are capable of being configured to a given set of functions is described.Experimental results suggest that the new cell behaves as well as the Actel 2 cell in terms of logic power but requires substantially less area and wiring overhead.

Universal logic gate for FPGA design

Conventional scan design imposes considerable area and delay overheads. To establish a scan chain in the test mode, multiplexers at the inputs of flip-flops and scan wires are added to the actual design. However, the functionality of the functional logic has not been utilized for the test purposes. We propose a low-overhead scan design methodology, called cost-free scan, which exploits the controllability of primary inputs to establish scan paths through the functional logic. We show how to analyze the circuit to determine all the free-scan flip-flops and select the best input vector to establish the maximum number of free-scan flip-flops for the scan chain design. Significant reduction in the scan overhead is achieved on ISCAS89 benchmarks. In full-scan designs, as many as 89% of the flip-flops are found free-scannable. In the partial-scan designs, we assume that selecting flip-flops for scan to break sequential cycles is used to increase circuit testability. Reduction can be as high as 97% in scan flip-flops needed to break sequential cycles.

Cost-free scan: a low-overhead scan path design

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