FPGA interconnection networks with capacitive boosting in strong and weak inversion




Eslami, Fatemeh

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Designers of Field-Programmable Gate Arrays (FPGAs) are always striving to improve the speed of their designs. The propagation delay of FPGA interconnection networks is a major challenge and continues to grow with newer technologies. FPGAs interconnection networks are implemented using NMOS pass transistor based multiplexers followed by buffers. The threshold voltage drop across an NMOS device degrades the high logic value, and results in unbalanced rising and falling edges, static power consumption due to the crowbar currents, and reduced noise margins. In this work, circuit design techniques to construct interconnection circuit with capacitive boosting are proposed. By using capacitive boosting in FPGAs interconnection networks, the signal transitions are accelerated and the crowbar currents of downstream buffers are reduced. In addition, buffers can be non-skewed or slightly skewed to improve noise immunity of the interconnection network. Results indicate that by using the presented circuit design technique, the propagation delay can be reduced by at least 10% versus prior art at the expense of a slight increase in silicon area. In addition, in a bid to reduce power consumption in reconfigurable arrays, operation in weak inversion region has been suggested. Current programmable interconnections cannot be directly used in this region due to a very poor propagation delay and sensitivity to Process-Voltage-Temperature (PVT) variations. This work also focuses on designing a common structure for FPGAs interconnection networks that can operate in both strong and weak inversion. We propose to use capacitive boosting together with a new circuit design technique, called Twins transmission gates in implementing FPGA interconnect multiplexers. We also propose to use capacitive boosting in designing buffers. This way, the operation region of the interconnection circuitry is shifted away from weak inversion toward strong inversion resulting in improved speed and enhanced tolerance to PVT variations. Simulation results indicate using capacitive boosting to implement the interconnection network can have a significant influence on delay and tolerance to variations. The interconnection network with capacitive boosting is at least 34% faster than prior art in weak inversion.



FPGA interconnection network, Level-restoring buffers, Capacitive boosting, Sub-threshold operation, Super-threshold operation