Efficient high-speed on-chip global interconnects

University dissertation from Institutionen för systemteknik

Abstract: The continuous miniaturization of integrated circuits has opened the path towards System-on-Chip realizations. Process shrinking into the nanometer regime improves transistor performancewhile the delay of global interconnects, connecting circuit blocks separated by a long distance, significantly increases. In fact, global interconnects extending across a full chip can have a delay corresponding to multiple clock cycles. At the same time, global clock skew constraints, not only between blocks but also along the pipelined interconnects, become even tighter. On-chip interconnects have always been considered RC-like, that is exhibiting long RC-delays. This has motivated large efforts on alternatives such as on-chip optical interconnects, which have not yet been demonstrated, or complex schemes utilizing on-chip F-transmission or pulsed current-mode signaling.In this thesis, we show that well-designed electrical global interconnects, behaving as transmission lines, have the capacity of very high data rates (higher than can be delivered by the actual process) and support near velocity-of-light delay for single-ended voltage-mode signaling, thus mitigating the RC-problem. We critically explore key interconnect performance measures such as data delay, maximum data rate, crosstalk, edge rates and power dissipation. To experimentally demonstrate the feasibility and superior properties of on-chip transmission line interconnects, we have designed and fabricated a test chip carrying a 5 mm long global communication link. Measurements show that we can achieve 3 Gb/s/wire over the 5 mm long, repeaterless on-chip bus implemented in a standard 0.18 ?m CMOS process, achieving a signal velocity of 1/3 of the velocity of light in vacuum.To manage the problems due to global wire delays, we describe and implement a Synchronous Latency Insensitive Design (SLID) scheme, based on source-synchronous data transfer between blocks and data re-timing at the receiving block. The SLIDtechnique not onlymitigates unknown globalwire delays, but also removes the need for controlling global clock skew. The high-performance and high robustness capability of the SLID-method is practically demonstrated through a successful implementation of a SLID-based, 5.4 mm long, on-chip global bus, supporting 3 Gb/s/wire and dynamically accepting ± 2 clock cycles of data-clock skew, in a standard 0.18 ?m CMOS porcess.In the context of technology scaling, there is a tendency for interconnects to dominate chip power dissipation due to their large total capacitance. In this thesis we address the problem of interconnect power dissipation by proposing and analyzing a transition-energy cost model aimed for efficient power estimation of performancecritical buses. The model, which includes properties that closely capture effects present in high-performance VLSI buses, can be used to more accurately determine the energy benefits of e.g. transition coding of bus topologies. We further show a power optimization scheme based on appropriate choice of reduced voltage swing of the interconnect and scaling of receiver amplifier. Finally, the power saving impact of swing reduction in combination with a sense-amplifying flip-flop receiver is shown on a microprocessor cache bus architecture used in industry.