Doctor of Philosophy

dissertationCommunication surpasses computation as the power and performance bottleneck in forthcoming exascale processors. Scaling has made transistors cheap, but on-chip wires have grown more expensive, both in terms of latency as well as energy. Therefore, the need for low energy, high performance interconnects is highly pronounced, especially for long distance communication. In this work, we examine two aspects of the global signaling problem. The first part of the thesis focuses on a high bandwidth asynchronous signaling protocol for long distance communication. Asynchrony among intellectual property (IP) cores on a chip has become necessary in a System on Chip (SoC) environment. Traditional asynchronous handshaking protocol suffers from loss of throughput due to the added latency of sending the acknowledge signal back to the sender. We demonstrate a method that supports end-to-end communication across links with arbitrarily large latency, without limiting the bandwidth, so long as line variation can be reliably controlled. We also evaluate the energy and latency improvements as a result of the design choices made available by this protocol. The use of transmission lines as a physical interconnect medium shows promise for deep submicron technologies. In our evaluations, we notice a lower energy footprint, as well as vastly reduced wire latency for transmission line interconnects. We approach this problem from two sides. Using field solvers, we investigate the physical design choices to determine the optimal way to implement these lines for a given back-end-of-line (BEOL) stack. We also approach the problem from a system designer's viewpoint, looking at ways to optimize the lines for different performance targets. This work analyzes the advantages and pitfalls of implementing asynchronous channel protocols for communication over long distances. Finally, the innovations resulting from this work are applied to a network-on-chip design example and the resulting power-performance benefits are reported

The Future of Formal Methods and GALS Design

AbstractThe System-on-Chip era has arrived, and it arrived quickly. Modular composition of components through a shared interconnect is now becoming the standard, rather than the exotic. Asynchronous interconnect fabrics and globally asynchronous locally synchronous (GALS) design has been shown to be potentially advantageous. However, the arduous road to developing asynchronous on-chip communication and interfaces to clocked cores is still nascent. This road of converting to asynchronous networks, and potentially the core intellectual property block as well, will be rocky. Asynchronous circuit design has been employed since the 1950's. However, it is doubtful that its present form will be what we will see 10 years hence. This treatise is intended to provoke debate as it projects what technologies will look like in the future, and discusses, among other aspects, the role of formal verification, education, the CAD industry, and the ever present tradeoff between greed and fear

Synchronous Elasticization: Considerations For Correct Implementation and MiniMIPS Case Study

Abstract—Latency insensitivity is a promising design paradigm in the nanometer era since it has potential benefits of increased modularity and robustness to variations. Synchronous elasticization is one approach (among others) of transforming an ordinary clocked circuit into a latency insensitive design. This paper presents practical considerations of elasticizing reconvergent fanouts. It also investigates the suitability of previously published as well as new join and fork implementations for usage in the elastic control network. We demonstrate that elasticization comes at a cost. Measurements of a MiniMIPS processor fabricated in a 0.5 µm node show that elasticization results in area and dynamic and idle power penalties of 29%, 13 % and 58.3%, respectively, without any loss in performance. These measurements do not exploit the capability of pipeline bubbles that occur if one needs to have unpredictable interface latency, or to insert extra bubbles into a pipeline due to wire delays. We finally show the architectural performance advantage of eager over lazy protocols in the presence of bubbles in the MiniMIPS

SAS: Source Asynchronous Signaling Protocol for Asynchronous Handshake Communication Free From Wire Delay Overhead

Abstract — Asynchronous handshake protocol communication is accomplished by sending data down a communication link coupled with data validity information. Flow control is established by acknowledging the receipt of data, thereby enabling transmission of new data down the link. Handshake protocols operate at target cycle times based on system operational requirements. When the communication delay down wires increases beyond a certain point, the latency in sending the request and acknowledge signals across the link becomes longer than the target cycle time. This reduces the communication bandwidth below the desired value. This deleterious effect is particularly conspicuous on long links and network-on-chip communication. A method of enabling full communication bandwidth on wires with arbitrary delay when employing handshake communication is provided. This method supports end-to-end communication across links with arbitrarily large but finite latency without limiting the bandwidth, so long as line variation can be reliably controlled. This paper introduces the new SAS protocol, provides an efficient implementation, and reports the resultant significant energy and bandwidth improvements over conventional handshaking methods. I

Leveraging the geometric properties of on-chip transmission line structures to improve interconnect performance: A case study in 65nm

Abstract — Implementation of low energy, low latency transmission line interconnects on a network-on-chip presents the circuit designer with a variety of structural design choices. This work presents a study of the comparative effects of changing the wire geometries on the latency, energy dissipated, area, and noise properties of the transmission lines. These results will aid the engineer in the design and performance analysis of the global interconnect and foster a quantitative understanding of the wave signaling properties in the RLC regime. Energy dissipation in wires has become a primary bottleneck for the continued scaling of future interconnect networks. Packet switching on high radix network topologies exacerbates this problem due to the energy and latency overhead contributed by switches and routers. Alternative network fabrics utilizing low latency, low energy buses hav