Steven J. E. Wilton

System-on-Chip: Reuse and Integration

Over the past ten years, as integrated circuits became increasingly more complex and expensive, the industry began to embrace new design and reuse methodologies that are collectively referred to as system-on-chip (SoC) design. In this paper, we focus on the reuse and integration issues encountered in this paradigm shift. The reusable components, called intellectual property (IP) blocks or cores, are typically synthesizable register-transfer level (RTL) designs (often called soft cores) or layout level designs (often called hard cores). The concept of reuse can be carried out at the block, platform, or chip levels, and involves making the IP sufficiently general, configurable, or programmable, for use in a wide range of applications. The IP integration issues include connecting the computational units to the communication medium, which is moving from ad hoc bus-based approaches toward structured network-on-chip (NoC) architectures. Design-for-test methodologies are also described, along with verification issues that must be addressed when integrating reusable components.

A Flexible Power Model for FPGAs

This paper describes a flexible power model for FPGAs. The model estimates the dynamic, short circuit, and leakage power for a wide variety of FPGA architectures. Such a model will be essential in the design and research of next-generation FPGAs, where power will be one of the primary optimization goals. The model has been integrated into the VPR CAD flow, and is available to the research community for use in FPGA architectural and CAD tool experimentation.

Tradeoffs in two-level on-chip caching

The performance of two-level on-chip caching is investigated for a range of technology and architecture assumptions. The area and access time of each level of cache is modeled in detail. The results indicate that for most workloads, two-level cache configurations (with a set-associative second level) perform marginally better than single-level cache configurations that require the same chip area once the first-level cache sizes are 64KB or larger. Two-level configurations become even more important in systems with no off-chip cache and in systems in which the memory cells in the first-level caches are multiported and hence larger than those in the second-level cache. Finally, a new replacement policy called two-level exclusive caching is introduced. Two-level exclusive caching improves the performance of two-level caching organizations by increasing the effective associativity and capacity.

A detailed power model for field-programmable gate arrays

Power has become a critical issue for field-programmable gate array (FPGA) vendors. Understanding the power dissipation within FPGAs is the first step in developing power-efficient architectures and computer-aided design (CAD) tools for FPGAs. This article describes a detailed and flexible power model which has been integrated in the widely used Versatile Place and Route (VPR) CAD tool. This power model estimates the dynamic, short-circuit, and leakage power consumed by FPGAs. It is the first flexible power model developed to evaluate architectural tradeoffs and the efficiency of power-aware CAD tools for a variety of FPGA architectures, and is freely available for noncommercial use. The model is flexible, in that it can estimate the power for a wide variety of FPGA architectures, and it is fast, in that it does not require extensive simulation, meaning it can be used to explore a large architectural space. We show how the model can be used to investigate the impact of various architectural parameters on the energy consumed by the FPGA, focusing on the segment length, switch block topology, lookuptable size, and cluster size.

On the Interaction Between Power-Aware FPGA CAD Algorithms

As Field-Programmable Gate Array (FPGA) power consumptioncontinues to increase, lower power FPGA circuitry, architectures,and Computer-Aided Design (CAD) tools need to be developed.Before designing low-power FPGA circuitry, architectures, orCAD tools, we must first determine where the biggest savings (interms of energy dissipation) are to be made and whether thesesavings are cumulative. In this paper, we focus on FPGA CADtools. Specifically, we describe a new power-aware CAD flow forFPGAs that was developed to answer the above questions.Estimating energy using very detailed post-route power and delaymodels, we determine the energy savings obtained by our power-awaretechnology mapping, clustering, placement, and routingalgorithms and investigate how the savings behave when thealgorithms are applied concurrently. The individual savings of thepower-aware technology-mapping, clustering, placement, androuting algorithms were 7.6%, 12.6%, 3.0%, and 2.6%respectively. The majority of the overall savings were achievedduring the technology mapping and clustering stages of the power-awareFPGA CAD flow. In addition, the savings were mostlycumulative when the individual power-aware CAD algorithmswere applied concurrently with an overall energy reduction of 22.6%.

The Impact of Pipelining on Energy per Operation in Field-Programmable Gate Arrays

This paper investigates experimentally the quantitative impact of pipelining on energy per operation for two representative FPGA devices: a 0.13μm CMOS high density/high speed FPGA (Altera Stratix EP1S40), and a 0.18μm CMOS low-cost FPGA (Xilinx XC2S200). The results are obtained by both measurements and execution of vendor-supplied tools for power estimation. It is found that pipelining can reduce the amount of energy per operation by between 40% and 90%. Further reduction in energy consumption can be achieved by power-aware clustering, although the effect becomes less pronounced for circuits with a large number of pipeline stages.

Dynamic voltage scaling for commercial FPGAs

A methodology for supporting dynamic voltage scaling (DVS) on commercial FPGAs is described. A logic delay measurement circuit (LDMC) is used to determine the speed of an inverter chain for various operating conditions at run time. A desired LDMC value, intended to match the critical path of the operating circuit plus a safety margin, is then chosen; a closed loop control scheme is used to maintain the desired LDMC value as chip temperature changes, by automatically adjusting the voltage applied to the FPGA. We describe experiments using this technique on various circuits at different clock frequencies and temperatures to demonstrate its utility and robustness. Power savings between 4% and 54% for the VINT supply are observed

SMAP: heterogeneous technology mapping for area reduction in FPGAs with embedded memory arrays

It has become clear that large embedded configurable memory arrays will be essential in future FPGAs. Embedded arrays provide high-density high-speed implementations of the storage parts of circuits. Unfortunately, they require the FPGA vendor to partition the device into memory and logic resources at manufacture-time. This leads to a waste of chip area for customers that do not use all of the storage provided. This chip area need not be wasted, and can in fact be used very efficiently, if the arrays are configured as large multi-output ROMs, and used to implement logic. In order to efficiently use the embedded arrays in this way, a technology mapping algorithm that identifies parts of circuits that can be efficiently mapped to an embedded array is required. In this paper, we describe such an algorithm. The new tool, called SMAP, packs as much circuit information as possible into the available memory arrays, and maps the rest of the circuit into four-input lookup-tables. On a set of 29 sequential and combinational benchmarks, the tool is able to map, on average, 60 4-LUTs into a single 2-Kbit memory array. If there are 16 arrays available, it can map, on average, 358 4-LUTs to the 16 arrays.

Programmable logic IP cores in SoC design: opportunities and challenges

As SoC design enters into mainstream usage, the ability to make post-fabrication changes will become more and more attractive. This ability can be realized using programmable logic cores. These cores are like any other IP in the SoC design methodology, except that their function can be changed after fabrication. This paper outlines ways in which programmable logic cores can simplify SoC design, and describes some of the challenges that must be overcome if the use of programmable logic cores is to become a mainstream design technique.

Floating-Point FPGA: Architecture and Modeling

This paper presents an architecture for a reconfigurable device that is specifically optimized for floating-point applications. Fine-grained units are used for implementing control logic and bit-oriented operations, while parameterized and reconfigurable word-based coarse-grained units incorporating word-oriented lookup tables and floating-point operations are used to implement datapaths. In order to facilitate comparison with existing FPGA devices, the virtual embedded block scheme is proposed to model embedded blocks using existing field-programmable gate array (FPGA) tools. This methodology involves adopting existing FPGA resources to model the size, position, and delay of the embedded elements. The standard design flow offered by FPGA and computer-aided design vendors is then applied and static timing analysis can be used to estimate the performance of the FPGA with the embedded blocks. On selected floating-point benchmark circuits, our results indicate that the proposed architecture can achieve four times improvement in speed and 25 times reduction in area compared with a traditional FPGA device.