Globally Asynchronous Locally Synchronous Design Research Articles

A GALS design based on multi-frequency clocking for digital switching noise reduction

In this paper, we investigate two design techniques to reduce the digital switching noise on the power supply lines: a multi-frequency clocking (MFC) and a globally asynchronous locally synchronous (GALS) design technique. By deploying an MFC design, we can spread the switching current peaks over a frequency band, which results in the supply noise reduction. With the use of a GALS technique, we partition a large synchronous design into some smaller locally synchronous modules (LSMs) which are clocked by their own local clocks. Each local clock signal can be different from the other in terms of frequency or phase. Thus, we can also distribute the digital switching activities inside circuits over time, which leads to the supply noise reductions. The experimental results on a commercial Field Programmable Gate Array (FPGA) Spartan-3 (XC3S400-TQ144) show that the noise reduction rates can be achieved up to 17.4 dB for a GALS system with 8 LSMs, and 19.2 dB for the joint use of the MFC and GALS techniques with 4 LSMs. Moreover, as a real design example, a DES crypto processor with the deployment of GALS design and multi-frequency clocking strategy is also implemented and evaluated. The noise reduction rate is achieved about 19.5 dB at the fundamental frequency.

Asynchronous Bypass Channels for Multi-Synchronous NoCs: A Router Microarchitecture, Topology, and Routing Algorithm

Network-on-chip (NoC) designs have emerged as a replacement for traditional shared-bus designs for on-chip communication. As with all current very large scale integration designs, however, reducing power consumption in NoCs is a critical challenge. One approach to reduce power consumption is to dynamically scale the voltage and frequency of each network node or groups of nodes (DVFS). Another approach is to replace the balanced clock tree with a globally-asynchronous, locally-synchronous (GALS) clocking scheme. In both DVFS and GALS designs, the chip as a whole is multi-synchronous. As the NoCs interconnecting those nodes must communicate across these clock domain boundaries, they tend to have high latencies as packets must be synchronized at the intermediate nodes. In this paper, we propose a novel router microarchitecture which offers superior performance with respect to typical synchronizing router designs for multi-synchronous networks. Our approach features asynchronous bypass channels which allow flit traversal of intermediate nodes within the network without the latching or synchronization overheads of typical designs. We also propose a new network topology and routing algorithm that leverage the advantages of the bypass channel offered by our router design. We present a detailed analysis of design decisions which affect the performance of the asynchronous bypass channel network. Our experiments show that our design improves the performance of a conventional synchronizing design with similar resources by up to 26% at low loads and increases saturation throughput by up to 50% for a uniform random traffic.

The Future of Formal Methods and GALS Design

The 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.

From synchronous to GALS: A new architecture for FPGAs

The conflictual demand of faster and larger designs is increasingly difficult to answer by the advances of solid state technology alone. At some point, it is expected that designers and manufacturers will have to give up the traditional synchronous design methodology for a Globally Asynchronous Locally Synchronous (GALS) one. Such changes imply more synchronization constraints, but also more flexibility. Consequently, this paper proposes a novel Field-Programmable Gate Arrays (FPGA) architecture that is compatible with existing devices and that can also support GALS designs. The main objective is simple: the proposed architecture must appear unchanged for synchronous design, but it must also include a minimal amount of basic components to prevent metastability for efficient asynchronous communications. Thus, the paper presents the constraint equations required to implement such a circuit. It also presents a pausible clock generator application and simulation results for the proposed architecture. All results demonstrate that with a few additional customized circuits, a standard FPGA cell can become appropriate for GALS methodologies.

Guest Editors' Introduction: GALS Design and Validation

Globally asynchronous, locally synchronous (GALS) design has grown in popularity in both academia and industry. Breaking the synchrony assumption in digital design is often unsettling for designers, and to alleviate the difficulty, researchers in EDA have been proposing various GALS-based solutions. However, the tools, verification techniques, and testing methodologies for asynchronous designs are not as widespread as for synchronous digital design, leading to the hitherto limited usage of GALS design approaches. This special issue introduces some of the basic issues of GALS design and validation in the hardware domain. The hope is that this special issue will generate more interest by researchers and industry practitioners in creating design tools, techniques, and validation methodologies for GALS design.

A Survey and Taxonomy of GALS Design Styles

Single-clocked digital systems are largely a thing of the past. Although most digital circuits remain synchronous, many designs feature multiple clock domains, often running at different frequencies. Using an asynchronous interconnect decouples the timing issues for the separate blocks. Systems employing such schemes are called globally asynchronous, locally synchronous (GALS). To minimize time to market, large SoC designs must integrate many functional blocks with minimal design effort. These blocks are usually designed using standard synchronous methods and often have different clocking requirements. A GALS approach can facilitate fast block reuse by providing wrapper circuits to handle interblock communication across clock domain boundaries. SoCs may also achieve power savings by clocking different blocks at their minimum speeds. For example, Scott et al. describe the advantages of GALS design for an embedded-processor peripheral bus.

Globally Asynchronous, Locally Synchronous Circuits: Overview and Outlook

For more than 20 years, significant research effort was concentrated on globally asynchronous, locally synchronous (GALS) design methodologies. But despite several successful implementations, GALS has had little impact on industry products. This article presents different GALS techniques and architectures. The authors also analyze the actual challenges and problems for wider adoption of the currently proposed GALS methods. Their analysis shows that significant improvement can be achieved in terms of system integration and EMI reduction. On the other hand, for power savings, only marginal improvements to the existing techniques can be expected. Additionally, introduction of a GALS approach leads to relatively small area increases, and in some cases even causes certain performance losses. The authors present major examples of GALS implementations. Finally, they outline some directions for future development of GALS techniques and their design flow. It is quite clear that the GALS design and test flow must be improved and more automated. Furthermore, the attendant performance degradations must be limited--for example, high data throughput must be ensured through very low hardware overhead for the GALS circuitry.

System integration by request-driven GALS design

A novel request-driven globally asynchronous locally synchronous (GALS) technique for the system integration of complex digital blocks is proposed. For this new GALS technique, an asynchronous wrapper compliant is developed and evaluated. This proposed GALS technique is applied to a baseband processor compatible with the wireless LAN standard IEEE 802.11a. The developed GALS baseband processor chip is fabricated and measured. Besides improvements of the system integration process, a 5 dB reduction in electromagnetic interference, 30% reduction in instantaneous supply current variation, and similar dynamic power consumption as in the synchronous baseband processor is achieved. Modern mobile communication devices contain highly complex digital structures. An ever-increasing system com- plexity is leading to growing challenges in the area of system integration, power management and noise reduction. In order to cope with these problems, different methods have been proposed. One of the most promising techniques at present is the globally asynchronous locally synchronous (GALS) methodology for system integration. The GALS methodology was first proposed over twenty years ago by Chapiro (1) , and in the last few years has regained in popularity with several new methods being developed. A GALS circuit consists of several modules. Each module works synchronously and is surrounded by an asynchronous wrapper (AW) that has an asynchronous interface to other modules. Together, a wrapper and its enclosed module form a GALS block. Most GALS propo- sals are based on a pausible clocking scheme as can be seen in the works of Muttersbach (2) , Moore et al. (3) , and Bormann and Cheung (4) . Those GALS proposals

Synchro-Tokens: A Deterministic GALS Methodology for Chip-Level Debug and Test

This paper describes a novel deterministic globally-asynchronous locally-synchronous (GALS) methodology called synchro-tokens. Wrappers around the synchronous blocks keep the system globally asynchronous while ensuring that each transition, although arriving at a nondeterministic time, is sensed by the synchronous block during a deterministic cycle of the local clock. This determinism facilitates debug and test methodologies, such as the use of stored-pattern testers, which are effective only when the system behavior is predictable and repeatable. Applications of synchro-tokens to GALS systems with two or more synchronous blocks and one or more asynchronous data channels are shown. Synchro-tokens supports both pipelined and unpipelined channels and a variety of clock generation methodologies. Novel schematic level designs of the wrapper components in a 180-nm technology are used to compare the performance of several different deterministic GALS design styles.