1 Seminar on High-Speed Asynchronous Pipelines Montek Singh Thursdays 10-11, SN325

2 Lecture 1: Introduction  What is asynchronous design? Why do we want to study it?  What is pipelining? How can it be used to design really fast hardware?

3 Introduction: Clocked Digital Design Most current digital systems are synchronous: Clock: a global signal that paces operation of all components Clock: a global signal that paces operation of all components clock Benefit of clocking: enables discrete-time representation all components operate exactly once per clock tick all components operate exactly once per clock tick component outputs need to be ready by next clock tick component outputs need to be ready by next clock tick  allows “glitchy” or incorrect outputs between clock ticks

4 Microelectronics Trends Current and Future Trends: Significant Challenges Large-Scale “Systems-on-a-Chip” (SoC) Large-Scale “Systems-on-a-Chip” (SoC)  100 Million ~ 1 Billion transistors/chip Very High Speeds Very High Speeds  multiple GigaHertz clock rates Explosive Growth in Consumer Electronics Explosive Growth in Consumer Electronics  demand for ever-increasing functionality …  … with very low power consumption (limited battery life) Higher Portability/Modularity/Reusability Higher Portability/Modularity/Reusability  “plug ’n play” components, robust interfaces

5 Challenges to Clocked Design Breakdown of Single-Clock Paradigm: Chip will be partitioned into multiple timing domains Chip will be partitioned into multiple timing domains Increasing Difficulties with Clocked Design: Clock distribution: will require significant designer effort Clock distribution: will require significant designer effort Performance bottleneck: a single slow component Performance bottleneck: a single slow component Clock burns large fraction of chip power Clock burns large fraction of chip power Fixed clock rate: poor match for Fixed clock rate: poor match for  designing reusable components  interfacing with mixed-timing environments

6 What is Asynchronous Design?  Digital design with no centralized clock  Synchronization using local “handshaking” Synchronous System (Centralized Control) Asynchronous System (Distributed Control) handshakinginterface clock

7 Why Asynchronous Design?  Higher Performance May obtain “average-case” operation (not “worst-case”) May obtain “average-case” operation (not “worst-case”) Avoids overheads of multi-GHz clock distribution Avoids overheads of multi-GHz clock distribution  Lower Power No clock power expended No clock power expended Inactive components consume negligible power Inactive components consume negligible power  Better Electromagnetic Compatibility Smooth radiation spectra: no clock spikes Smooth radiation spectra: no clock spikes Much less interference with sensitive receivers [e.g., Philips pagers] Much less interference with sensitive receivers [e.g., Philips pagers]  Greater Flexibility/Modularity Naturally adapt to varied environments Naturally adapt to varied environments Supports reusable components Supports reusable components

8 Challenges of Asynchronous Design communication must be hazard-free! communication must be hazard-free! special design challenge = “hazard-free synthesis” special design challenge = “hazard-free synthesis”  Testability Issues: absence of clock means no “single-stepping” absence of clock means no “single-stepping”  Lack of Commercial CAD Tools: chicken-and-egg problem chicken-and-egg problem  Hazards: potential “glitches” on wire clean signals hazardous signals clock tick no problem for clocked systems

9 Asynchronous Design: Past & Present Async Design: In existence for 50 years, but … … many recent technical advances: Hazard-Free Circuit Design: Hazard-Free Circuit Design:  several practical techniques for controllers [Stanford/Columbia] Design for Testability: Design for Testability:  several test solutions, e.g. Philips Research Maturing Computer-Aided-Design (“CAD”) Tools: Maturing Computer-Aided-Design (“CAD”) Tools:  software tools for automated design [Philips,Columbia,Manchester] Successful Fabricated Chips: Successful Fabricated Chips:  embedded processors, high-speed pipelines, consumer electronics…

10 Recent Commercial Interest Several commercial asynchronous chips: Philips: asynchronous 80c51 microcontrollers Philips: asynchronous 80c51 microcontrollers  used in commercial pagers [1998] and cell phones [2000] Univ. of Manchester: async ARM processor [2000] Univ. of Manchester: async ARM processor [2000] Motorola: async divider in PowerPC chip [2000] Motorola: async divider in PowerPC chip [2000] HAL: async floating-point divider HAL: async floating-point divider  in HAL-I and II processors [early 1990’s] Recent experimental chips: IBM, Sun and Intel: IBM, Sun and Intel:  fast pipelines, arbiters, instruction-length decoder… IBM/Columbia Univ.: asynchronous digital FIR filter IBM/Columbia Univ.: asynchronous digital FIR filter Several recent startups: Theseus Logic, ADD, AmuCo… Theseus Logic, ADD, AmuCo…

11 Seminar Focus Overall Goal: Asynchronous Design for Very High-Speed Systems Focus: High-Throughput Pipelines Motivation: Pipelining is at the heart of nearly all high-performance digital systems high-performance digital systems Additional Benefits: Low power Low power Interfacing with mixed systems Interfacing with mixed systems Modular and scalable design Modular and scalable design

12 A “coarse-grain” pipeline (e.g. simple processor) A “fine-grain” pipeline (e.g. pipelined adder) fetchdecodeexecute Background: Pipelining What is Pipelining?: Breaking up a complex operation on a stream of data into simpler sequential operations + Throughput: significantly increased – Latency: somewhat degraded Storage elements (latches/registers) Throughput = #data items processed/second

13 Seminar Focus (contd.) Particular Focus: Extremely fine-grain pipelines “gate-level” pipelining = use narrowest possible stages “gate-level” pipelining = use narrowest possible stages each stage consists of only a single level of logic gates each stage consists of only a single level of logic gates  some of the fastest existing digital pipelines to date Application areas: multimedia hardware (graphics accelerators, video DSP’s, …) multimedia hardware (graphics accelerators, video DSP’s, …)  naturally pipelined systems, throughput is critical  input is often “bursty” optical networking optical networking  serializing/deserializing FIFO’s genomic string matching? genomic string matching?  KMP style string matching: variable skip lengths

14 Homework Problem Alice and Bob live on opposite sides of a wide river: Alice is supposed to send a message (say, a “Yes”/”No”) across to Bob around midnight. Both have flashlights, but neither owns a watch. What should they do? Suggest several strategies, and discuss pros and cons of each. AliceBob