1. 1 Clockless Logic or How do I make hardware fast, power- efficient, less noisy, and easy-to-design? Montek Singh Thu, Jan 8, 2004

2. 2 Course Information (1) Course Number: COMP290-084 Time and Place Tue/Thu 3:30-4:45pm, Sitterson Hall 325 Tue/Thu 3:30-4:45pm, Sitterson Hall 325  Any conflicts? Instructor Montek Singh Montek Singh montek@cs.unc.edu (not singh@cs!) montek@cs.unc.edu (not singh@cs!) montek@cs.unc.edu SN 245, 962-1832 SN 245, 962-1832 Teaching Assistant None None Course Web Page http://www.cs.unc.edu/~montek http://www.cs.unc.edu/~montek

3. 3 Course Information (2) Prerequisites: undergraduate knowledge of: digital logic, algorithms, discrete math (sets and graphs) undergraduate knowledge of: digital logic, algorithms, discrete math (sets and graphs) you are assumed to know the following topics: you are assumed to know the following topics:  digital logic: Boolean algebra, logic gates, and latches and registers  algorithms: search techniques, enumeration, divide and conquer, and time complexity  discrete math: elementary set theory and graph theory no knowledge of advanced circuit design or of VLSI is assumed no knowledge of advanced circuit design or of VLSI is assumed  relevant topics will be covered in class as needed  VLSI primer included in this class

4. 4 Course Information (3) Textbook (required): Principles of Asynchronous Circuit Design  A Systems Perspective. Principles of Asynchronous Circuit Design  A Systems Perspective.  Jens Sparsø and Steve Furber (eds.). Kluwer. (ASK ME!) Textbooks (optional): Principles of CMOS VLSI Design: A Systems Perspective Principles of CMOS VLSI Design: A Systems Perspective  Weste and Eshraghian. Addison-Wesley, 1993. Computer Aids for VLSI Design Computer Aids for VLSI Design  Steven M. Rubin. Static Free Software. http://www.rulabinsky.com/cavd (Free, online) http://www.rulabinsky.com/cavd Other Reading Material: Lecture notes Lecture notes Papers and technical reports supplied by instructor Papers and technical reports supplied by instructor

5. 5 Course Information (4) Course Content: Introduction to clockless logic Introduction to clockless logic  Benefits and challenges  Data representation, and control signaling Graphical representation of asynchronous systems Graphical representation of asynchronous systems  Petri nets, state transition graphs, burst-mode machines, etc. Algorithms for logic synthesis Algorithms for logic synthesis  Combinational  Sequential VLSI design primer VLSI design primer Design techniques Design techniques  High-performance: fine-grain pipelining  Low-power Formal methods Formal methods  Performance analysis  Verification Case studies of real-world asynchronous processors Case studies of real-world asynchronous processors

6. 6 Course Information (4) Grading Homework: 30% Homework: 30% Project: 40% Project: 40%  your choice of topic: from pure algorithms to VLSI design Presentation: 15% Presentation: 15% Class participation: 15% Class participation: 15% Honor Code is in effect encouraged to discuss ideas/concepts encouraged to discuss ideas/concepts work handed in must be your own work handed in must be your own acknowledge all help acknowledge all help

7. 7 Lecture 1: Introduction  What is asynchronous design?  Why do we want to study it?  How is data represented in an asynchronous system?  How is information exchanged?

8. 8 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

9. 9 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

10. 10 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  challenge: gluing together multiple timing domains –glue logic is susceptible to “metastability” (=incorrect values transferred) and latency overheads Increasing Difficulties with Clocked Design: Clock distribution: requires significant designer effort Clock distribution: requires significant designer effort Performance bottleneck: a single slow component Performance bottleneck: a single slow component Clock burns large fraction of chip power (~40-70%) Clock burns large fraction of chip power (~40-70%) Fixed clock rate: poor match for Fixed clock rate: poor match for  designing reusable components  interfacing with mixed-timing environments

11. 11 What is Asynchronous Design?  Digital design with no centralized clock  Synchronization using local “handshaking” Asynchronous System (Distributed Control) handshakinginterface Synchronous System (Centralized Control) clock

12. 12 Why Asynchronous Design? (1)  Higher Performance May obtain “average-case” operation (not “worst-case”) May obtain “average-case” operation (not “worst-case”)  not limited by slowest component 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, smartcards] Much less interference with sensitive receivers [e.g., Philips pagers, smartcards]  Greater Flexibility/Modularity Naturally adapt to variable-speed environments Naturally adapt to variable-speed environments Supports reusable components Supports reusable components

13. 13 Why Asynchronous Design? (2)  The world already is mostly asynchronous! Events at the level of (or in between) large-scale systems are asynchronous Events at the level of (or in between) large-scale systems are asynchronous  several seconds to several milliseconds  e.g., PC-printer communication, keyboard inputs, network comm. Events at the board level (or between chips) are often asynchronous Events at the board level (or between chips) are often asynchronous  milliseconds to 100 nanoseconds  e.g., CPU-memory interface, interface with I/O subsystem (interrupts) Events within a chip, at the level of functional units (e.g., adders, control logic) are currently synchronous Events within a chip, at the level of functional units (e.g., adders, control logic) are currently synchronous  several nanoseconds to 100 picoseconds Events at the level of a single logic gate are asynchronous Events at the level of a single logic gate are asynchronous  10 picoseconds Events at the quantum level are asynchronous Events at the quantum level are asynchronous  picoseconds to femtoseconds  So, why bother with clocks at all?! make everything asynchronous  greater elegance and robustness make everything asynchronous  greater elegance and robustness

14. 14 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

15. 15 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…

16. 16 Recent Commercial Interest Several commercial asynchronous chips: Philips: asynchronous 80c51 microcontrollers Philips: asynchronous 80c51 microcontrollers  used in commercial pagers [1998] and smartcards [2001] 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/UNC: asynchronous digital FIR filter IBM/Columbia/UNC: asynchronous digital FIR filter Several recent startups: Theseus Logic, Fulcrum, Self-Timed Solutions… Theseus Logic, Fulcrum, Self-Timed Solutions…

17. 17 A 5-minute 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

18. 18 got it Solution 1 Alice uses 2 lamps: 1 to indicate that she is ready with the message, and 1 to indicate that she is ready with the message, and 1 for the message itself 1 for the message itself Bob uses 1 lamp: to indicate that he has received the message to indicate that he has received the message Alice Bob ready yes/no

19. 19 Solution 2 Alice uses 2 lamps: Green lamp to indicate “yes” Green lamp to indicate “yes” Red lamp to indicate “no” Red lamp to indicate “no” Bob uses 1 lamp: to indicate that he has received the message to indicate that he has received the message got it Alice Bob no yes

20. 20 Solution 3 What if Alice and Bob could keep time? Alice uses 1 lamp for the message: At 12 midnight: turns on lamp if message = “yes” At 12 midnight: turns on lamp if message = “yes” At 12:01: turns lamp off At 12:01: turns lamp off Bob needs no lamps! Takes down the message between 12 and 12:01 Takes down the message between 12 and 12:01 Pros: Fewer signals, lesser processing needed Cons: Alice and Bob must keep their clocks closely synchronized If Bob’s watch is off by a minute, incorrect communication possible If Bob’s watch is off by a minute, incorrect communication possible