1 Clockless Logic Montek Singh Thu, Jan 13, 2004

2 Preliminaries  How is data represented in an asynchronous system?  How is information exchanged?: control signaling (handshake styles)

3 Data Encoding: “Bundled Data” Single-rail “Bundled Datapath”: simplest approach widely used widely usedFeatures: datapath: 1 wire per bit (e.g. standard sync blocks) datapath: 1 wire per bit (e.g. standard sync blocks) matched delay: produces delayed “done” signal matched delay: produces delayed “done” signal  worst-case delay: longer than slowest path +Practical style: can reuse sync components ; small area –Fixed (worst-case) completion time done indicates valid data valid data bit 1 request bit n bit 1 bit m done matcheddelay function block

4 Bundled Data: Completion Sensing Delay Matching: either single worst-case delay either single worst-case delay or, fine-grain delay or, fine-grain delay requestdone bank of delays MUX delay selector Speculative completion: choose delay “on the fly” choose delay “on the fly” start with shortest delay; increase as needed start with shortest delay; increase as needed

5 +provides robust data-dependent completion –needs completion detectors Data Encoding: Dual-Rail Dual-rail: uses 2 wires per data bit bit n bit 1 bit m bit 1 Each Dual-Rail Pair: provides both data value and validity

6 Dual-Rail: Completion Sensing Dual-Rail Completion Detector: combines dual-rail signals combines dual-rail signals indicates when all bits are valid (or reset) indicates when all bits are valid (or reset) C Done OR bit 0 OR bit 1 OR bit n  OR together 2 rails per bit  Merge results using a Müller “C-element” C-element: if all inputs=1, output  1 if all inputs=1, output  1 if all inputs=0, output  0 if all inputs=0, output  0 else, maintain output value else, maintain output valueC-element: if all inputs=1, output  1 if all inputs=1, output  1 if all inputs=0, output  0 if all inputs=0, output  0 else, maintain output value else, maintain output value

7 4-Phase: requires 4 events per handshake Handshaking Styles: 4-phase Request Acknowledge start event done get ready for next event ready for next event +“Level-sensitive”  simpler logic implementation –Overhead of “return-to-zero” (RTZ or resetting) extra events which do no useful computation extra events which do no useful computation

8 +Elegant: no return-to-zero –Slower logic implementation: logic primitives are inherently level-sensitive, not event-based (at least in CMOS) logic primitives are inherently level-sensitive, not event-based (at least in CMOS) Handshaking Styles: 2-phase 2-Phase: requires 2 events per handshake a.k.a. transition signaling a.k.a. transition signaling Request Acknowledge start event done start next event next event done

9 +No return-to-zero (like 2-phase) +Level-based implementation (like 4-phase) –Need a timing constraint on pulse with Handshaking Styles: Pulse Mode Pulse Mode: combines benefits of 2-phase and 4-phase use pulses to represent events use pulses to represent events Request Acknowledge start event done start next event next event done

10 +Efficient protocol: no return-to-zero, level-based –Need aggressive low-level design techniques  much effort to ensure reliability Handshaking Styles: Single-Track Single-Track: combines req and ack onto single wire! one wire used for bidirectional communication one wire used for bidirectional communication  sender raises, receiver lowers req + ack Request Acknowledgereqreqackack

11 Handshaking + Data Representation Several combinations possible: dual-rail 4-phase, single-rail 4-phase, dual-rail 2-phase, and single- rail 2-phase dual-rail 4-phase, single-rail 4-phase, dual-rail 2-phase, and single- rail 2-phase Example: dual-rail 4-phase dual-rail data: functions as an implicit “request” dual-rail data: functions as an implicit “request” 4-phase cycle: between acknowledge and implicit request 4-phase cycle: between acknowledge and implicit request bit m bit 1 ack A B

12 Other Data Representation Styles  Level-Encoded Dual-Rail (LEDR) 2 wires per bit: “data” and “phase” 2 wires per bit: “data” and “phase” exactly one wire per bit changes value exactly one wire per bit changes value  if new value is different, “data” wire changes value  else “phase” wire change value  M-of-N Codes N wires used for a data word N wires used for a data word M wires (M <= N) change value M wires (M <= N) change value Values of N and M: have impact on… Values of N and M: have impact on…  information transmitted, power consumed and logic complexity  Knuth codes, Huffman codes, … data phase

13 Which to use? Depends on several performance parameters: speed speed  single-rail vs. dual-rail –single-rail may be faster (if designed aggressively) –dual-rail may be faster (if completion times vary widely)  2-phase vs. 4-phase –2-phase may be faster (if logic overhead is small) –4-phase may be faster (if overhead of return-to-zero is small) power consumption power consumption  2-phase typically has fewer gate transitions (  lower power) amount of logic used (#gates/wires/pins  chip area) amount of logic used (#gates/wires/pins  chip area)  single-rail needs fewer gates/wires/pins design and verification effort design and verification effort  dual-rail, 1-of-N, M-of-N, Knuth codes…: –delay-insensitive: robust in the presence of arbitrary delays  single-rail: requires greater timing verification effort