1 Clockless Logic: Dynamic Logic Pipelines (contd.)  Drawbacks of Williams’ PS0 Pipelines  Lookahead Pipelines

2 Drawbacks of PSO Pipelining 1. Poor throughput: long cycle time: 6 events per cycle long cycle time: 6 events per cycle data “tokens” are forced far apart in time data “tokens” are forced far apart in time 2. Limited storage capacity: max only 50% of stages can hold distinct tokens max only 50% of stages can hold distinct tokens data tokens must be separated by at least one spacer data tokens must be separated by at least one spacer Our Research Goals: address both issues still maintain very low latency still maintain very low latency

3 Recent Approaches 3 novel styles for high-speed async pipelining: MOUSETRAP Pipelines [Singh/Nowick, TAU-00, ICCD-01] MOUSETRAP Pipelines [Singh/Nowick, TAU-00, ICCD-01] “Lookahead Pipelines” (LP) [Singh/Nowick, Async-00] “Lookahead Pipelines” (LP) [Singh/Nowick, Async-00] “High-Capacity Pipelines” (HC) [Singh/Nowick, WVLSI-00] “High-Capacity Pipelines” (HC) [Singh/Nowick, WVLSI-00] Goal: significantly improve throughput of PS0 Two Distinct Strategies: LP: introduce protocol optimizations LP: introduce protocol optimizations  “shave off” components from critical cycle HC: fundamentally new protocol HC: fundamentally new protocol  greater concurrency: “loosely-coupled” stages  

4Outline è New Asynchronous Pipelines: MOUSETRAP Pipelines MOUSETRAP Pipelines è Lookahead Pipelines (LP) High-Capacity Pipelines (HC) High-Capacity Pipelines (HC) Dynamic circuit style Static circuit style

5 Lookahead Pipelines: Strategy #1 Use non-neighbor communication: stage receives information from multiple later stages stage receives information from multiple later stages allows “early evaluation” allows “early evaluation” Benefit: stage gets head-start on next cycle

6 Lookahead Pipelines: Strategy #2 Use early completion detection: completion detector moved before stage (not after) completion detector moved before stage (not after) stage indicates “early done” in parallel with computation stage indicates “early done” in parallel with computation Benefit: again, stage gets head-start on next cycle early completion detector

7 Lookahead Pipelines: Overview 5 New Designs: è“Dual-Rail” Data Signaling: LP3/1: “early evaluation” LP3/1: “early evaluation” LP2/2: “early done” LP2/2: “early done” LP2/1: “early evaluation” + “early done” LP2/1: “early evaluation” + “early done”  “Single-Rail” Bundled-Data Signaling: LP SR 2/2: “early done” LP SR 2/2: “early done” LP SR 2/1: “early evaluation” + “early done” LP SR 2/1: “early evaluation” + “early done”

8 Optimization = “early evaluation” each stage has two control inputs: from stages N+1 and N+2 each stage has two control inputs: from stages N+1 and N+2 Idea: shorten precharge phase terminate precharge early: when N+2 is done evaluating terminate precharge early: when N+2 is done evaluating Dual-Rail Design #1: LP3/1 Data in Data out PC Eval From N+2 N N+1 N+2 Processing Block Completion Detector

9 LP3/1 Protocol LP3/1 Protocol PRECHARGE N: when N+1 completes evaluation PRECHARGE N: when N+1 completes evaluation EVALUATE N: when N+2 completes evaluation EVALUATE N: when N+2 completes evaluation New! Enables “early evaluation!” 4 N evaluates N+1 evaluates N+2 indicates “done” N+2 evaluates N N+1 N+2 N+1 indicates “done” 3

10 PS0PS0 LP3/1LP3/1 LP3/1: Comparison with PS NN+1N+2 NN+1N+2 Enables “early evaluation!” 1 1 evaluates evaluates 2 2 evaluates evaluates 3 3 evaluates evaluates Only 4 events in cycle! 6 events in cycle PRECHARGE N: when N+1 completes evaluation 3 indicates “done” 3 EVALUATE N: when N+2 completes evaluation EVALUATE N: when N+1 completes precharging

LP3/1 Performance Cycle Time = saved path Savings over PS0: 1 Precharge + 1 Completion Detection

12 LP3/1: Inside a Stage Precharge when PC=1 (and Eval=0) Precharge when PC=1 (and Eval=0) Evaluate “early” when Eval=1 (or PC=0) Evaluate “early” when Eval=1 (or PC=0) PC (From Stage N+1) Eval (From Stage N+2) NAND A NAND gate merges 2 control inputs: Problem: “early” Eval=1 is non-persistent!  may be de-asserted before stage completes evaluation! Problem: “early” Eval=1 is non-persistent!  may be de-asserted before stage completes evaluation! Merging 2 Control Inputs: “early Eval” “old Eval”

13 LP3/1 Timing Constraints: Example Observation: PC=0 soon after Eval=1, and is persistent Solution: no change!  use PC as safe “takeover” for Eval! Timing Constraint: PC=0 must arrive before Eval de-asserted  simple one-sided timing requirement  other constraints as well… all easily satisfied in practice PC (From Stage N+1) Eval (From Stage N+2) NAND Problem (cont.): “early” Eval=1 non-persistent

14 Dual-Rail Design #2: LP2/2 Optimization = “early done” Idea: move completion detector before processing block Idea: move completion detector before processing block  stage indicates when “about to” precharge/evaluate Processing Block “early” Completion Detector Data in Data out “early done”

15 LP2/2 Completion Detector Modified completion detectors needed: Done =1 when stage starts evaluating, and inputs valid Done =1 when stage starts evaluating, and inputs valid Done =0 when stage starts precharging Done =0 when stage starts precharging  asymmetric C-element C Done OR bit 0 OR bit 1 OR bit n + + +PC

LP2/2 Protocol Completion Detection: performed in parallel with evaluation/precharge of stage N evaluates N+1 evaluates N N+1 N+2 2 “early done” of N+1 eval 3 3 “early done” of N+2 eval “early done” of N+1 prech

17 LP2/2 Performance LP2/2 savings over PS0: 1 Evaluation + 1 Precharge Cycle Time =

18 Dual-Rail Design #3: LP2/1 Hybrid of LP3/1 and LP2/2. Combines: early evaluation of LP3/1 early evaluation of LP3/1 early done of LP2/2 early done of LP2/2 Cycle Time =

19 Lookahead Pipelines: Overview 5 New Designs: è“Dual-Rail” Data Signaling: LP3/1: “early evaluation” LP3/1: “early evaluation” LP2/2: “early done” LP2/2: “early done” LP2/1: “early evaluation” + “early done” LP2/1: “early evaluation” + “early done”  “Single-Rail” Bundled-Data Signaling: LP SR 2/2: “early done” LP SR 2/2: “early done” LP SR 2/1: “early evaluation” + “early done” LP SR 2/1: “early evaluation” + “early done”

20 Single-Rail Design: LP SR 2/1 Derivative of LP2/1, adapted to single-rail:  bundled-data: matched delays instead of completion detectors delaydelay delay “Ack” to previous stages is “tapped off early”  once in evaluate (precharge), dynamic logic insensitive to input changes

21 PC and Eval are combined exactly as in LP3/1 Inside an LP SR 2/1 Stage “done” generated by an asymmetric C-element done =1 when stage evaluates, and data inputs valid done =1 when stage evaluates, and data inputs valid done =0 when stage precharges done =0 when stage precharges PC (From Stage N+1) Eval (From Stage N+2) NAND aC + “ack” “req” in data in data out “req” out matched delay done

22 LP SR 2/1 Protocol Cycle Time = N evaluates N+2 evaluates N+2 indicates “done” N N+1 N+2 2 N+1 evaluates N+1 indicates “done”

23Results Designed/simulated FIFO’s for each pipeline style Experimental Setup: design: 4-bit wide, 10-stage FIFO design: 4-bit wide, 10-stage FIFO technology: 0.6  HP CMOS technology: 0.6  HP CMOS operating conditions: 3.3 V and 300°K operating conditions: 3.3 V and 300°K

24 dual-rail single-rail Comparison with Williams’ PS0  LP2/1: >2X faster than Williams’ PS0  LP SR 2/1: 1.2 Giga items/sec

25 Comparison: LP SR 2/1 vs. Molnar FIFO’s LP SR 2/1 FIFO: 1.2 Giga items/sec Adding logic processing to FIFO:  simply fold logic into dynamic gate  little overhead Comparison with Molnar FIFO’s: asp* FIFO: 1.1 Giga items/sec asp* FIFO: 1.1 Giga items/sec  more complex timing assumptions  not easily formalized  requires explicit latches, separate from logic!  adding logic processing between stages  significant overhead micropipeline: 1.7 Giga items/sec micropipeline: 1.7 Giga items/sec  two parallel FIFO’s, each only 0.85 Giga/sec  very expensive transition latches  cannot add logic processing to FIFO!

26 datapath width = 32 dual-rail bits! Practicality of Gate-Level Pipelining When datapath is wide:  Can often split into narrow “streams”  comp. d et. f airly low cost!  Use “localized” completion detector for each stream: for each stream: need to examine only a few bits need to examine only a few bits  small fan-in  small fan-in send “done” to only a few gates send “done” to only a few gates  small fan-out  small fan-outdone fan-out=2 comp. det. fan-in = 2

27Conclusions Introduced several new dynamic pipelines: Use two novel protocols: Use two novel protocols: –“early evaluation” –“early done” Especially suitable for fine-grain (gate-level) pipelining Especially suitable for fine-grain (gate-level) pipelining Very high throughputs obtained: Very high throughputs obtained: –dual-rail: >2X improvement over Williams’ PS0 –single-rail: 1.2 Giga items/second in 0.6  CMOS Use easy-to-satisfy, one-sided timing constraints Use easy-to-satisfy, one-sided timing constraints Robustly handle arbitrary-speed environments Robustly handle arbitrary-speed environments –overcome a major shortcoming of Williams’ PS0 pipelines Recent Improvement: Even faster single-rail pipeline (WVLSI’00)