Asynchronous Circuits Jordi Cortadella Universitat Politècnica de Catalunya, Barcelona Collège de...

Asynchronous Circuits

Jordi CortadellaUniversitat Politècnica de Catalunya, Barcelona

Collège de FranceMay 14th, 2013

Goals

• Convince ourselves that:– designing an asynchronous circuit is easy– synchronous and asynchronous circuits are similar– asynchronous circuits bring new advantages

• Not to discourage designers with exotic and sophisticated asynchronous schemes

Collège de France 2013 Asynchronous circuits 2

Clocking

Collège de France 2013 Asynchronous circuits

Nvidia KeplerTM GK110

• How to distribute the clock?

• How to determine the clockfrequency?

• How to implement robustcommunications?

• How to reduce and manageenergy?

3

28nm, 7.1B transistors, 550mm2, 2688 CUDA cores,Base clock: 836MHz, Memory clock: 6GHz

Synchronous circuits

Synchronous circuit


CombinationalLogic

Flip

Flo

ps

Flip

Flo

ps

PLL

6

12112

Synchronous circuit


CL

Two competing paths:• Launching path• Capturing path

Launching path < Capturing path + Period

CLKtree + CL < CLKtree + Period

CL < Period (no clock skew)

2PLL

7

Source-synchronous


CLKgen matched delay matched delay matched delay

• No global clock required

• More tolerance to PVT variations

• Period > longest combinational path

• Good for acyclic pipelines

Launching path

Capturing path

8

CLKgen

?

Source-synchronous with forks and joins


How to synchronize incoming events?

9

C element (Muller 1959)


CA

BC

A

B

C

A B C0 0 00 1 C1 0 C1 1 1

10

C element (Muller 1959)


A

B C

A

B

C

A B C0 0 00 1 C1 0 C1 1 1

MAJ

11

(many implementations exist)

Multi-input C element


C

C

C

C

C

C

a1

a2

a3

a4

a5

a6

a7

c

12

Completion detection

Completion detection


CLKgen

fixed delay

The fixed delay must be longer than theworst-case logic delay (plus variability)

Q: could we detect when a computation has completed ASAP ?

14

A 1 SP 0 SP 1 SP 1 SP

Delay-insensitive codes: Dual Rail• Dual rail: every bit encoded with two signals


A.t A.f A0 0 Spacer0 1 01 0 11 1 Not used

A.t

A.f

15

Dual-Rail AND gate


A B C

SP SP SP

0 - 0

- 0 0

SP 1 SP

1 SP SP

1 1 1

A

BC

A.t

A.f

B.t

B.f

C.t

C.f

16

Dual-Rail Inverter


A Z

SP SP

0 1

1 0

A.t

A.f

Z.t

Z.f

17

Dual-Rail AND/OR gate


A

BC

A.t

A.f

B.t

B.f

C.t

C.f

A

BC

A.f

A.t

B.f

B.t

C.f

C.tA

BC

18

Dual rail: completion detection

Dual-rail logic

•••

•••


00

00

00

00

00

00

00

00

00

00

00

00

00

01

10

10

10

01

01

01

10

01

10

10

01

01


Dual-rail logic

•••

•••

C done

Completion detection tree




AND

OR

INV

AND

CLKgen

21



AND

OR

INV

AND

C

22

C

Single rail data vs. dual railSome back-of-the-envelope estimations:


Single rail Dual RailArea 1 2Delay 1 << 1Static power 1 2Dynamic power < 0.2 2

Dual rail:• Good for speed• Large area• High power comsumption

23

Handshaking

Handshaking


CLKgen unknown delay

Assume that the source module can provide data at any rate:

• When should the CLK generator send an event if the

internal delays of the circuit are unknown?

Solution: handshaking

25

Handshaking


I have data

I want data

Data

Request

Acknowledge

26

Asynchronous elastic pipeline

C

ReqIn ReqOut

AckIn AckOut

C C C

• David Muller’s pipeline (late 50’s)• Sutherland’s Micropipelines (Turing award, 1989)


Multiple inputs and outputs


Multiple inputs and outputs


delay

29

Channel-based communication• A channel contains data and handshake wires


DataReq

Ack

30

DataReq

Ack

Two-phase protocol

• Every edge is active• It may require double-edge triggered flip-flops or

pulse generators


Data 1 Data 2 Data 3

Req

Ack

Data

Data transfer Data transfer

31

Four-phase protocol

• Valid data on the active edge of Req• Req/Ack must return to zero before the next transfer• Different variations of the 4-phase protocol exist


Data 1 Data 2 Data 3

Req

Ack

Data

Data transfer Data transfer

32

How to memorize?


CombinationalLogic LL

delay

CC

? ?

2-phase or 4-phase ?

33

How to memorize?



delay

CC

Pulsegenerator

2-phase

34

How to memorize?



delay

CC 4-phase

35

Performance analysis

Ring oscillators


CC

CC

C

• Every ring requires an odd number of inverters

• The cycle period is determined by the slowest ring

• The cycle period is adapted to the operating conditions(temperature, voltage)

37

1

2 3 4

5

6 7

Why asynchronous?

Modularity• Time-independent functional composability

– Performance may be affected (but not functionality)


A BDataReq

AckB’

Tracking variability


matched delay

Tracking variability

delay

best typ worst

multi-corner matched delay

critical paths

Good correlation for:

• Process variability (systematic)

• Global voltage fluctuations

• Temperature

• Aging (partially)Collège de France 2013 Asynchronous circuits 42

Margins

Gate and wire delays (typ) P V T AgingPLLJitter

Skew

Rigid Clocks:

Cycle period

Gate and wire delays (typ) P V TA

gin

g

Elastic Clocks:

Skew

Cycle period

Margin reduction

Speed-up / Power savings


wasted timecomputation time

Rigid clock

computation time

Cycle period

Cycle period

Elastic clock

Clock elasticity


Voltage scaling and power savings

-24%-14%

3 ARM926 coreson the same die


Design Automation

Design automation paradigms• Synthesis of asynchronous controllers

– Logic synthesis from Petri nets orasynchronous FSMs

• Syntax-directed translation– Correct-by-construction composition of handshake

components

• De-synchronization– Automatic transformation from synchronous to

asynchronousCollège de France 2013 Asynchronous circuits 47

Synthesis of asynchronous controllers


DSr

LDS

LDTACK

D

DTACK

LDS+ LDTACK+ D+ DTACK+ DSr- D-

DTACK-

LDS-LDTACK-

DSr+

Synthesis of asynchronous controllers


LDS+ LDTACK+ D+ DTACK+ DSr- D-

DTACK-

LDS-LDTACK-

DSr+

DTACKD

DSr

LDS

LDTACK

Example: Petrify

Syntax-directed translation


(A || B) ; C

P = (A || B) ; C



par

A B

C

A || B

seq

P = (A || B) ; C



seq

par

A B

C

P = (A || B) ; C



A B

P = (A ; B) seq



c := a + b +

c

a b



→

SEQ

xR

R

RWMUX

→

yR

R

RWMUX

*

DMX-

DMX-

DMX <>

DMX <

do

→→ @

áá ññ→

out

int = type [0..255]& gcd: main proc (in? chan <<int,int>> & out! chan int)begin x, y: var int| forever do in?<<x,y>>

; do x <> y then if x < y then y:=y-x else x:=x-y fi od

; out!x odend

Sources:

J. Kessels and A. Peeters.DESCALE: A Design Experiment for a SmartCard Application Consuming Low Energy,in Principles of Asynchronous Circuit Design, A Systems Perspective,Eds., J. Sparso and S. Furber, Kluwer Academic Publishers, 2001.

P.A.Beerel, R.O. Ozdag and M. Ferretti.A Designer’s Guide to Asynchronous VLSI,Cambridge University Press, 2010. 55

De-synchronization• Strategy: substitute the clock tree

by local clocks and handshakes

• Combinational logic and latches are not modified

• More tolerance to variability– Similar area, less power and/or more speed

• Cortadella, Kondratyev, Lavagno and Sotiriou. Desynchronization: Synthesis of asynchronous circuits from synchronous specifications.IEEE TCAD, Oct 2006.


Synchronous operation


CLKgen

Transforming a synchronous circuit into asynchronous (automatically)

57

De-synchronization


Transforming a synchronous circuit into asynchronous (automatically)

59

Conclusions• Asynchrony offers flexibility in time

– Modularity– Dynamic adaptability– Tolerance to variability

• Better optimization of power/performance

• Why isn’t it an important trend in circuit design?– Lack of commercial EDA support (timing sign-off)– Designers do not feel comfortable with “unpredictable” timing– Other aspects: testing, verification, …

• De-synchronization might be a viable solutionCollège de France 2013 Asynchronous circuits 61

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