Introduction to CMOS VLSI Design Lecture 19: Design for Skew David Harris Harvey Mudd College Spring...

Introduction toCMOS VLSI

Design

Lecture 19: Design for Skew

David Harris

Harvey Mudd College

Spring 2004

19: Design for Skew Slide 2CMOS VLSI Design

Outline Clock Distribution Clock Skew Skew-Tolerant Static Circuits Traditional Domino Circuits Skew-Tolerant Domino Circuits


Clocking Synchronous systems use a clock to keep

operations in sequence– Distinguish this from previous or next– Determine speed at which machine operates

Clock must be distributed to all the sequencing elements– Flip-flops and latches

Also distribute clock to other elements – Domino circuits and memories


Clock Distribution On a small chip, the clock distribution network is just

a wire– And possibly an inverter for clkb

On practical chips, the RC delay of the wire resistance and gate load is very long– Variations in this delay cause clock to get to

different elements at different times– This is called clock skew

Most chips use repeaters to buffer the clock and equalize the delay– Reduces but doesn’t eliminate skew


Example Skew comes from differences in gate and wire delay

– With right buffer sizing, clk1 and clk2 could ideally arrive at the same time.

– But power supply noise changes buffer delays

– clk2 and clk3 will always see RC skew

3 mm

1.3 pF

3.1 mmgclk

clk1

0.5 mm

clk2clk3

0.4 pF 0.4 pF


Review: Skew Impact

F1

F2

clk

clk clk

Combinational Logic

Tc

Q1 D2

Q1

D2

tskew

CL

Q1

D2

F1

clk

Q1

F2

clk

D2

clk

tskew

tsetup

tpcq

tpdq

tcd

thold

tccq

setup skew

sequencing overhead

hold skew

pd c pcq

cd ccq

t T t t t

t t t t

Ideally full cycle is

available for work Skew adds sequencing

overhead Increases hold time too


Cycle Time Trends Much of CPU performance comes from higher f

– f is improving faster than simple process shrinks– Sequencing overhead is bigger part of cycle

0 .0 1

0 .1

1

1 0

1 0 0

8 0 3 8 68 0 4 8 6P e n tiu mP e n tiu m II / III

Spe

cInt

95

1985 1988 1991 1994 1997 2000

1.2 0.8 0 .6 0.35 2.0

Process

100

200

500VD D = 5 VDD = 3.3

VDD = 2.5

50Fan

out-

of-4

(F

O4)

Inve

rter

Del

ay (

ps)

0.25

1 0

1 0 0

1 0 0 0

8 0 3 8 68 0 4 8 6P e n tiu mP e n tiu m II / II I

MH

z

1 9 8 8 1 9 9 1 1 9 9 4 1 9 9 7 2 0 0 01 9 8 5

1 0

1 0 0

1 9 8 5 1 9 8 8 1 9 9 1 1 9 9 4 1 9 9 7

8 0 3 8 68 0 4 8 6P e n tiu mP e n tiu m II / II IF

O4

inve

rte

r de

lays

/ cy

cle

50

20

2 0 0 0


Solutions Reduce clock skew

– Careful clock distribution network design– Plenty of metal wiring resources

Analyze clock skew– Only budget actual, not worst case skews– Local vs. global skew budgets

Tolerate clock skew– Choose circuit structures insensitive to skew


Clock Dist. Networks Ad hoc Grids H-tree Hybrid


Clock Grids Use grid on two or more levels to carry clock Make wires wide to reduce RC delay Ensures low skew between nearby points But possibly large skew across die


Alpha Clock Grids

PLL

gclk grid

Alpha 21064 Alpha 21164 Alpha 21264

gclk grid

Alpha 21064 Alpha 21164 Alpha 21264


H-Trees Fractal structure

– Gets clock arbitrarily close to any point– Matched delay along all paths

Delay variations cause skew A and B might see big skew A B


Itanium 2 H-Tree Four levels of buffering:

– Primary driver– Repeater– Second-level

clock buffer– Gater

Route around

obstructionsPrimary Buffer

Repeaters

Typical SLCBLocations


Hybrid Networks Use H-tree to distribute clock to many points Tie these points together with a grid

Ex: IBM Power4, PowerPC– H-tree drives 16-64 sector buffers– Buffers drive total of 1024 points– All points shorted together with grid


Skew Tolerance Flip-flops are sensitive to skew because of hard edges

– Data launches at latest rising edge of clock– Must setup before earliest next rising edge of clock– Overhead would shrink if we can soften edge

Latches tolerate moderate amounts of skew– Data can arrive anytime latch is transparent


Skew: Latches

Q1

L1

1

2

L2 L3

1 12

CombinationalLogic 1

CombinationalLogic 2

Q2 Q3D1 D2 D3

sequencing overhead

1 2 hold nonoverlap skew

borrow setup nonoverlap skew

2

,

2

pd c pdq

cd cd ccq

c

t T t

t t t t t t

Tt t t t

2-Phase Latches

setup skew

sequencing overhead

hold skew

borrow setup skew

max ,pd c pdq pcq pw

cd pw ccq

pw

t T t t t t t

t t t t t

t t t t

Pulsed Latches


Dynamic Circuit Review Static circuits are slow because fat pMOS load input Dynamic gates use precharge to remove pMOS

transistors from the inputs– Precharge: = 0 output forced high– Evaluate: = 1 output may pull low

A B

Y

C DY

A B C D

A

B

C

D


Domino Circuits Dynamic inputs must monotonically rise during

evaluation– Place inverting stage between each dynamic gate– Dynamic / static pair called domino gate

Domino gates can be safely cascaded

A

W

B

X

domino AND

dynamicNAND

staticinverter


Domino Timing Domino gates are 1.5 – 2x faster than static CMOS

– Lower logical effort because of reduced Cin

Challenge is to keep precharge off critical path Look at clocking schemes for precharge and eval

– Traditional schemes have severe overhead– Skew-tolerant domino hides this overhead


Traditional Domino Ckts Hide precharge time by ping-ponging between half-

cycles– One evaluates while other precharges– Latches hold results during precharge

Tc

Sta

tic

Dyn

amic

Latc

h

clk

Sta

tic

Dyn

amic

clkS

tatic

Dyn

amic

clk

Dyn

amic

clk clk

Sta

tic

Dyn

amic

Latc

h

Sta

tic

Dyn

amic

Sta

tic

Dyn

amic

Dyn

amic

clk clk clk clk clk

clk

clk

tpdq tpdq

2pd c pdqt T t


Clock Skew Skew increases sequencing overhead

– Traditional domino has hard edges– Evaluate at latest rising edge– Setup at latch by earliest falling edge

Sta

tic

Dyn

amic

Latc

hclk

Sta

tic

Dyn

amic

clkD

ynam

icclk clk

Sta

tic

Dyn

amic

Latc

h

Sta

tic

Dyn

amic

Dyn

amic

clk clk clk clk

clk

clk

tskewtsetup

setup skew2 2pd ct T t t


Time Borrowing Logic may not exactly fit half-cycle

– No flexibility to borrow time to balance logic between half cycles

Traditional domino sequencing overhead is about 25% of cycle time in fast systems!

Sta

tic

Dyn

amic

Latc

hclk

Sta

tic

Dyn

amic

clk clk

Sta

tic

Dyn

amic

Latc

h

Sta

tic

Dyn

amic

clk clk clk

clk

clk

tskewtsetup


Relaxing the Timing Sequencing overhead caused by hard edges

– Data departs dynamic gate on late rising edge– Must setup at latch on early falling edge

Latch functions– Prevent glitches on inputs of domino gates– Holds results during precharge

Is the latch really necessary?– No glitches if inputs come from other domino– Can we hold the results in another way?


Skew-Tolerant Domino Use overlapping clocks to eliminate latches at phase

boundaries.– Second phase evaluates using results of first

a

Sta

tic

Dyn

amic

1

Sta

tic

Dyn

amic

2

b c d

a

1

2

b

c

a

1

2

b

c

No latch atphase boundary


Full Keeper After second phase evaluates, first phase precharges Input to second phase falls

– Violates monotonicity? But we no longer need the value Now the second gate has a floating output

– Need full keeper to hold it either high or low

weak fullkeepertransistors

f

X

H


Time Borrowing Overlap can be used to

– Tolerate clock skew– Permit time borrowing

No sequencing overhead

tskew

Sta

tic

Dyn

amic

Sta

tic

Dyn

amic

Sta

tic

Dyn

amic

Dyn

amic

Sta

tic

Dyn

amic

Sta

tic

Dyn

amic

Sta

tic

Dyn

amic

Dyn

amic

Sta

tic

Sta

tic

1

2

1 1 1 1 1 2 2 2

Phase 1 Phase 2

toverlap

tborrow

pd ct T


Multiple Phases With more clock phases, each phase overlaps more

– Permits more skew tolerance and time borrowing

Sta

tic

Dyn

amic

Sta

tic

Dyn

amic

Sta

tic

Dyn

amic

Dyn

amic

Sta

tic

Dyn

amic

Sta

tic

Dyn

amic

Sta

tic

Dyn

amic

Dyn

amic

Sta

tic

Sta

tic

3

4

1 1 2 2 3 3 4 4

Phase 1 Phase 2 Phase 3 Phase 4

1

2


Clock Generation

clken

1

2

3

4


Summary Clock skew effectively increases setup and hold

times in systems with hard edges Managing skew

– Reduce: good clock distribution network– Analyze: local vs. global skew– Tolerate: use systems with soft edges

Flip-flops and traditional domino are costly Latches and skew-tolerant domino perform at full

speed even with moderate clock skews.

Introduction to CMOS VLSI Design Lecture 19: Design for Skew David Harris Harvey Mudd College Spring...

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