S. Reda EN160 SP’07

Design and Implementation of VLSI Systems(EN1600)

Lecture 21: Dynamic Combinational Circuit Design

S. Reda EN160 SP’07

Dynamic logic• Dynamic gates uses a clocked pMOS pullup• Two modes: precharge and evaluate

1

2A Y

4/3

2/3

AY

1

1

AY

Static Pseudo-nMOS Dynamic

Precharge Evaluate

Y

Precharge

• Dynamic circuit operation is divided into two modes: precharge and evaluate

S. Reda EN160 SP’07

What if the input is ON during precharge?

• What if pulldown network is ON during precharge?– Contention arises because both pMOS and nMOS will be ON

• Use series evaluation transistor to prevent fight.

AY

foot

precharge transistor Y

inputs

Y

inputs

footed unfooted

f f

S. Reda EN160 SP’07

Logic effort for dynamic circuits

Very fast with very low logical effort

S. Reda EN160 SP’07

Dynamic circuits have a problem: Monotonicity requirement

• Dynamic gates require monotonically rising inputs during evaluation– 0 → 0

– 0 → 1

– 1 → 1

– But not 1 → 0

Precharge Evaluate

Y

Precharge

A

Output should rise but does not

violates monotonicity during evaluation

AY

foot

precharge transistor

S. Reda EN160 SP’07

Implications of Monotonicity

• But dynamic gates produce monotonically falling outputs during evaluation

• Illegal for one dynamic gate to drive another!

S. Reda EN160 SP’07

Domino Logic

• Follow dynamic stage with inverting static gate– Dynamic / static pair is called domino gate– Produces monotonic outputs

Precharge Evaluate

W

Precharge

X

Y

Z

A

BC

C

AB

W XY Z =

XZH H

A

W

B C

X Y Z

domino AND

dynamicNAND

staticinverter

S. Reda EN160 SP’07

Domino optimizations

• Each domino gate triggers next one, like a string of dominos toppling over

• Gates evaluate sequentially but precharge in parallel• Thus evaluation is more critical than precharge• HI-skewed static stages can perform logic

S0

D0

S1

D1

S2

D2

S3

D3

S4

D4

S5

D5

S6

D6

S7

D7

YH

8-input multiplexer built from two 4-input dynamic multiplexers

S. Reda EN160 SP’07

Dual-Rail Domino

• Domino only performs noninverting functions:– AND, OR but not NAND, NOR, or XOR

• Dual-rail domino solves this problem– Takes true and complementary inputs

– Produces true and complementary outputs

sig_h sig_l Meaning

0 0 Precharged

0 1 ‘0’

1 0 ‘1’

1 1 invalid

S. Reda EN160 SP’07

Leakage problems

• Dynamic node floats high during evaluation– Transistors are leaky (IOFF 0)

– Dynamic value will leak away over time– Formerly miliseconds, now nanoseconds!

• Use keeper to hold dynamic node– Must be weak enough not to fight evaluation

A

H

2

2

1 kX

Y

weak keeper

S. Reda EN160 SP’07

Charge sharing

• Dynamic gates suffer from charge sharing

B = 0

AY

x

Cx

CYA

x

Y

Charge sharing noise

• Solution: add secondary precharge transistors• Typically need to precharge every other node

• Big load capacitance CY helps as well

B

AY

x

secondaryprechargetransistor

S. Reda EN160 SP’07

Domino Summary

• Domino logic is attractive for high-speed circuits– 1.5 – 2x faster than static CMOS– But many challenges: Monotonicity, leakage, charge sharing,

noise, and high dynamic power

• Widely used in high-performance microprocessors

Static CMOS Ratioed Circuits Cascode Voltage Switch Logic Pass-transistor Circuits Dynamic Circuits

Circuit Families