Transcript Lecture 10: Circuit Families

Lecture 10:
Circuit
Families
Outline
 Pseudo-nMOS Logic
 Dynamic Logic
 Pass Transistor Logic
10: Circuit Families
CMOS VLSI Design 4th Ed.
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Introduction
 What makes a circuit fast?
– I = C dV/dt -> tpd  (C/I) DV
– low capacitance
– high current
4
B
– small swing
4
A
 Logical effort is proportional to C/I
1
1
 pMOS are the enemy!
– High capacitance for a given current
 Can we take the pMOS capacitance off the input?
 Various circuit families try to do this…
10: Circuit Families
CMOS VLSI Design 4th Ed.
Y
3
Pseudo-nMOS
 In the old days, nMOS processes had no pMOS
– Instead, use pull-up transistor that is always ON
 In CMOS, use a pMOS that is always ON
– Ratio issue
– Make pMOS about ¼ effective strength of
pulldown network
1.8
1.5
load
P/2
1.2
P = 24
Ids
Vout 0.9
Vout
16/2
Vin
0.6
P = 14
0.3
P=4
0
0
0.3
0.6
0.9
1.2
1.5
1.8
Vin
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CMOS VLSI Design 4th Ed.
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Pseudo-nMOS Gates
 Design for unit current on output
to compare with unit inverter.
 pMOS fights nMOS
Y
inputs
f
Inverter
Y
A
NAND2
gu
gd
gavg
pu
pd
pavg
=
=
=
=
=
=
10: Circuit Families
A
B
gu
g
Y gd
avg
pu
pd
pavg
NOR2
=
=
=
=
=
=
A
CMOS VLSI Design 4th Ed.
B
gu
gd
gavg
Y pu
pd
pavg
=
=
=
=
=
=
5
Pseudo-nMOS Gates
 Design for unit current on output
to compare with unit inverter.
 pMOS fights nMOS
Y
inputs
f
Inverter
2/3
Y
A
4/3
NAND2
gu
gd
gavg
pu
pd
pavg
= 4/3
= 4/9
= 8/9
= 6/3
= 6/9
= 12/9
10: Circuit Families
gu
g
Y gd
avg
8/3
pu
pd
8/3
pavg
2/3
A
B
NOR2
= 8/3
= 8/9
= 16/9
= 10/3
= 10/9
= 20/9
2/3
A
CMOS VLSI Design 4th Ed.
4/3 B
gu
gd
gavg
Y pu
4/3 pd
pavg
= 4/3
= 4/9
= 8/9
= 10/3
= 10/9
= 20/9
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Pseudo-nMOS Design
 Ex: Design a k-input AND gate using pseudo-nMOS.
Estimate the delay driving a fanout of H
Pseudo-nMOS





G = 1 * 8/9 = 8/9
F = GBH = 8H/9
P = 1 + (4+8k)/9 = (8k+13)/9
N=2
4 2 H 8k  13
1/N
D = NF + P = 3  9
10: Circuit Families
In1
1
Ink
1
CMOS VLSI Design 4th Ed.
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H
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Pseudo-nMOS Power
 Pseudo-nMOS draws power whenever Y = 0
– Called static power P = IDDVDD
– A few mA / gate * 1M gates would be a problem
– Explains why nMOS went extinct
 Use pseudo-nMOS sparingly for wide NORs
 Turn off pMOS when not in use
en
Y
A
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B
C
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Ratio Example
 The chip contains a 32 word x 48 bit ROM
– Uses pseudo-nMOS decoder and bitline pullups
– On average, one wordline and 24 bitlines are high
 Find static power drawn by the ROM
– Ion-p = 36 mA, VDD = 1.0 V
 Solution:
Ppull-up  VDD I pull-up  36 μW
Pstatic  (31  24) Ppull-up  1.98 mW
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Dynamic Logic
 Dynamic gates uses a clocked pMOS pullup
 Two modes: precharge and evaluate
2
A

2/3
Y
1
Y
1
A
Static
4/3
Pseudo-nMOS

Precharge
Y
A
1
Dynamic
Evaluate
Precharge
Y
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CMOS VLSI Design 4th Ed.
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The Foot
 What if pulldown network is ON during precharge?
 Use series evaluation transistor to prevent fight.
precharge transistor

Y


Y
inputs
A
Y
inputs
f
f
foot
footed
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CMOS VLSI Design 4th Ed.
unfooted
11
Logical Effort
Inverter

unfooted
NAND2
1
gd
pd
footed
2
2
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2
B
2
gd
pd
= 2/3
= 3/3

1
1
B
Y
gd
pd
= 2/3
= 3/3
A
1
gd
pd
= 1/3
= 3/3
1
Y
1
Y
A
A
= 1/3
= 2/3


1
Y
1
Y
A

NOR2
A
3
B
3
3

1
Y
gd
pd
= 3/3
= 4/3
CMOS VLSI Design 4th Ed.
A
2
B
2
2
gd
pd
= 2/3
= 5/3
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Monotonicity
 Dynamic gates require monotonically rising inputs
during evaluation

– 0 -> 0
A
– 0 -> 1
– 1 -> 1
violates monotonicity
– But not 1 -> 0
during evaluation
A

Precharge
Evaluate
Precharge
Y
Output should rise but does not
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CMOS VLSI Design 4th Ed.
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Monotonicity Woes
 But dynamic gates produce
monotonically falling
outputs during evaluation
 Illegal for one dynamic gate
to drive another!
A=1

A
Y

Precharge
Evaluate
Precharge
X
X
X monotonically falls during evaluation
Y
Y should rise but cannot
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CMOS VLSI Design 4th Ed.
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Domino Gates
 Follow dynamic stage with inverting static gate
– Dynamic / static pair is called domino gate
– Produces monotonic outputs

Precharge
Evaluate
Precharge
domino AND
W
W
X
Y
Z
X
A
B
C

Y
Z
dynamic static
NAND inverter

A
B
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

W
X
H
C
CMOS VLSI Design 4th Ed.
Y
H
Z
=
A
B

X
Z
C
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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
S1
S2
S3
D0
D1
D2
D3
H
Y

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S4
S5
S6
S7
D4
D5
D6
D7
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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
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
Y_l
inputs
f
Y_h
f

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Example: AND/NAND
 Given A_h, A_l, B_h, B_l
 Compute Y_h = AB, Y_l = AB
 Pulldown networks are conduction complements

Y_l
A_h
= A*B
A_l
B_l
Y_h
= A*B
B_h

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CMOS VLSI Design 4th Ed.
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Example: XOR/XNOR
 Sometimes possible to share transistors

Y_l
= A xnor B
A_h
Y_h
A_l
A_l
B_l
B_h
A_h
= A xor B

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CMOS VLSI Design 4th Ed.
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Leakage
 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
weak keeper

A
1 k
X
H
Y
2
2
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CMOS VLSI Design 4th Ed.
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Charge Sharing
 Dynamic gates suffer from charge sharing


A
Y
CY
x
A
Y
B=0
Cx
Charge sharing noise
x
CY
Vx  VY 
VDD
Cx  CY
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CMOS VLSI Design 4th Ed.
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Secondary Precharge
 Solution: add secondary precharge transistors
– Typically need to precharge every other node
 Big load capacitance CY helps as well

Y
A
secondary
precharge
transistor
x
B
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CMOS VLSI Design 4th Ed.
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Noise Sensitivity
 Dynamic gates are very sensitive to noise
– Inputs: VIH  Vtn
– Outputs: floating output susceptible noise
 Noise sources
– Capacitive crosstalk
– Charge sharing
– Power supply noise
– Feedthrough noise
– And more!
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CMOS VLSI Design 4th Ed.
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Power
 Domino gates have high activity factors
– Output evaluates and precharges
• If output probability = 0.5, a = 0.5
– Output rises and falls on half the cycles
– Clocked transistors have a = 1
 Leads to very high power consumption
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CMOS VLSI Design 4th Ed.
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Domino Summary
 Domino logic is attractive for high-speed circuits
– 1.3 – 2x faster than static CMOS
– But many challenges:
• Monotonicity, leakage, charge sharing, noise
 Widely used in high-performance microprocessors in
1990s when speed was king
 Largely displaced by static CMOS now that power is
the limiter
 Still used in memories for area efficiency
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CMOS VLSI Design 4th Ed.
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Pass Transistor Circuits
 Use pass transistors like switches to do logic
 Inputs drive diffusion terminals as well as gates
 CMOS + Transmission Gates:
– 2-input multiplexer
– Gates should be restoring
S
S
A
A
S
S
Y
Y
B
B
S
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CMOS VLSI Design 4th Ed.
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LEAP
 LEAn integration with Pass transistors
 Get rid of pMOS transistors
– Use weak pMOS feedback to pull fully high
– Ratio constraint
S
A
S
L
Y
B
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CMOS VLSI Design 4th Ed.
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CPL
 Complementary Pass-transistor Logic
– Dual-rail form of pass transistor logic
– Avoids need for ratioed feedback
– Optional cross-coupling for rail-to-rail swing
S
A
S
L
Y
L
Y
B
S
A
S
B
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CMOS VLSI Design 4th Ed.
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Pass Transistor Summary
 Researchers investigated pass transistor logic for
general purpose applications in the 1990’s
– Benefits over static CMOS were small or negative
– No longer generally used
 However, pass transistors still have a niche in
special circuits such as memories where they offer
small size and the threshold drops can be managed
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CMOS VLSI Design 4th Ed.
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