Transcript logical effort

Lecture 6:
Logical
Effort
Outline






Logical Effort
Delay in a Logic Gate
Multistage Logic Networks
Choosing the Best Number of Stages
Example
Summary
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CMOS VLSI Design 4th Ed.
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Introduction
 Chip designers face a bewildering array of choices
– What is the best circuit topology for a function?
– How many stages of logic give least delay?
– How wide should the transistors be?
???
 Logical effort is a method to make these decisions
– Uses a simple model of delay
– Allows back-of-the-envelope calculations
– Helps make rapid comparisons between alternatives
– Emphasizes remarkable symmetries
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Example
 Ben Bitdiddle is the memory designer for the Motoroil 68W86,
an embedded automotive processor. Help Ben design the
decoder for a register file.
A[3:0] A[3:0]
32 bits
6: Logical Effort
CMOS VLSI Design 4th Ed.
16 words
4:16 Decoder
 Decoder specifications:
– 16 word register file
– Each word is 32 bits wide
– Each bit presents load of 3 unit-sized transistors
– True and complementary address inputs A[3:0]
– Each input may drive 10 unit-sized transistors
 Ben needs to decide:
– How many stages to use?
– How large should each gate be?
– How fast can decoder operate?
16
Register File
4
Delay in a Logic Gate
 Express delays in process-independent unit d  d abs

 Delay has two components: d = f + p
  3RC
 f: effort delay = gh (a.k.a. stage effort)
 3 ps in 65 nm process
– Again has two components
60 ps in 0.6 mm process
 g: logical effort
– Measures relative ability of gate to deliver current
– g  1 for inverter
 h: electrical effort = Cout / Cin
– Ratio of output to input capacitance
– Sometimes called fanout
 p: parasitic delay
– Represents delay of gate driving no load
– Set by internal parasitic capacitance
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CMOS VLSI Design 4th Ed.
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Delay Plots
=f+p
= gh + p
 What about
NOR2?
2-input
NAND
6
Normalized Delay: d
d
Inverter
5
g=1
p=1
d=h+1
4
3
g = 4/3
p=2
d = (4/3)h + 2
Effort Delay: f
2
1
Parasitic Delay: p
0
0
1
2
3
4
5
Electrical Effort:
h = Cout / Cin
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Computing Logical Effort
 DEF: Logical effort is the ratio of the input
capacitance of a gate to the input capacitance of an
inverter delivering the same output current.
 Measure from delay vs. fanout plots
 Or estimate by counting transistor widths
2
Y
2
A
Y
1
Cin = 3
g = 3/3
6: Logical Effort
2
A
2
B
2
A
4
B
4
Cin = 4
g = 4/3
CMOS VLSI Design 4th Ed.
Y
1
1
Cin = 5
g = 5/3
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Catalog of Gates
 Logical effort of common gates
Gate type
Number of inputs
1
2
3
4
n
NAND
4/3
5/3
6/3
(n+2)/3
NOR
5/3
7/3
9/3
(2n+1)/3
2
2
2
2
4, 4
6, 12, 6
8, 16, 16, 8
Inverter
Tristate / mux
XOR, XNOR
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1
2
CMOS VLSI Design 4th Ed.
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Catalog of Gates
 Parasitic delay of common gates
– In multiples of pinv (1)
Gate type
Number of inputs
1
2
3
4
n
NAND
2
3
4
n
NOR
2
3
4
n
4
6
8
2n
4
6
8
Inverter
Tristate / mux
XOR, XNOR
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1
2
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Example: Ring Oscillator
 Estimate the frequency of an N-stage ring oscillator
Logical Effort:
Electrical Effort:
Parasitic Delay:
Stage Delay:
Frequency:
6: Logical Effort
31 stage ring oscillator in
g=1
0.6 mm process has
h=1
frequency of ~ 200 MHz
p=1
d=2
fosc = 1/(2*N*d) = 1/4N
CMOS VLSI Design 4th Ed.
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Example: FO4 Inverter
 Estimate the delay of a fanout-of-4 (FO4) inverter
d
Logical Effort:
Electrical Effort:
Parasitic Delay:
Stage Delay:
6: Logical Effort
g=1
h=4
p=1
d=5
The FO4 delay is about
300 ps in 0.6 mm process
15 ps in a 65 nm process
CMOS VLSI Design 4th Ed.
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Limitations to Linear Delay Model
 Input and output slope
 Input arrival times
 Velocity saturation
VDD  Vt  Vc

IdsatN series

Idsat
VDD  Vt  NVc
 Voltage dependence
k

k
k
CVDD
VDD  Vt 
CVDD

VDD  Vt* 
kC
Vt*
1
VDD
CVDD
Ioff 10
6: Logical Effort

VDD
S
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Limitations to Linear Delay Model
 Gate source capacitance (series transistor
capacitances)
 Bootstrapping (Miller effect)
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Multistage Logic Networks
 Logical effort generalizes to multistage networks
 Path Logical Effort
G
gi

 Path Electrical Effort
g1 = 1
h1 = x/10
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Cin-path
F   f i   gi hi
 Path Effort
10
H
Cout-path
x
g2 = 5/3
h2 = y/x
y
g3 = 4/3
h3 = z/y
z
g4 = 1
h4 = 20/z
CMOS VLSI Design 4th Ed.
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Multistage Logic Networks
 Logical effort generalizes to multistage networks
 Path Logical Effort
G
gi

 Path Electrical Effort
H
Cout  path
Cin  path
F   f i   gi hi
 Path Effort
 Can we write F = GH?
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CMOS VLSI Design 4th Ed.
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Paths that Branch
 No! Consider paths that branch:
15
G
H
GH
h1
h2
F
=1
5
= 90 / 5 = 18
= 18
= (15 +15) / 5 = 6
= 90 / 15 = 6
= g1g2h1h2 = 36 = 2GH
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90
90
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Branching Effort
 Introduce branching effort
– Accounts for branching between stages in path
b
Con path  Coff path
Con path
B   bi
Note:
h
i
 BH
 Now we compute the path effort
– F = GBH
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Multistage Delays
 Path Effort Delay
DF   f i
 Path Parasitic Delay
P   pi
 Path Delay
D   d i  DF  P
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Designing Fast Circuits
D   d i  DF  P
 Delay is smallest when each stage bears same effort
fˆ  gi hi  F
1
N
 Thus minimum delay of N stage path is
1
N
D  NF  P
 This is a key result of logical effort
– Find fastest possible delay
– Doesn’t require calculating gate sizes
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Gate Sizes
 How wide should the gates be for least delay?
fˆ  gh  g CCoutin
gi Couti
 Cini 
fˆ
 Working backward, apply capacitance
transformation to find input capacitance of each gate
given load it drives.
 Check work by verifying input cap spec is met.
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Example: 3-stage path
 Select gate sizes x and y for least delay from A to B
x
x
A
8
6: Logical Effort
x
CMOS VLSI Design 4th Ed.
y
45
y
B
45
21
Example: 3-stage path
x
x
A
8
x
y
45
y
Logical Effort
Electrical Effort
Branching Effort
Path Effort
Best Stage Effort
Parasitic Delay
Delay
6: Logical Effort
B
45
G = (4/3)*(5/3)*(5/3) = 100/27
H = 45/8
B=3*2=6
F = GBH = 125
fˆ  3 F  5
P=2+3+2=7
D = 3*5 + 7 = 22 = 4.4 FO4
CMOS VLSI Design 4th Ed.
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Example: 3-stage path
 Work backward for sizes
y = 45 * (5/3) / 5 = 15
x = (15*2) * (5/3) / 5 = 10
x
y
x
A P:
84
N: 4
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45
45
P:
x 4
N: 6
P:
y 12
N: 3
CMOS VLSI Design 4th Ed.
B
B
45
45
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Best Number of Stages
 How many stages should a path use?
– Minimizing number of stages is not always fastest
 Example: drive 64-bit datapath with unit inverter
Initial Driver
1
1
1
1
8
4
2.8
16
8
D = NF1/N + P
= N(64)1/N + N
23
Datapath Load
N:
f:
D:
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64
1
64
65
CMOS VLSI Design 4th Ed.
64
2
8
18
64
3
4
15
Fastest
64
4
2.8
15.3
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Derivation
 Consider adding inverters to end of path
– How many give least delay?
Logic Block:
n1Stages
Path Effort F
n1
D  NF   pi   N  n1  pinv
1
N
N - n1 ExtraInverters
i 1
1
1
1
D
  F N ln F N  F N  pinv  0
N
 Define best stage effort
F
1
N
pinv   1  ln    0
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Best Stage Effort

pinv   1  ln    0 has no closed-form solution
 Neglecting parasitics (pinv = 0), we find  = 2.718 (e)
 For pinv = 1, solve numerically for  = 3.59
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Sensitivity Analysis
D(N) /D(N)
 How sensitive is delay to using exactly the best
1.6
number of stages?
1.51
1.4
1.26
1.2
1.15
1.0
( =2.4)
(=6)
0.0
0.5
0.7
1.0
1.4
2.0
N/ N
 2.4 <  < 6 gives delay within 15% of optimal
– We can be sloppy!
– I like  = 4
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CMOS VLSI Design 4th Ed.
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Example, Revisited
 Ben Bitdiddle is the memory designer for the Motoroil 68W86,
an embedded automotive processor. Help Ben design the
decoder for a register file.
A[3:0] A[3:0]
32 bits
6: Logical Effort
CMOS VLSI Design 4th Ed.
16 words
4:16 Decoder
 Decoder specifications:
– 16 word register file
– Each word is 32 bits wide
– Each bit presents load of 3 unit-sized transistors
– True and complementary address inputs A[3:0]
– Each input may drive 10 unit-sized transistors
 Ben needs to decide:
– How many stages to use?
– How large should each gate be?
– How fast can decoder operate?
16
Register File
28
Number of Stages
 Decoder effort is mainly electrical and branching
Electrical Effort:
H = (32*3) / 10 = 9.6
Branching Effort:
B=8
 If we neglect logical effort (assume G = 1)
Path Effort:
F = GBH = 76.8
Number of Stages:
N = log4F = 3.1
 Try a 3-stage design
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CMOS VLSI Design 4th Ed.
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Gate Sizes & Delay
Logical Effort:
Path Effort:
Stage Effort:
Path Delay:
Gate sizes:
A[3] A[3]
10
10
A[2] A[2]
10
10
A[1] A[1]
10
10
G = 1 * 6/3 * 1 = 2
F = GBH = 154
fˆ  F 1/ 3  5.36
D  3 fˆ  1  4  1  22.1
z = 96*1/5.36 = 18
y = 18*2/5.36 = 6.7
A[0] A[0]
10
10
y
z
word[0]
96 units of wordline capacitance
y
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z
word[15]
CMOS VLSI Design 4th Ed.
30
Comparison
 Compare many alternatives with a spreadsheet
 D = N(76.8 G)1/N + P
Design
NOR4
N
1
G
3
P
4
D
234
NAND4-INV
2
2
5
29.8
NAND2-NOR2
2
20/9
4
30.1
INV-NAND4-INV
NAND4-INV-INV-INV
3
4
2
2
6
7
22.1
21.1
NAND2-NOR2-INV-INV
4
20/9
6
20.5
NAND2-INV-NAND2-INV
4
16/9
6
19.7
INV-NAND2-INV-NAND2-INV
5
16/9
7
20.4
NAND2-INV-NAND2-INV-INV-INV
6
16/9
8
21.6
6: Logical Effort
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Review of Definitions
Term
Stage
Path
number of stages
1
N
logical effort
g
G   gi
electrical effort
h  CCoutin
H
branching effort
b
effort
f  gh
F  GBH
effort delay
f
DF   f i
parasitic delay
p
P   pi
delay
d f p
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Con-path Coff-path
Con-path
CMOS VLSI Design 4th Ed.
Cout-path
Cin-path
B   bi
D   d i  DF  P
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Method of Logical Effort
1)
2)
3)
4)
5)
Compute path effort
Estimate best number of stages
Sketch path with N stages
Estimate least delay
Determine best stage effort
N  log 4 F
1
N
D  NF  P
ˆf  F N1
gi Couti
Cini 
fˆ
6) Find gate sizes
6: Logical Effort
F  GBH
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Some Key Insights
 Allows one to compare alternative topologies quickly
 NAND structures are usually faster than NOR
 Paths are faster when the effort delays of each
stage are about the same and their value is around
four.
 Path delay is insensitive to modest deviations from
optimum. Stage efforts of 2.4 – 6 give only 15%
deviation.
 Using stage efforts greater than four reduces area
and power at a slight cost in speed.
 Fewer stages or larger gates generally do not help.
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Some Key Insights
 The delay of a well-designed path is about log4F
FO4 inverter delays. Each quadrupling of the load
adds about one FO4 inverter delay to the path.
 The logical effort of each input of a gate increases
through no fault of its own delay as the number of
inputs grows. Maximum is about 4 series transistors.
 Inverters or 2 input NAND gates are best for driving
nodes with a large branching effort.
 When a path forks and one leg is more critical than
the others, buffer the noncritical path.
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Some Key Insights
 In paths that branch, each fork should contribute
equal delay.
 The stage efforts f can be made equal for each
stage, but load capacitance can change.
 To minimize delay, upsize gates on nodes with large
fixed capacitances to reduce the effort borne by the
gate, while only slightly increasing the effort borne
by the predecessor.
 A standard cell library offers a discrete set of gates.
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Limits of Logical Effort
 Chicken and egg problem
– Need path to compute G
– But don’t know number of stages without G
 Simplistic delay model
– Neglects input rise time effects
 Interconnect
– Iteration required in designs with wire
 Maximum speed only
– Not minimum area/power for constrained delay
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Delay Models
 Slope-based linear model.
 Nonlinear delay model (table look-up)
 Current-source model
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CMOS VLSI Design 4th Ed.
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Pitfalls and Fallacies
 Defining gate delay for an unloaded gate
 Trying to increase speed by increasing the size of
transistors in a path.
– Especially if someone else is designing the
previous gate 
 Trying to increase speed by using as few stages as
possible.
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CMOS VLSI Design 4th Ed.
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Historical Perspective
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Summary
 Logical effort is useful for thinking of delay in circuits
– Numeric logical effort characterizes gates
– NANDs are faster than NORs in CMOS
– Paths are fastest when effort delays are ~4
– Path delay is weakly sensitive to stages, sizes
– But using fewer stages doesn’t mean faster paths
– Delay of path is about log4F FO4 inverter delays
– Inverters and NAND2 best for driving large caps
 Provides language for discussing fast circuits
– But requires practice to master
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