4. FFaann--IInn CCoonnssiiddeerraattiioonnss
A B C D
A
B C3
CL Distributed RC model
(Elmore delay)
C C2
D C1
tdHL = 0.69 Reqn(C1+2C2+3C3+4CL)
Propagation delay deteriorates
rapidly as a function of fan-in –
quadratically in the worst case.
5. ttpp aass aa FFuunnccttiioonn ooff FFaann--IInn
1250
1000
750
500
250
quadratic
Gates with a
fan-in
greater than
4 should be
avoided.
0
2 4 6 8 10 12 14 16
fan-in
tp
HL
td
tp(psec)
23. LLooggiiccaall EEffffoorrtt
z
MMuullttiissttaaggee llooggiicc nneettwwoorrkkss
10
A w
x y 20
B
g=1
f=w/10
g=4/3
f=x/w
g=5/3
f=y/x
g=1
f=z/y
g=5/3
f=20/z
•Path Parasitic Delay
P pi
•Path Delay
D P gi fi
•How do we minimize D? How do we select the sizing?
24. LLooggiiccaall EEffffoorrtt
PPaatthh eeffffoorrtt iiss aann iinnddiirreecctt mmeeaassuurree ooff tthhee ppaatthh ddeellaayy
•Path Electrical Effort F
Cout
Cin
fi
•Path Logical Effort G gi
•Path Effort H GF
• The above does not include any considerationof the effect of
fanout within the path
• H counts only the fanout of the output
• We need to express the branching behavior along the path
26. LLooggiiccaall EEffffoorrtt
PPaatthh EEffffoorrtt
H GFB gi
PPaatthh DDeellaayy
D P gi fi
fi gi fi
MMiinniimmiizzeedd wwhheenn eeaacchh ssttaaggee ddeellaayy iiss eeqquuaall
gi fi hˆ
D P Nhˆ
P N
N
H
N
H
28. LLooggiiccaall EEffffoorrtt
1
EExxaammppllee
10
A w
B
x y
z 20
g=1
f=w/10
g=4/3
f=x/w
G g
g=5/3
f=y/x
4 5 5
1
g=1
f=z/y
100
g=5/3
f=20/z
F
Cout
Cin
B 1
i
3 3
20
2
10
3 27
H GFB
100
2 1
200
27 27
29. LLooggiiccaall EEffffoorrtt
200
5
27
3 3
z
10
A w
x y 20
B
g=1
f=w/10
g=4/3
f=x/w
hˆ
ˆ
g=5/3
f=y/x
1.49
5 20
g=1
f=z/y
g=5/3
f=20/z
h g5 f5
3 z
1.49 z 22.3
hˆ g f 1
z
4
y
1.49 y 15.0
hˆ g f
5
y
3 x
1.49 x 16.8
hˆ g f 4 x
1.49 w 15.0
2 2
3 w
N
H
4