TKT-1527 Digital System Design Issues
Designing for Low Power
Mary Jane Irwin ( www.cse.psu.edu/~mji )
www.cse.psu.edu/~cg477
[Adapted from Rabaey’s Digital Integrated Circuits, ©2002, J. Rabaey et al.]
CSE477 L12&13 Low Power.1
Irwin&Vijay, PSU, 2002
Review: Designing Fast CMOS Gates

Transistor sizing

Progressive transistor sizing


Transistor ordering


fet closest to the output is smallest of series fets
put latest arriving signal closest to the output
Logic structure reordering

replace large fan-in gates with smaller fan-in gate network

Logical effort

Buffer (inverter) insertion


separate large fan-in from large CL with buffers
uses buffers so there are no more than four TGs in series
CSE477 L12&13 Low Power.2
Irwin&Vijay, PSU, 2002
Why Power Matters

Packaging costs

Power supply rail design

Chip and system cooling costs

Noise immunity and system reliability

Battery life (in portable systems)

Environmental concerns


Office equipment accounted for 5% of total US commercial
energy usage in 1993
Energy Star compliant systems
CSE477 L12&13 Low Power.3
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Why worry about power? -- Power Dissipation
Lead microprocessors power continues to increase
Power (Watts)
100
P6
Pentium ®
10
8086 286
1
8008
4004
486
386
8085
8080
0.1
1971
1974
1978
1985
1992
2000
Year
Power delivery and dissipation will be prohibitive
Source: Borkar, De Intel
CSE477 L12&13 Low Power.4
Irwin&Vijay, PSU, 2002
Why worry about power? -- Chip Power Density
Sun’s
Surface
Power Density (W/cm2)
10000
Rocket
Nozzle
1000
…chips might become hot…
Nuclear
Reactor
100
8086 Hot Plate
10 4004
P6
8008 8085
Pentium®
386
286
486
8080
1
1970
1980
1990
Year
2000
2010
Source: Borkar, De Intel
CSE477 L12&13 Low Power.5
Irwin&Vijay, PSU, 2002
Chip Power Density Distribution
Al-SiC+ Epoxy Die Attach
WillametteMap
Power Distribution
Power
On-Die Temperature
110
250
100
100
200-250
90
150-200
80
100-150
50-100
70
0-50
60
Temperature (C)
150
Heat Flux (W/cm2)
200
50
50
40
0

Power density is not uniformly distributed across the chip

Silicon is not a good heat conductor

Max junction temperature is determined by hot-spots
 Impact on packaging, w.r.t. cooling
CSE477 L12&13 Low Power.6
Irwin&Vijay, PSU, 2002
Power and Energy Figures of Merit

Power consumption in Watts


Peak power




determines power ground wiring designs
sets packaging limits
impacts signal noise margin and reliability analysis
Energy efficiency in Joules


determines battery life in hours
rate at which power is consumed over time
Energy = power * delay


Joules = Watts * seconds
lower energy number means less power to perform a
computation at the same frequency
CSE477 L12&13 Low Power.7
Irwin&Vijay, PSU, 2002
Power versus Energy
Power is height of curve
Watts
Lower power design could simply be slower
Approach 1
Approach 2
Watts
time
Energy is area under curve
Two approaches require the same energy
Approach 1
Approach 2
time
CSE477 L12&13 Low Power.8
Irwin&Vijay, PSU, 2002
PDP and EDP
Power-delay product (PDP) = Pav * tp = (CLVDD2)/2



PDP is the average energy consumed per switching event
(Watts * sec = Joule)
lower power design could simply be a slower design
Energy-delay product (EDP) = PDP * tp = Pav * tp2


EDP is the average energy
consumed multiplied by the
computation time required
takes into account that one
can trade increased delay
for lower energy/operation
(e.g., via supply voltage
scaling that increases delay,
but decreases energy
consumption)
Energy-Delay (normalized)

15
energy-delay
10
energy
5
delay
0
0.5

allows one to understand tradeoffs better
CSE477 L12&13 Low Power.9
1
1.5
2
2.5
Vdd (V)
Irwin&Vijay, PSU, 2002
Understanding Tradeoffs

Which design is the “best” (fastest, coolest, both) ?
Lower
EDP
b
c
a
d
1/Delay
better
CSE477 L12&13 Low Power.11
Irwin&Vijay, PSU, 2002
CMOS Energy & Power Equations
E = CL VDD2 P01 + tsc VDD Ipeak P01 + VDD
Ileakage
f01 = P01 * fclock
P = CL VDD2 f01 + tscVDD Ipeak f01 + VDD Ileakage
Dynamic
power
CSE477 L12&13 Low Power.12
Short-circuit
power
Leakage
power
Irwin&Vijay, PSU, 2002
Dynamic Power Consumption
Vdd
Vin
Vout
CL
Energy/transition = CL *
VDD2
f01
* P01
Pdyn = Energy/transition * f = CL * VDD2 * P01 * f
Pdyn = CEFF * VDD2 * f
where CEFF = P01 CL
Not a function of transistor sizes!
Data dependent - a function of switching activity!
CSE477 L12&13 Low Power.13
Irwin&Vijay, PSU, 2002
Lowering Dynamic Power
Capacitance:
Function of fan-out,
wire length, transistor
sizes
Supply Voltage:
Has been dropping
with successive
generations
Pdyn = CL VDD2 P01 f
Activity factor:
How often, on average,
do wires switch?
CSE477 L12&13 Low Power.14
Clock frequency:
Increasing…
Irwin&Vijay, PSU, 2002
Short Circuit Power Consumption
Vin
Isc
Vout
CL
Finite slope of the input signal causes a direct
current path between VDD and GND for a short
period of time during switching when both the
NMOS and PMOS transistors are conducting.
CSE477 L12&13 Low Power.15
Irwin&Vijay, PSU, 2002
Short Circuit Currents Determinates
Esc = tsc VDD Ipeak P01
Psc = tsc VDD Ipeak f01

Duration and slope of the input signal, tsc
 Ipeak


determined by
the saturation current of the P and N transistors which
depend on their sizes, process technology, temperature, etc.
strong function of the ratio between input and output slopes
- a function of CL
CSE477 L12&13 Low Power.16
Irwin&Vijay, PSU, 2002
Impact of CL on Psc
Isc  0
Vin
Isc  Imax
Vout
CL
Vin
Vout
CL
Large capacitive load
Small capacitive load
Output fall time significantly
larger than input rise time.
Output fall time substantially
smaller than the input rise
time.
CSE477 L12&13 Low Power.17
Irwin&Vijay, PSU, 2002
Ipeak as a Function of CL
2.5
x 10-4
CL = 20 fF
2
When load capacitance
is small, Ipeak is large.
1.5
CL = 100 fF
1
0.5
0
0
-0.5
2
Short circuit dissipation
is minimized by
CL = 500 fF
matching the rise/fall
times of the input and
4
6
x 10-10 output signals - slope
engineering.
time (sec)
500 psec input slope
CSE477 L12&13 Low Power.18
Irwin&Vijay, PSU, 2002
Psc as a Function of Rise/Fall Times
8
When load capacitance
is small (tsin/tsout > 2 for
VDD > 2V) the power is
dominated by Psc
7
VDD= 3.3 V
6
5
4
VDD = 2.5 V
3
2
1
VDD = 1.5V
0
0
2
tsin/tsou
If VDD < VTn + |VTp| then
Psc is eliminated since
both devices are never
on at the same time.
4
t
W/Lp = 1.125 m/0.25 m
W/Ln = 0.375 m/0.25 m
CL = 30 fF
CSE477 L12&13 Low Power.19
normalized wrt zero input
rise-time dissipation
Irwin&Vijay, PSU, 2002
Leakage (Static) Power Consumption
VDD Ileakage
Vout
Drain junction
leakage
Gate leakage
Sub-threshold current
Sub-threshold current is the dominant factor.
All increase exponentially with temperature!
CSE477 L12&13 Low Power.20
Irwin&Vijay, PSU, 2002
Leakage as a Function of VT

Continued scaling of supply voltage and the subsequent
scaling of threshold voltage will make subthreshold
conduction a dominate component of power dissipation.
10-2
ID (A)

10-7
VT=0.4V
VT=0.1V
10-12
0
0.2
0.4
0.6
0.8
An 90mV/decade VT
roll-off - so each
255mV increase in
VT gives 3 orders of
magnitude reduction
in leakage (but
adversely affects
performance)
1
VGS (V)
CSE477 L12&13 Low Power.21
Irwin&Vijay, PSU, 2002
TSMC Processes Leakage and VT
CL018
G
CL018
LP
CL018
ULP
CL018
HS
CL015
HS
CL013
HS
Vdd
1.8 V
1.8 V
1.8 V
2V
1.5 V
1.2 V
Tox (effective)
42 Å
42 Å
42 Å
42 Å
29 Å
24 Å
Lgate
0.16 m
0.16 m
0.18 m
0.13 m
0.11 m
0.08 m
IDSat (n/p)
(A/m)
600/260
500/180
320/130
780/360
860/370
920/400
20
1.60
0.15
300
1,800
13,000
0.42 V
0.63 V
0.73 V
0.40 V
0.29 V
0.25 V
30
22
14
43
52
80
Ioff (leakage)
(A/m)
VTn
FET Perf.
(GHz)
From MPR, 2000
CSE477 L12&13 Low Power.22
Irwin&Vijay, PSU, 2002
Exponential Increase in Leakage Currents
10000
Ileakage(nA/m)
1000
0.25
0.18
0.13
0.1
100
10
1
30
40
50
60
70
80
Temp(C)
90
100
110
From De,1999
CSE477 L12&13 Low Power.23
Irwin&Vijay, PSU, 2002
Review: Energy & Power Equations
E = CL VDD2 P01 + tsc VDD Ipeak P01 + VDD
Ileakage
f01 = P01 * fclock
P = CL VDD2 f01 + tscVDD Ipeak f01 + VDD Ileakage
Dynamic power
(~90% today and
decreasing
relatively)
CSE477 L12&13 Low Power.24
Short-circuit
power
(~8% today and
decreasing
absolutely)
Leakage power
(~2% today and
increasing)
Irwin&Vijay, PSU, 2002
Power and Energy Design Space
Constant
Throughput/Latency
Energy
Design Time
Variable
Throughput/Latency
Non-active Modules
Logic Design
Active
Reduced Vdd
Run Time
DFS, DVS
Clock Gating
Sizing
Multi-Vdd
(Dynamic
Freq, Voltage
Scaling)
Sleep Transistors
Leakage
+ Multi-VT
Multi-Vdd
+ Variable VT
Variable VT
CSE477 L12&13 Low Power.25
Irwin&Vijay, PSU, 2002
Dynamic Power as a Function of Device Size
Device sizing affects dynamic energy consumption


The optimal gate sizing factor
(f) for dynamic energy is
smaller than the one for
performance, especially for
large F’s


gain is largest for networks with large overall effective fan-outs (F
= CL/Cg,1)
e.g., for F=20,
fopt(energy) = 3.53 while
fopt(performance) = 4.47
If energy is a concern avoid
oversizing beyond the
optimal
1.5
F=1
normalized energy

F=2
1
F=5
0.5
F=10
F=20
0
1
2
3
4
f
5
6
7
From Nikolic, UCB
CSE477 L12&13 Low Power.26
Irwin&Vijay, PSU, 2002
Dynamic Power Consumption is Data Dependent

Switching activity, P01, has two components


A static component – function of the logic topology
A dynamic component – function of the timing behavior (glitching)
2-input NOR Gate
A
B
Out
0
0
1
0
1
0
1
0
0
1
1
0
CSE477 L12&13 Low Power.27
Static transition probability
P01 = Pout=0 x Pout=1
= P0 x (1-P0)
With input signal probabilities
PA=1 = 1/2
PB=1 = 1/2
NOR static transition probability
= 3/4 x 1/4 = 3/16
Irwin&Vijay, PSU, 2002
NOR Gate Transition Probabilities

Switching activity is a strong function of the input signal
statistics

PA and PB are the probabilities that inputs A and B are one
A
B
0
A
B
CL
PA
1 0
PB
1
P01 = P0 x P1 = (1-(1-PA)(1-PB)) (1-PA)(1-PB)
CSE477 L12&13 Low Power.28
Irwin&Vijay, PSU, 2002
Transition Probabilities for Some Basic Gates
NOR
OR
NAND
AND
XOR
P01 = Pout=0 x Pout=1
(1 - (1 - PA)(1 - PB)) x (1 - PA)(1 - PB)
(1 - PA)(1 - PB) x (1 - (1 - PA)(1 - PB))
PAPB x (1 - PAPB)
(1 - PAPB) x PAPB
(1 - (PA + PB- 2PAPB)) x (PA + PB- 2PAPB)
0.5 A
0.5 B
X
Z
For X: P01 = P0 x P1 = (1-PA) PA
= 0.5 x 0.5 = 0.25
For Z: P01 = P0 x P1 = (1-PXPB) PXPB
= (1 – (0.5 x 0.5)) x (0.5 x 0.5) = 3/16
CSE477 L12&13 Low Power.30
Irwin&Vijay, PSU, 2002
Inter-signal Correlations

Determining switching activity is complicated by the fact
that signals exhibit correlation in space and time

reconvergent fan-out
(1-0.5)(1-0.5)x(1-(1-0.5)(1-0.5)) = 3/16
0.5
A
0.5
B
X
Z
Reconvergent
(1- 0.75 x 0.5) x (0.75 x 0.5) = 0.234
Incorrect!
P(Z=1) = P(B=1) & P(A=1 | B=1)

Have to use conditional probabilities
CSE477 L12&13 Low Power.32
Irwin&Vijay, PSU, 2002
Logic Restructuring

Logic restructuring: changing the topology of a logic
network to reduce transitions
AND: P01 = P0 x P1 = (1 - PAPB) x PAPB
0.5
A
B
0.5
(1-0.25)*0.25 = 3/16
7/64
W
X
15/256
C
F
0.5
D
0.5
0.5 A
0.5 B
0.5
C
0.5 D
3/16
Y
15/256
F
Z
3/16
Chain implementation has a lower overall switching activity
than the tree implementation for random inputs
Ignores glitching effects
CSE477 L12&13 Low Power.33
Irwin&Vijay, PSU, 2002
Input Ordering
(1-0.5x0.2)x(0.5x0.2)=0.09
0.5
A
B
0.2
X
C
0.1
F
0.2
B
C
0.1
(1-0.2x0.1)x(0.2x0.1)=0.0196
X
A
0.5
F
Beneficial to postpone the introduction of signals with a
high transition rate (signals with signal probability
close to 0.5)
CSE477 L12&13 Low Power.35
Irwin&Vijay, PSU, 2002
Glitching in Static CMOS Networks

Gates have a nonzero propagation delay resulting in
spurious transitions or glitches (dynamic hazards)

glitch: node exhibits multiple transitions in a single cycle before
settling to the correct logic value
A
B
X
Z
C
ABC
101
000
X
Z
Unit Delay
CSE477 L12&13 Low Power.37
Irwin&Vijay, PSU, 2002
Glitching in an RCA
Cin
S14
S15
S0
S1
S2
S Output Voltage (V)
3
S3
2
S4
Cin
S2
S15
S5
1
S10
S1
S0
0
0
CSE477 L12&13 Low Power.38
2
4
6
Time (ps)
8
10
12
Irwin&Vijay, PSU, 2002
Balanced Delay Paths to Reduce Glitching

Glitching is due to a mismatch in the path lengths in
the logic network; if all input signals of a gate change
simultaneously, no glitching occurs
0
0
F1
0
0
0
0
1
F1
1
F2 2
F3
0
0
F3
F2
1
So equalize the lengths of timing paths through logic
CSE477 L12&13 Low Power.39
Irwin&Vijay, PSU, 2002
Power and Energy Design Space
Constant
Throughput/Latency
Energy
Design Time
Variable
Throughput/Latency
Non-active Modules
Logic Design
Active
Reduced Vdd
Run Time
DFS, DVS
Clock Gating
Sizing
Multi-Vdd
(Dynamic
Freq, Voltage
Scaling)
Sleep Transistors
Leakage
+ Multi-VT
Multi-Vdd
+ Variable VT
Variable VT
CSE477 L12&13 Low Power.40
Irwin&Vijay, PSU, 2002
Dynamic Power as a Function of VDD


Decreasing the VDD
decreases dynamic
energy consumption
(quadratically)
But, increases gate
delay (decreases
performance)
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
VDD (V)

Determine the critical path(s) at design time and use high
VDD for the transistors on those paths for speed. Use a
lower VDD on the other gates, especially those that drive
large capacitances (as this yields the largest energy
benefits).
CSE477 L12&13 Low Power.41
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Multiple VDD Considerations

How many VDD? – Two is becoming common


Many chips already have two supplies (one for core and one for I/O)
When combining multiple supplies, level converters are
required whenever a module at the lower supply drives a
gate at the higher supply (step-up)

If a gate supplied with VDDL drives a gate at VDDH, the PMOS never
turns off
V
- The cross-coupled PMOS transistors
do the level conversion
- The NMOS transistor operate on a
reduced supply


Vin
DDH
VDDL
Vout
Level converters are not needed
for a step-down change in voltage
Overhead of level converters can be mitigated by doing conversions
at register boundaries and embedding the level conversion inside
the flipflop (see Figure 11.47)
CSE477 L12&13 Low Power.42
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Dual-Supply Inside a Logic Block

Minimum energy consumption is achieved if all logic
paths are critical (have the same delay)

Clustered voltage-scaling


Each path starts with VDDH and switches to VDDL (gray logic
gates) when delay slack is available
Level conversion is done in the flipflops at the end of the paths
CSE477 L12&13 Low Power.43
Irwin&Vijay, PSU, 2002
Power and Energy Design Space
Constant
Throughput/Latency
Energy
Design Time
Variable
Throughput/Latency
Non-active Modules
Logic Design
Active
Reduced Vdd
Run Time
DFS, DVS
Clock Gating
Sizing
Multi-Vdd
(Dynamic
Freq, Voltage
Scaling)
Sleep Transistors
Leakage
+ Multi-VT
Multi-Vdd
+ Variable VT
Variable VT
CSE477 L12&13 Low Power.44
Irwin&Vijay, PSU, 2002

Reducing the VT
increases the subthreshold leakage
current (exponentially)


90mV reduction in VT
increases leakage by an
order of magnitude
But, reducing VT
decreases gate delay
(increases performance)
ID (A)
Leakage as a Function of Design Time VT
VT=0.4V
VT=0.1V
0
0.2
0.4
0.6
0.8
1
VGS (V)

Determine the critical path(s) at design time and use low
VT devices on the transistors on those paths for speed.
Use a high VT on the other logic for leakage control.

A careful assignment of VT’s can reduce the leakage by as much
as 80%
CSE477 L12&13 Low Power.45
Irwin&Vijay, PSU, 2002
Dual-Thresholds Inside a Logic Block

Minimum energy consumption is achieved if all logic
paths are critical (have the same delay)

Use lower threshold on timing-critical paths


Assignment can be done on a per gate or transistor basis; no
clustering of the logic is needed
No level converters are needed
CSE477 L12&13 Low Power.46
Irwin&Vijay, PSU, 2002
Variable VT (ABB) at Run Time

VT = VT0 + (|-2F + VSB| - |-2F|)
For an n-channel device, the substrate is normally tied
to ground (VSB = 0)

 A negative
bias on VSB
causes VT to increase
 Adjusting
the substrate
bias at run time is called
adaptive body-biasing
(ABB)
Requires a dual well fab
process

0.9
0.85
0.8
0.75
0.7
0.65
0.6
0.55
0.5
0.45
0.4
-2.5
CSE477 L12&13 Low Power.47
-2
-1.5
-1
VSB (V)
-0.5
0
Irwin&Vijay, PSU, 2002