Download - LOW POWER DIGITAL DESIGN
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Rana, Ian Owen A.Rana, Ian Owen A.
Low-Power Low-Power Digital DesignDigital Design
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Agenda
● Recap● Power reduction on
Gate level Architecture level Algorithm level System level
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Recap: Problems of Power Dissipation
Continuously increasing performance demands
Increasing power dissipation of technical devices
Today: power dissipation is a main problem
High Power dissipation leads to:
High efforts for cooling
Increasing operational costs
Reduced reliability
High efforts for cooling
Increasing operational costs
Reduced reliability
Reduced time of operation
Higher weight (batteries)
Reduced mobility
Reduced time of operation
Higher weight (batteries)
Reduced mobility
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CL
Recap: Consumption in CMOS
Voltage (Volt, V) Water pressure (bar) Current (Ampere, A) Water quantity per second (liter/s) Energy Amount of Water
Energy consumption is proportional to capacitive load!
0
1
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Watts
time
Power is height of curve
Watts
time
Energy is area under curve
Approach 1
Approach 2
Approach 2
Approach 1
Recap: Energy and Power
Energy = Power * time for calculation = Power * Delay
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P = α f CL VDD2 + VDD Ipeak (P0→1 + P1→0 ) + VDD Ileak
Dynamic power(≈ 40 - 70% today and decreasing
relatively)
Short-circuit power(≈ 10 % today and
decreasing absolutely)
Leakage power(≈ 20 – 50 %
today and increasing)
Recap: Power Equations in CMOS
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Recap: Levels of Optimization
nach Massoud Pedram
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Recap: Logic Restructuring
Chain implementation has a lower overall switching activity than tree implementation for random inputs
BUT: Ignores glitching effects
Logic restructuring: changing the topology of a logic network to reduce transitions
A
BC
D F
AB
CD Z
FW
X
Y0.5
0.5
(1-0.25)*0.25 = 3/16
0.50.5
0.5
0.5
0.5
0.5
7/64 = 0.109
15/256
3/16
3/16 = 0.188
15/256
AND: P0→1 = P0 * P1 = (1 - PAPB) * PAPB
Source: Timmernann, 2007
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Recap: Input Ordering
Beneficial: postponing introduction of signals with a high transition rate (signals with signal probability close to 0.5)
A
BC
X
F
0.5
0.20.1
B
CA
X
F
0.2
0.10.5
(1-0.5x0.2)*(0.5x0.2)=0.09 (1-0.2x0.1)*(0.2x0.1)=0.0196
Source: Irwin, 2000
AND: P0→1 = (1 - PAPB) * PAPB
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ABC
X
Z
101 000
Unit Delay
AB
X
ZC
Recap: Glitching
Source: Irwin, 2000
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Design Layer: Gate Level
● Basic elements: Logic gates Sequential elements (flipflops, latches)
● Behavior of elements is described in libraries
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Device Sizing (= changing gate width)
Affects input capacitance Cin
Affects load capacitance Cload
Affects dynamic power consumption Pdyn
Optimal fanout factor f for Pdyn is smaller
than for performance (especially for
large loads)
e.g., for Cload=20, Cin=1
fcircuit = 20
fopt_energy = 3.53
fopt_performance = 4.47
For Low Power: avoid oversizing (f too
big) beyond the optimal
1 2 3 4 5 6 70
0.5
1
1.5
fanout fno
rmal
ized
ene
rgy fcircuit=1
fcircuit=2
fcircuit=5
fcircuit=10
fcircuit=20
Dynamic Power and Device Size
Source: Nikolic, UCB
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0
1
2
3
4
5
6
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
Supply voltage (VDD)
Re
lativ
e D
ela
y t d
0
2
4
6
8
10
Rel
ativ
e P
dyn
VDD versus Delay and Power
● Delay (td) and dynamic power consumption (Pdyn) are functions of VDD
tdPdyn
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Multiple VDD
● Main ideas: Use of different supply voltages within the same design
High VDD for critical parts (high performance needed)
Low VDD for non-critical parts (only low performance demands)
● At design phase: Determine critical path(s) (see upper next slide)
High VDD for gates on those paths
Lower VDD on the other gates (in non-critical paths)
For low VDD: prefer gates that drive large capacitances (yields
the largest energy benefits)
● Usually two different VDD (but more are possible)
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● Level converters: Necessary, when module at lower supply drives gate at higher
supply (step-up) If gate supplied with VDDL drives a gate supplied with VDDH
then PMOS never turns off Possible implementation:
Cross-coupled PMOS transistors NMOS transistor operate on
reduced supply
No need of level converters for step-down change in voltage
Reducing of overhead: Conversions at register boundaries Embedding of inside flipflop
Multiple VDD cont’d
VDDH
Vin
VoutVDDL
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Data Paths
● Data propagate through different data paths between registers (flipflops - FF)
● Paths mostly differ in propagation delay times● Frequency of clock signal (CLK) depends on path with longest
delay critical path
Paths
Path
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Data Paths: Slack
B
A
Y
C
time
all Inputs of G1arrived
G1 ready withevaluation
delay of G1
all inputs of G2arrived
Slack for G1
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Multiple VDD in Data Paths
● Minimum energy consumption when all logic paths are critical (same delay)
● Possible Algorithm: clustered voltage-scaling
Each path starts with VDDH and switches to VDDL (blue gates)
when slack is available Level conversion in flipflops at end of paths
Connected with VDDL
Connected with VDDH
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Design Layer: Architecture Level
● Also known as Register transfer level (RTL)● Base elements:
Register structures Arithmetic logic units (ALU) Memory elements
● Only behavior is described
(no inner structure)
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● Most popular method for power reduction of clock signals and functional units
● Gate off clock to idle functional units● Logic for generation of disable signal necessary Higher complexity of control logic Higher power consumption Critical timing critical for avoiding of
clock glitches at OR gate output Additional gate delay on clock signal
Clock Gating
Reg
clock
disable
Functionalunit
Source: Irwin, 2000
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Clock Gating cont’d
Source: Agarwal, 2007
D QD
CLK
● Clock-Gating in Low-Power Flip-Flop
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Clock Gating cont’d
● Clock gating over consideration of state in Finite-State-
Machines (FSM)
Combinational logic
LatchClock
activation logic
Flip
-flo
ps
PI
CLK
PO
Source: L. Benini and G. De Micheli,Dynamic Power Management, Boston: Springer, 1998.
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Clock Gating: Example
DSP/HIF
DEU
MIF
VDE
896Kb SRAM
Source: M. Ohashi, Matsushita, 2002
90% of FlipFlops clock-gated
70% power reduction by clock-gating
MPEG4 decoder
10
8.5mW
0 155
30.6mW
20 25
Without clock gating
With clock gating
Power [mW]
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0
1
2
3
4
5
6
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
Supply voltage (VDD)
Re
lativ
e D
ela
y t d
0
2
4
6
8
10
Rel
ativ
e P
dyn
Recap: VDD versus Delay and Power
Dynamic Power can be traded by delay
tdPdyn
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A Reference Datapath
Combinationallogic
OutputInputR
egi
ster
Re
gis
ter
CLKSupply voltage = Vref
Total capacitance switched per cycle = Cref
Clock frequency = fClk
Power consumption: Pref = CrefVref2fclk
Cref
Source: Agarwal, 2007
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Thank You!!! :)