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## L26: Power Estimation

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Battery technology

Microprocessor power dissipation

Power(W)

Alpha 21164

Alpha 21264

50

P III 500

45

P II 300

40

35

Alpha21064 200

30

25

P6 166

20

P5 66

15

P-PC604 133

10

i486 DX2 66

P-PC601 50

i486 DX25

5

i386 DX 16

i486 DX4 100

i286

i486 DX 50

P-PC750 400

1980

1985

1990

1995

2000

year

Capacity

(Watt-Hour/lb)

50

40

Is it possible?

30

20

Nickel-Cadmium

10

1975

1985

1995

2000

1965

year

Cost vs Power

Power

Strategy

Cost

Low power

processor

none

none

1 Watt

heat sink,

air flow

$1-5

3-5 Watt

Laptop

Computer

fan sink

$10-15

5-15 Watt

15+ Watt

exotic

$50+

Power Estimation

- Circuit Level Power Estimation
- Logic/module Level Power Estimation
- High Level Power Estimation
- Software Power Estimation

Power Estimation Techniques

- Circuit Simulation (SPICE): a set of input vectors, accurate, memory and time constraints
- Monte Carlo: randomly generated input patterns, normal distributed power per time interval T using a simulator switch level simulation (IRSIM): defined as no. of rising and falling transitions over total number of inputs
- Powermill (transistor level): steady-state transitions, hazards and glitches, transient short circuit current and leakage current; measures current density and voltage drop in the power net and identifies reliability problem caused by EM failures, ground bounce and excessive voltage drops.
- DesignPower (Synopsys): simulation-based analysis is within 8-15% of SPICE in terms of percentage difference (Probability-based analysis is within 15-20% of SPICE).

Power Estimation Techniques

- Static (non-Simulative) - useful for synthesis and architectural exploration
- Probability-based
- Entropy-based
- Dynamic (simulative) - useful for final power
- Direct
- Sampling-based
- Compaction-based
- Hybrid (high-level simulation + low-level analytical model evaluation)
- Power macromodels for datapath, control, memory
- Instruction-level models for microprocessors, DSPs

Previous work(1)

- Simulation based approach
- accurate and system independent
- pattern dependent and after implementation
- Direct simulation
- SPICE, transistor-level simulator, IRSIM
- Statistical simulation
- Monte Carlo simulation

Previous work(2)

- Non-simulation based approach
- library, stochastic, information theoretic model
- Behavioral-level approach
- library(parameter, area, delay, internal power dissipation)
- useful in comparing different adder and multiplier architecture for their switching activity
- stochastic
- using probability density function, joint probability density function.
- Information theoretic
- entropy

Previous work(3)

- Logic-level approach
- using signal probability
- zero-delay based approach
- OBDD

Circuit Level

- SPICE
- classical tool for power analysis of circuits
- large runtime for large circuits
- mainly used as the reference for other power estimation tools.
- PowerMill
- uses simplified electrical model of the transistor.
- operating conditions of transistor are stored as look-up tables.(interpolated by piecewise linear approximation.)
- stages - partitioned subcircuits, source/drain connected transistors
- 2-3 orders faster than SPICE

Introduction

- Glitch
- additional power is typically 20%

Circuit Level

- IRSIM
- event-driven, switch-level simulator
- modeled by capacitive nodes and transistors
- partitioning into stages is used.
- voltage level - High, Low, Undetermined

Partitioning into stages

Theoretical background

- Synchronous system controlled by global clock

Hierarchical approach to power estimation of combinational circuits(1)

- Estimate power of large circuit in a short time
- model sub-circuit
- compute steady-state prob.
- compute edge-activity using state-transition-diagram(std)
- compute energy
- State-Transition Diagram
- 2 input NOR

Hierarchical approach to power estimation of combinational circuits(3)

- Computation of steady-state prob.
- Compute edge prob.
- Make state-transition matrix
- Compute steady-state prob.
- Compute edge-activity
- Energy computation of each edge in the std
- Compute edge activity energy using SPICE

Hierarchical approach to power estimation of combinational circuits(4)

- Computation of output signal parameters
- compute x3 using std

of NOR 1

- compute energy for second

NOR using Wj calculated

NOR 1 and EAj obtained

for the second NOR

Hierarchical approach to power estimation of combinational circuits(5)

- Loading and routing considerations
- Recompute edge energy with concerning of load cap.
- Effect of loading can be taken into account

Power estimation of sequential circuits

- Sequential block has a combinational block and some storage elements like flip-flop
- Extend method to flip-flop

Experimental result(1)

- Power estimation of basic cells and multipliers

Logic Level

- Pattern dependent analysis
- example : Entice-Aspen System(‘94 Workshop on Low Power Design)
- cell characterization
- using SPICE simulation under different conditions
- parameters : supply voltage, input signal slope, operating temperature, fabrication process variation
- modeling styles : polynomials, tables, piecewise linear
- power vector : Set of logic values and signal transitions
- activity analysis
- using Verilog-XL simulator
- find event vector in the power vector set
- total energy = (Energy of power vector)(# of occurrences)

High Level Power Estimation

- RTL power estimation
- problem
- given an RTL circuit description consisting of m modules, and an input vector sequence of length N, estimate the average power consumption
- estimation process
- perform behavioral simulation and collect the input statistics for all modules in RTL descriptions
- evaluate the power macro-model equation for each module and sum over the modules
- implementation
- in the form of a power co-simulator
- collect input statistics from the output of behavioral simulator
- produce the power value at the end

Power macro model

census macro modeling

sampler macro modeling

adaptive macro modeling

Census macro modeling

input data statistics must be collected for every simulation cycle

very slow simulation

assumed input vectors

macro-model is biased

ex) pseudo-random, speech data, etc.

requires

input vectors

Power

estimate

Input

vectors

Power

estimate

Behavioral

Simulator

Sampler

Macro

Modeling

requires

input

vector

power

Behavioral

Simulator

Census

Macro

Modeling

Gate-level

Power

Simulation

input vectors

for

each module

Confidence level

and interval

Input

vectors

input vectors

for

each module

vectors

confidence

level

and interval

High Level Power EstimationSampler macro modeling

collects and analyzes input vectors for a relative small number of cycles

using statistical random sampling methods

Adaptive macro modeling

involves a gate-level simulator on a small number of cycles

improve the estimation accuracy

bias of the static macro models is reduced

High Level Power EstimationHigh Level Power Estimation

- Power estimation at high level
- statistical technique
- only consider the operations of a given type, number of bus, register and memory access
- power dissipation depends on
- data activity
- physical capacitance
- two approaches considering physical cap.
- develop analytic models for estimating the switched capacitance
- synthesis the circuit and then estimate the power dissipation of the circuit

Develop analytic models

develop analytic models for estimation

models is a function of the circuit complexity and technology/library parameters

key issue

estimation of the circuit complexity

Synthesis approach

procedure

quick synthesis

estimate power dissipation using RTL/gate-level estimation techniques

tends to be more accurate

requires the development of a quick synthesis capability

much more efficient than a full synthesis program in time

High Level Power EstimationSoftware Power Estimation

- Objective
- estimate the power dissipation of a piece of code
- Lower level method
- gate level power estimation
- Higher level method
- architectural power estimation
- bus switching activity
- instruction level power analysis

Gate level power estimation

most accurate method

too slow approach

usefulness

evaluate the power dissipation behavior of a processor design

characterize the processor for the more efficient instruction

Architectural power estimation

less precise but much fast

determine which system components are active in each execution cycle

Bus switching activity

bus activity is assumed to be representative of overall switching activity

computed from the sequence of op-codes, addresses, and data

Instruction level power analysis

characterize the power dissipation of instruction sequence

use for optimizing a program based on the power estimate

Software Power EstimationInstruction level power analysis

base cost

independent of the prior state of the processor

circuit state effects

take into account the effect of prior processor state

Instruction

Name

Base

Cost(pJ)

Circuit State Effects (pJ)

LOAD

DLOAD

ADD

MULT

LOAD;

ADD

LOAD;

MULT

LOAD

1.98

0.13

0.15

1.19

0.92

1.25

1.06

DLOAD

2.37

0.17

1.19

0.92

1.32

1.06

ADD

0.99

0.26

0.53

0.86

0.99

MULT

1.19

0.66

0.79

0.96

LOAD;ADD

2.10

0.40

0.53

LOAD;MULT

2.25

0.79

Software Power EstimationCircuit

State

Instruction

Base

DLOAD A<-x, B<-y

2.37

1.19

LOAD C<-x; MULT D<-A,B

2.25

1.06

0.99

ADD A<-C,D

0.99

Total

5.61

3.24

Power Optimization

- Modeling and Technology
- Sources of power consumption
- Switching component
- Short-circuit component
- Leakage component
- Static power
- Voltage Scaling
- Adiabatic switching
- Circuit Design Level
- Logic and Module Design Level
- Architecture and System Design Level
- Some Design Examples

Switching component

- energy for charge parasitic capacitors(gate, diffusion, and interconnect)
- ex1. In CMOS, output nodes are charged or discharged.
- ex2. Charge sharing

PMOS

network

High

NMOS

network

Low

Evaluation

Short circuit component

- finite rise and fall time direct current path between Vdd and GND

(Both NMOS and PMOS are turned on)

Vin

Is

Is

Vin

t1

t2

Leakage component

- reverse viased PN junction
- subthreshold current

Gate

Gnd

P+

P+

N

Reverse leakage current

Vdd

Static power

- although CMOS circuits consume power only when switching, some situations consume static power.
- reduced voltage levels feeding CMOS gates
- pseudo-NMOS logic style
- single PMOS pullup network(always ON because the gate is grounded)
- when the output is driven low, conducting path from the supply to ground is created.

Vdd-Vtn

Vdd

Weakly turned on

Vdd

Voltage Scaling

- Why?
- Lowering the supply voltage is most effective means of power reduction.
- Feature size of the process geometry decreases.
- Smaller process geometry requires the voltage to be lowered because of the thinner gate.
- Although the delay increases as the voltage is lowered, the small channel length of the advance process increases the circuit performance

Voltage Scaling

- Scaling from 5V to 3.3V
- External components(TTL) operate at 5V and the cost to interface with them made the voltage scaling difficult.
- The components from low-voltage industry such as LVTTL, CMOS, BiCMOS(which operate at 3.3V) make the voltage scaling with low cost.
- Scaling below 3.3V
- Depending the technology, the supply voltage can be lower than 3.3V.
- The supply voltage cannot be too close to the threshold voltage. significant speed loss.

Performance vs. Voltage scaling

- The lowest voltage possible without significant loss of performance is the voltage when the electron velocity get out of saturation.
- As the feature size is shrunken, the same electric field can be obtained even when the supply voltage is decreased. ( i.e, velocity saturation occurs at the lower supply voltage. )

electron

speed

Terminal speed

As process technology

is shrunken

Supply voltage

Adiabatic Switching

- Energy injected into a node with cap. C to a voltage, V, is Esig = CV2/2
- Energy drawn from the power supply Einj = QV = CV2
- Einj = 2 Esig : Half of the energy drawn from the supply is dissipated
- Also, Esig is dissipated when the node pulled low.
- All energy drawn from the supply is used only once before being discarded.

Adiabatic Switching

- Solution : charge the load from a supply that is at the same potential as the load
- Same supply voltage as the voltage of the load
- Charge transfer proceeds sufficiently slowly to not require a large potential drop Energy dissipation varies roughly with the inverse of the switching time
- Difficulties
- switching transitions must occur when there is no potential drop across the switching devices
- zero energy occurs with arbitrarily low speed switching : With realistic switching rates, the energy savings may not sufficient compared to the circuit complexity.

Adiabatic Switching

Va

X

X’

Va

Y’

Y

- Example : inverter
- 1. input(X, X’) set to a value value.
- 2. (evaluation) slow voltage ramp to Va from 0 to Vdd. One of Y and Y’ is adiabatically charged to Vdd.
- 3. (hold) Y and Y’ can be used as the inputs of other stage
- 4. (restore)ramp to Va from Vdd to 0.

X

Y

X’

X

X

X’

X’

Y’

Adiabatic Switching

- Other components of power consumption
- on-resistance of switches
- Process improvement(parasitic capacitance reduction) allows lower power consumption in adiabatic circuits
- Application
- When a small number of circuit nodes which significant capacitance driven by high voltage.
- Capacitive transcducers, LCD panels, etc.

Reduction of Switched Capacitance

- Reduce the switching activity of the digital circuits to the minimal level required to perform the computation saves the power.
- Methods
- power down mode of the chip
- gated clock
- circuit optimization to reduce transitions
- reduction of # of operations(algorithm change)
- data representation
- resource ordering
- logic style : Dynamic or Static
- layout optimization

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