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Why Low-Power Devices?

- Practical reasons

(Reducing power requirements of high throughput portable applications)

- Financial reasons

(Reducing packaging costs and achieving memory savings)

- Technological reasons

(Excessive heat prevents the realization of high density chips and limits their functionalities)

Application Fields

- Portable Electronics (PC, PDA, Wireless)
- IC Cost (Packaging and Cooling)
- Reliability (Electromigration, Latch-up)
- Signal Integrity (Switching Noise, DC Voltage Drop)
- Thermal Design
- Ultra-low-power applications
- Space missions (miniaturized satellites)

Different Constraints for Different Application Fields

- Portable devices: Battery life-time
- Telecom and military: Reliability (reduced power decreases electromigration, hence increases reliability)
- High volume products: Unit cost

(reduced power decreases packaging cost)

Driving Forces for Low-Power: Portable Applications

- The market of portable applications is growing rapidly.

Global Market for Cellular Phones

Driving Forces for Low-Power: Deep-Submicron Technology

- ADVANTAGES
- Smaller geometries
- Higher clock frequencies

- DISADVANTAGES
- Higher power consumption
- Lower reliability

Driving Forces for Low-Power:Battery Limitations

- Battery maximum power and capacity increases 10%-15% per year
- Increasing gap with respect to power demand

What has worked up to now?

- Voltage and process scaling
- Design methodologies
- Power-aware design flows and tools, trade area forlower power
- Architecture Design
- Power down techniques
- Clock gating, dynamic power management
- Dynamic voltage scaling based on workload
- Power conscious RT/ logic synthesis
- Better cell library design and resizing methods
- Cap. reduction, threshold control, transistor layout

P(t)

RMS power

Average power

Energy

t

Power Metrics- Average power: Related to battery lifetime.
- Peak power: Related to reliability and thermal failure
- RMS power: Related to cycle-by-cycle power
- Energy=power time: Related to power-delay product.

Why CMOS?

VDD

- Intrinsically low power consuming

(when the input is static, there is no power consumption)

- Reference technology
- Ease of design.

PMOS

Vin

Vout

NMOS

Basic CMOS configuration

VSS

A biased NMOS gate

VGS=VTn

+ + + + + + + +

p

n

n

When VGS VTn , the n-channel is developed and the device can start operation by applying a positive VDS

Power Dissipation in CMOS Circuits

Ptotal = Pswitching + Pshort-circuit + Pleakage

Due to charging and discharging capacitors (dynamic power consumption)

Due to direct paths

Due to leaking diodes and transistors

%75

%20

%5

Energy Consumed (related to Battery Power)

Energy consumed due to a complete cycle 010.

Dynamic Power Consumption(related to Battery Power)

- Average power consumption by a node cycling at each period T:

(each period has a 01 or a 1 0 transition)

- Average power consumed by a node with partial activity
- (only a fraction of the periods has a transition)

Dynamic Power Consumption(related to Inverter)

- Average power consumption by a node cycling at each period T:

(each period has a 01 or a 1 0 transition)

- Average power consumed by a node with partial activity
- (only a fraction of the periods has a transition)

Dynamic Power Consumption

- Define effective capacitance Ceff:
- To minimize switching power
- Reduce VDD
- Reduce Ceff

Factors Influencing Ceff

- Circuit function
- Circuit technology
- Input probabilities
- Circuit topology

Some Basic Definitions

- Signal probability of a signal g(t) is given by

- Signal activity of a logic signal g(t) is given by

where ng(t) is the number of transitions of g(t) in the time interval between –T/2 and T/2.

Factors Influencing Ceff:Circuit Function

- Assume that there are M mutually independent signals g1, g2,...gM each having a signal probability Pi and a signal activity Ai, for i n.
- For static CMOS, the signal probability at the output of a gate is determined according to the probability of 1s (or 0s) in the logic description of the gate

P1

P1

P1P2

1-(1-P1)(1-P2)

P1

1-P1

P2

P2

Factors Influencing Ceff:Circuit Function (Static CMOS)

- Transistors connected to the same input are turning on and off simultaneously when the input changes
- CLof a static CMOS gate is charged to VDD any time a 01 transition at the output node is required.
- CL of a static CMOS gate is discharged to ground any time a 1 0 transition at the output node is required.

NOR Gate

Y

B

Factors Influencing Ceff:Circuit Function (Static CMOS)- Two-input NOR gate
- Assume only one input transition per cycle is allowed
- Assume inputs are equiprobable: pA=pB=1/2.
- The probability for the output to be 1 is

pY=(1-pA)(1-pB)=1/4

- The probability for the output to be 0 is

pY’=1-pY=3/4

Factors Influencing Ceff:Circuit Function (Static CMOS)

- State transition diagram of the NOR gate

Y

B

Factors Influencing Ceff:Circuit Function (Static CMOS)- Two-input XOR gate
- Assume only one input transition per cycle is allowed
- Assume inputs are equiprobable: pA=pB=1/2.
- The probability for the output to be 1 is

pY=(1-pA)pB+(1-pB)pA=1/2

- The probability for the output to be 0 is

pY’=1-pY=1/2

Factors Influencing Ceff:Circuit Function (Static CMOS)

- State transition diagram of the NOR gate

Factors Influencing Ceff:Circuit Function (Dynamic CMOS)

- At each cycle, MD is precharged to VDD.
- CL is precharged to VDD at each clock cycle
- It is discharged to ground any time a 1 0 transition at the output node is required.

MD

Y

B

Factors Influencing Ceff:Circuit Function (Dynamic CMOS)- Two-input NOR gate
- Assume only one input transition per cycle is allowed
- Assume inputs are equiprobable: pA=pB=1/2.
- The probability for the output to be discharged is

pY’=3/4

- The probability of CL to be re-charged at the next cycle is pY’.

Factors Influencing Ceff:Circuit Function (Dynamicvs Static)

- dynamic CMOS static CMOS:
- Ceff (dynamic CMOS) Ceff (static CMOS)
- Power due to glitching is much smaller in dynamic CMOS than it is in static CMOS.
- In static CMOS, the transition probability depends on both input probabilities and previous state.
- In dynamic CMOS, the transition probability depends on solely input probabilities.
- In static CMOS, the gate output does not switch if the inputs do not change between subsequent cycles.
- In dynamic CMOS, the gate output may switch even if the inputs do not change between subsequent cycles.

Y

B

Factors Influencing Ceff:Input Probabilities (Static CMOS)- Two-input NOR gate
- Assume only one input transition per cycle is allowed
- Assume inputs are not equiprobable: pA, pB
- The probability for the output to be 1 is

pY=(1-pA)(1-pB)

- The probability for the output to be 0 is

pY’=1-pY

Factors Influencing Ceff:Input Probabilities (Static CMOS)

- The probability for the output of a NOR gate to have a 01 transition:

Factors Influencing Ceff:Input Probabilities (Static CMOS)

- Signal probability calculation:
- For each input signal and gate output in the circuit, assign a unique variable
- Starting from at the inputs and proceeding to the outputs, write the expression for the output of each gate as a function of its input expression
- Suppress all exponents in a given expression to obtain the correct probability for that signal (Recall that an exponent of a binary number is also a binary number)

Factors Influencing Ceff:Input Probabilities (Static CMOS)

- Signal activity calculation: Boolean Difference

- It signifies the condition under which output f is sensitized to input xi
- If the primary inputs to function f are not spatially correlated, the signal activity at f is

Factors Influencing Ceff:Input Probabilities (Static CMOS)

- Signal activity through basic gates

P1 , A1

P2 A1 + P1 A2

P1 , A

A

P2 , A2

P1 , A1

(1-P2 ) A1 + (1-P1) A2

P2 , A2

- Signal activity is used to determine dynamic power due to glitches.

Factors Influencing Ceff:Circuit Topology

- Circuit topology may have high impact on Ceff
- Example: Chain and Tree implementation of a four input NAND gate
- Assume static CMOS
- Assume all inputs are equiprobable.

Factors Influencing Ceff:Circuit Topology

- Globally chain implementation has a lower switching activity in the static behavior of the circuit.
- Timing skew between signals may cause hazards resulting in extra power dissipation.
- Consider 11101011 in chain circuit with unit delay of each gate.

Factors Influencing Ceff:Circuit Topology

- The chain circuit suffers from hazards, but the tree circuit does not (due to its balanced paths)
- Dynamic CMOS is glitch-free because the gate output can make at most one power consuming transition per clock cycle.

Power Dissipation in CMOS Circuits

Ptotal = Pswitching + Pshort-circuit + Pleakage

Due to charging and discharging capacitors (dynamic power consumption)

Due to direct paths

Due to leaking diodes and transistors

%75

%20

%5

Power Dissipation due to Short Circuit Currents

- Short circuit current flows when both devices are on simultaneously:

- Short circuit current, Isc, flows from VDD to ground
- The power dissipated by a CMOS gate due to short circuit current is

Power Dissipation due to Short Circuit Currents

because both transistors will never be on simultaneously

- Isc is significant when the input rise-fall time is much larger than the output rise-fall times.
- When input and output rise-fall times are equal, Isc tends to zero
- Isc =0 when

Power Dissipation due to Short Circuit Currents

- The energy dissipated by a CMOS gate due to short circuit current is

- If the current isc(t) is a triangle, then

Power Dissipation due to Short Circuit Currents

Weight of short circuit power in the total power consumption

Power Dissipation due to Short Circuit Currents

Weight of short circuit power of the total energy

Power Dissipation due to Short Circuit Currents

- Short circuit component of the total power consumption may be important and may increase while scaling
- Short circuit power increases with signal transition times at inputs
- Short circuit power decreases with signal increasing load capacitance (because Imax decreases)

Power Dissipation in CMOS Circuits

Ptotal = Pswitching + Pshort-circuit + Pleakage

Due to charging and discharging capacitors (dynamic power consumption)

Due to direct paths

Due to leaking diodes and transistors

%75

%20

%5

Power Dissipation due to Leakage Currents

- Leakage currents are important in the systems with long periods of inactivity
- Reverse bias diode current through the transistor drain= IL
- Subthreshold current through the channel of an off transistor= Ids

Power Reduction Methods:Voltage Supply Scaling

- Historically most adapted method is the reduction of voltage supply, VDD

%75

%20

%5

Power Reduction Methods:Voltage Supply Scaling

- Gate delay, Td, increases as VDD decreases!

The circuit cannot be switched very fast!

Additional Power Reduction Methods

- Preserving circuit speed and computational throughput mandatory.
- Two solutions:
- Threshold voltage scaling
- Architecture driven voltage scaling based on
- Pipelining
- Parallelization

Power Reduction Methods:Threshold Voltage Scaling

- Reduce threshold voltage while reducing supply voltage:
- Example:
- Circuit A: VDD=1.5V, VTh=1V
- Circuit B: VDD=0.9V, VTh=0.5V
- Circuits A and B approximately have the same performance

Power Reduction Methods:Threshold Voltage Scaling

- Td increases as VDD approaches to VTh

Power Reduction Methods:Threshold Voltage Scaling

- If threshold voltage scaling is required, low-threshold MOS devices must be used for the design.
- The limit on the threshold voltage scaling is imposed by the noise margin and the increase of the subthreshold current (Ids)
- Tradeoff between Pswitching (decreases as VTh decreases) and Pleakage (increases as VTh decreases)

Power Reduction Methods:Architecture Driven Supply Voltage Scaling

- Strategy:

1. Modify the architecture of the system so as to make it faster.

2. Reduce VDD so as to restore the original speed. Power consumption has decreased.

- The most common architectural changes rely on the exploitation of parallelization and pipelining.
- Drawback:

The additional circuitry required to compensate the speed degradation may dominate, and the power consumption may increase.

- Consequence:

Parallelism and pipelining do not always pay-off.

Parallel and Pipelined Architectures

Example: Reference Adder-Comparator

Parallel and Pipelined Architectures

- Supply voltage: VRef = 5V .
- Assume the worst-case delay (through adder and comparator) to be 25nsec.
- Best clock period allowed: TRef = 25nsec.
- Total effective capacitance: Cref
- Power consumption:

PRef = 0.5 CRefV 2Ref (Tref)-1

- CRef has been obtained assuming equiprobable inputs.
- For maximum throughput, no voltage scaling is allowed (no additional delay) no power reduction can be obtained.

Parallel Architectures

Ppar=0.36Pref

Parallel Architectures

- Best possible clock period: 25nsec.
- However, the required throughput is guaranteed if the clock period is doubled:

TPar = 50nsec.

- The speed of the adder and the comparator can thus be halved.
- Supply voltage : VPar = 2.9V = O.58 Vref
- Total effective capacitance: CPar= 2.15 CRef

(factor 2.15 instead of 2 is due to extra routing).

- Since TPar = 2 TRef, power consumption is

PPar =O.36 PRef

Parallel Architectures

- Parallelism does not pay off when VDD approaches to VTh

Pipelined Architectures

Ppar=0.39Pref

Pipelined Architectures

- Power consumption is reduced by a factor of 2.5, approximately as in the case of parallel realization.
- Area penalty is much more Iimited than in the parallel case.
- The use of pipelining also reduces the sequential depth of the circuit, thus reducing power dissipation due to hazards and critical races.

Parallel-Pipelined Architectures

Ppar=0.2Pref

Comments on Parallel and Pipelined Architectures

- Total reduction of dynamic power that can be applied through voltage scaling is always quite remarkable
- Yet, there are some cases where modifying the clock frequency or the relative speed of some components through architectural changes is infeasible (Example: Time-multiplexed architectures like DSP and microprocessors.

Different Supply Voltages for Different Units

- Partition the chip into multiple sub-units each of which is designed to operate at a specific supply voltage

3V

5V

5V

SLOW

3V

FAST

5V

SLOW

SLOW

3V

3V

SLOW

3V

Other Methods in Power Reduction

- Supply voltage scaling is not the only possible solution to reduce power consumption.
- Considerable results can be obtained through minimization of Ceff.
- Ceff is proportional to switching= Ceff= CL
- Design and synthesis techniques have been developed to reduce both the capacitive Ioad, CL and the switching activity, , at all stages of the design process.

Power Analysis

- Fast and accurate analysis in the design process
- Power budgeting
- Knowledge-based architectural and implementation decisions
- Package selection
- Power hungry module identification
- Detailed and comprehesive analysis at the later stages
- Satisfaction of power budget and constraints
- Hot spots

Average vs Detailed Power Analysis

- Average Power Analysis is for
- Heat dissipation
- Power budgeting
- Package selection
- Implementation trade-offs for power
- Detailed Power Analysis is for
- Determining power supply specifications
- Voltage drop
- Hot spots
- Peak power

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