Download ECE 331 – Digital System Design

ECE 331 – Digital System Design
Power Dissipation
and
Additional Design Constraints
(Lecture #14)
The slides included herein were taken from the materials accompanying
Fundamentals of Logic Design, 6th Edition, by Roth and Kinney,
and were used with permission from Cengage Learning.
Material to be covered …
Supplemental
Chapter 8: Sections 1 – 5
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Power Dissipation
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Power Dissipation
•
Each integrated circuit (IC) dissipates power
•
PT = PS + PD
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PT = total power dissipated by IC
–
PS = static or quiescent power dissipation
–
PD = dynamic power dissipation
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Static Power Dissipation
•
PS = VCC * ICC
–
VCC = supply voltage
–
ICC = quiescent supply current
–
PS = static power consumption
•
ICC and VCC are specified in the datasheet for
the integrated circuit (IC).
•
For CMOS devices, PS is very small.
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74LS00 Datasheet
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Static Power Dissipation
•
Example: 74LS00 (Quad 2-input NAND)
–
Supply voltage
•
–
–
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4.75 V <= VCC <= 5.25 V
Supply current
•
High output:
ICCmax = 1.6 mA
•
Low output:
ICCmax = 4.4 mA
Maximum static power dissipation
•
High output:
•
Low output:
PS = 8.4 mW
PS = 23.1 mW
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Static Power Dissipation
–
Duty Cycle
•
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Clock signal typically has 50% duty cycle
PS = PS_high * thigh + PS_low * tlow
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PS_high = 8.4 mW
•
PS_low = 23.1 mW
•
Assume 50% duty cycle (high / low half the time)
•
PS = 8.4 mW * 0.5 + 23.1 mW * 0.5 = 15.8 mW
•
Assume 60% duty cycle (high 60% of the time)
•
PS = 8.4 mW * 0.6 + 23.1 mW * 0.4 = 14.28 mW
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Dynamic Power Dissipation

For TTL devices, PD is negligible compared to PS.


For CMOS devices, PD dominates PT.


Assume PS = 0
PD >> PS
PD in CMOS circuits arises from the movement of
charge into and out of the device capacitance.
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Dynamic Power Dissipation


In CMOS devices, charge is stored in the

CPD = power dissipation capacitance
(internal)

CL = capacitance of the load and wires
(external)
These capacitors are in parallel


CT = CPD + CL
The stored charge (on these capacitors) is

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QT = CT * VDD = (CPD + CL) * VDD
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Dynamic Power Dissipation


The charge moves into and out of the
capacitors on every transition of the output.

Low → High

High → Low
Current = movement of charge

IAVG = (CPD + CL) * VDD * fT


Where fT = output frequency
PD = IAVG * VDD = (CPD + CL) * V2DD * fT
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74HC00 Datasheet
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Dynamic Power Dissipation

Example: 74HC00 (Quad 2-input NAND)

VDD = 5V

CPD = 20 pF, CL = 50 pF
PD = (20 + 50 pF) * (5V)2 * fT
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fT (Hz)
PD
1K
1.8 mW
1M
1.8 mW
100M
180 mW
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74HC00 Datasheet
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Total Power Dissipation


For the 74HC00, PS is determined as follows

VCC = 5V

ICC = 20 mA

PS = VCC * ICC = 5V * 20 mA = 100 mA
The PT is then determined from
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
PT = PS + PD

where PD is a function of fT
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Total Power Dissipation
•
PT = PS + PD
•
Compare PT for Quad 2-input NAND (74xx00)
•
0 Hz
1 MHz
100 MHz
TTL
15.8 mW
15.8 mW
15.8 mW
CMOS
100 mW
1.805 mW
180 mW
Compare TTL and CMOS
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TTL
CMOS
PS
VCC * ICC
VDD * IDD
PD
~0W
(CPD + CL) * V2DD * fT
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Hazards
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Hazards
When the input to a combinational logic circuit changes,
unwanted switching transients may appear on the output.
These transients occur when different paths from input to
output have different propagation delays.
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Hazards
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Hazards



When analyzing combinational logic circuits for
hazards we will consider the case where only one
input changes at a time.
Under this condition, a static 1-hazard occurs when
the input change causes one product term (in a SOP
expression) to transition from 1 to 0 and another
product term to transition from 0 to 1.
Both product terms can be transiently 0, resulting in
the static 1-hazard.
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Hazards


Under the same condition, a static 0-hazard occurs
when the input change causes one sum term (in a
POS expression) to transition from 0 to 1 and
another sum term to transition from 1 to 0.
Both sum terms can be transiently 1, resulting in the
static 0-hazard.
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Detecting Static 1-Hazards
We can detect hazards in a two-level AND-OR circuit using
the following procedure:
1. Write down the sum-of-products expression for the circuit.
2. Plot each term on the map and loop it.
3. If any two adjacent 1′s are not covered by the same loop, a 1-hazard
exists for the transition between the two 1′s. For an n-variable map,
this transition occurs when one variable changes and the other n – 1
variables are held constant.
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Detecting Static 1-Hazards
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Removing Static 1-Hazards
redundant, but necessary
to remove hazard
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Detecting Static 0-Hazards
We can detect hazards in a two-level OR-AND circuit using
the following procedure:
1. Write down the product-of-sums expression for the circuit.
2. Plot each sum term on the map and loop the zeros.
3. If any two adjacent 0′s are not covered by the same loop, a 0-hazard
exists for the transition between the two 0′s. For an n-variable map,
this transition occurs when one variable changes and the other n – 1
variables are held constant.
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Detecting Static 0-Hazards
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Removing Static 0-Hazards
How many redundant gates are necessary to remove the 0-hazards?
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Hazards
Exercise:
Design a hazard-free combinational logic circuit
to implement the following logic function
F(A,B,C) = A'.C' + A.D + B.C.D'
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Hazards
Exercise:
Design a hazard-free combinational logic circuit
to implement the following logic function
F(A,B,C) = (A'+C').(A+D).(B+C+D')
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Hazards

Two-level AND-OR circuits (SOP) cannot have
static 1-Hazards.


Why?
Two-level OR-AND circuits (POS) cannot have
static 0-Hazards.

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Why?
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Questions?
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