Transcript Properties of Digital Circuits

Properties of Digital Circuits
 Quantized States
 Voltage Transfer Characteristic (VTC)
 Voltage Levels
 Noise Margins
 Current Levels
 Fan-In and Fan-out
 Power Dissipation
 Propagation Delay
 Power-Delay Product
 IC Packaging
 Technology Trends of MOS Microprocessors and
Memories
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Some Key Lecture Objectives
 Before performing the Properties of Digital Circuits practicum,
we need to look at the key properties of digital circuits used to
compare one technology with another.
 How voltage levels of a technology not only define the logic
states of a technology, but also lead to a definition of noise
immunity for a technology.
 An understanding of how current levels, “loading,” power
dissipation, and speed of operation are all inter-related.
Reference:
Fundamentals of Digital Logic, Chap 1, Sec 3.8, and App E.4.
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Transistor-Transistor-Logic (TTL)
TTL dominated the IC market from the 1960’s to
early 1980’s. This picture changed in the 1980s
due to two major factors:

Discrete logic gates in static CMOS
competitive in speed at a lower power cost.

The advent of programmable logic components such
as PLDs and FPGAs made it possible to program
complex random logic functions (equivalent to
hundreds of TTL gates) on a single component. This
results in a large reduction in board real-estate cost,
while adding flexibility.
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became
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CMOS

CMOS stands for Complementary Metal-Oxide
Semiconductor. MOS field-effect transistors (nchannel and p-channel) are used to construct logic
gates.
 FETs are voltage controlled and operate nearly as an
ideal switch.
 MOSFETs advantages: Lower power consumption
than BJTs so billons of devices can be packed onto a
single chip (MOSFETs or what are now known as
FinFETs have dimensions in the deep sub-micron
range).
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CMOS
MOSFET
Gate Length
FinFET
Gate Length
Currently, how small is the gate length of a FinFET?
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Quantized States
 Binary System
possible states: 1 or 0, on or off
positive logic: 1= high, 0 = low
negative logic: 1= low, 0 = high
 Future Systems – multilevel logic
Ex: if three different on-off levels, then
23 = 8 logic states
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VTC of a TTL Inverter
Vi
Vo
Voltage Levels (VOH, VOL, VIL,VIH) are defined at dVo/dVi = -1
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VTC for a CMOS Inverter
Vout
V OH = V DD
Slope = –1
V OL = 0 V
VT
V IL
V IH
V
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( V DD – V T ) V DD
Vin
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Table of Voltage Levels for
TTL Families
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PSPICE Schematic of LS7404 TTL Inverter
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PSPICE VTC Simulation of the LS7404
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PSPICE VTC Simulation of the LS7404
VIL = 0.7 V
VIH = 1.18 V
VOH = 4 V
VOL = 0.15 V
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Digital Properties Practicum
Measured VTC of LS7404
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Voltage Level Definitions
 VIH – High Level Input Voltage
Vinput ≥ VIH to be recognized as a “1”
 TTL Ex:
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VIH = 2V, thus at the input
a “1” is between 2V and VCC
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Voltage Level Definitions
 VOH – High Level Output Voltage
 TTL Ex:
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VOH = 2.4V, thus at the output
a “1” is between 2.4V and VCC
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Voltage Level Definitions
 VIL – Low Level Input Voltage
Vinput ≤ VIL to be recognized as a “0”
 TTL Ex:
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VIL = 0.8V, thus at the input
a “0” is between 0V and 0.8V
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Voltage Level Definitions
 VOL – Low Level Output Voltage
 TTL Ex:
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VOL = 0.4V, thus at the output
a “0” is between 0V and 0.4V
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Logic Level Matching
Levels at output of one gate must be sufficient
to drive next gate.
VOH > VIH
VOL < VIL
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Voltage Level Definitions
 VLS – logic swing at the output
VLS = VOH - VOL
 Ideally, should be as large as possible. Higher VLS reduces
ambiguity in the output logic state and increases noise
immunity.
 VLS defines the range of output voltages that define the
unknown state, “X” state.
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Voltage Level Definitions
 VTW – transition width at the input
VTW = VIH - VIL
 Ideally, should be as small as possible. Lower VTW
reduces ambiguity in the input logic state and increases
noise immunity.
 VTW defines the range of input voltages that define the
unknown state, “X” state.
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Voltage Levels Summary
“1”
“X”
“0”
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“1”
“X”
“0”
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Noise in Digital Circuits
• Noise – unwanted variations of voltages and currents at the
logic nodes
• from two wires placed side by side
– capacitive coupling
• voltage change on one wire can
influence signal on the neighboring wire
• cross talk
– inductive coupling
• current change on one wire can
influence signal on the neighboring wire
v(t)
i(t)
• from noise on the power and ground supply rails
– can influence signal levels in the gate
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VDD
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Noise Margin Definitions
 NM’s define the amount of noise immunity in the
high or low level logic state.
 NMH - High Level Noise Margin
NMH = VOH - VIH
 NML - Low Level Noise Margin
NML = VIL - VOL
 Ideally, the NM’s should be as large and as equal
as possible.
 NM of a technology = min{NMH , NML}
 TTL Ex: NMH = 2.4V - 2V = 0.4V
NML = 0.8V – 0.4V = 0.4V
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Noise Superimposed on TTL Signals
OR
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Noise Margin Question
A certain logic family has the following voltage parameters:
VIH = 1.18 V, VOH = 4 V, VIL = 0.7 V, VOL = 0.15 V
What is the largest positive-going noise spike that can be
tolerated?
NML = VIL – VOL = 0.55V
What is the largest negative-going noise spike that can be
tolerated?
NMH = VOH – VIH = 2.82V
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PSPICE Noise Simulation Demonstrating the
Concept of Noise Margin (LS7404)
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PSPICE Noise Simulation
V(IN) = Source + Noise
VOH
VOL
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Vpeak-to-peak
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PSPICE Noise Simulation
V(OUT) for Vpeak= 0, 0.5, 1, 3.1V
Vpeak=0V
Vpeak=1V
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Vpeak=0.5V
Vpeak=3.1V
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The Ideal Inverter
The ideal gate should have
–
–
–
–
–
infinite slope (gain) in the transition region
high and low noise margins equal to half the logic swing
a gate threshold located in the middle of the logic swing
input and output impedances of infinity and zero, respectively
infinite output drive capability (infinite output current or fanout)
VOUT
VOH = VCC
Ideal VTC
VCC = VSUPPLY
VTW=0 (Impossible to be in X state)
NMH = NML = VCC/2
dVo
VLS=VCC
dVi
Ri = 
 
Ro = 0
Fanout = 
VOL = 0V
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VIL=VIH= VCC/2
VIN
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Current Level Conventions
Current out of a terminal is given as a negative value in Data Sheets.
Current into a terminal is given as positive value in Data Sheets.
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Table of Current Levels for
TTL Families
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Current Sourcing Versus Current Sinking
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Current Level Definitions
IOL – Low Level Output Current
“0”
IOUT
Maximum current that an output terminal
of a gate can sink when the output is “0”.
If |IOUT| > |IOL|, then eventually
VOUT > VOL
TTL Ex: IOL = +16mA, VOUT ≤ 0.4V
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Current Level Definitions
IIL – Low Level Input Current
“0”
IIN
Maximum current that an input terminal of a
gate can source when the input is “0”.
TTL Ex: IIL = -1.6mA, VIN ≤ 0.8V
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Current Level Definitions
IOH – High Level Output Current
“1”
IOUT
Maximum current that an output terminal of
a gate can source when the output is “1”.
If |IOUT| > |IOH|, then eventually
VOUT < VOH
TTL Ex: IOH = -400mA, VOUT ≥ 2.4V
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Current Level Definitions
IIH – High Level Input Current
“1”
IIN
Maximum current that an input terminal
of a gate can sink when the input is “1”.
TTL Ex: IIH = +40mA, VIN ≥ 2.0V
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TTL Example – LED Load
Which circuit would you use to drive an LED?
(Assume that VLED = ~1.7 V.)
I
+
1.7 V
I
-
+
1.7 V
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TTL Example – LED Load
Analysis of the first circuit:
I
+
1.7 V
For this circuit, Vout ≥2.4V and thus I ≥ 2mA > IOH ! (IOH = -0.4mA)
This TTL logic device can’t supply this amount of current since the
output voltage will decrease as the output current exceeds IOH.
Thereby, a “1” state at the output is no longer guaranteed.
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TTL Example – LED Load
Analysis of the second circuit:
I
-
+
1.7 V
If Vout = 0.4V, then I = 9 mA < IOL. ( IOL = 16 mA )
If Vout = 0V, then I = 10 mA < IOL.
The necessary output current can be supplied.
For TTL, the magnitude of IOL > the magnitude of IOH
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Fan-Out and Fan-In

Fan-out – number of load gates
connected to the output of the
driving gate

gates with large fan-out are slower
N

Fan-in – the number of inputs to
the gate

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M
gates with large fan-in are bigger
and slower
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Static or DC Fan-Out
Fan-Out (N) is defined as the number of
loads (gate inputs) that can be driven by a
single gate output at DC or low frequencies.
Low Level Fan-out
NL =
IOL of the driving gate
High Level Fan-out
NH =
I IL of the load gate
IOH of the driving gate
I IH of the load gate
N = min{NL,NH}
Ex: For most TTL families, N = NL.
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TTL Static Fan-Out Example
NL 
16mA
 10
1.6mA
NH 
400 m A
 10
40m A
N  10
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Dynamic Fan-Out
Additional current
due to the capacitive
load.
The impedance of a capacitor, (wC)-1, decreases as the frequency
increases. Therefore, the load current increases as the frequency
increases. This additional load current must be taken into account
when determining fan-out at high frequency.
Dynamic Fan-Out << Static Fan-Out
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Power Dissipation Definitions
Instantaneous power:
p(t) = v(t)i(t) = Vsupplyi(t)
Peak power:
Ppeak = Vsupplyipeak
Average power:
Vsupply
1 t T
Pav   p(t )dt 
T t
T
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
t T
t
isupply ( t ) dt
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Power Dissipation
 Power consumption: how much energy is consumed
per operation and how much heat the circuit
dissipates
 power supply sizing (determined by peak power)
Ppeak = Vsupplyipeak
– battery lifetime (determined by average power dissipation)
p(t) = v(t)i(t) = Vsupplyi(t)
Pavg= 1/T  p(t) dt = Vsupply/T  isupply(t) dt
– packaging and cooling requirements
 Two important components:
static (DC) and dynamic
CMOS Example: PAV = VDD Ileakage + CL VDD2 f
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TTL Supply Currents
• The power supply current
is dependent on the
output state.
• ICCL > ICCH since
 IOL > IOH
 IIL > IIH
Note: these values are for a 7400 NAND Gate chip (total current for 4
NAND gates)
Power Dissipation of a TTL Gate
PAV  PCCAV  PEEAV
PAV 
I CCH ( gate )  I CCL ( gate )
2
VCC 
I EEH ( gate )  I EEL ( gate )
2
VEE
For VEE  0
PAV 
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I CCH ( gate )  I CCL ( gate )
2
VCC
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Power Dissipation of a TTL Gate
Example
What is the power dissipation of a single TTL NAND
gate (7400)?
8mA
I CCH ( gate) 
 2mA
4
22mA
I CCL ( gate) 
 5.5mA
4
2mA  5.5mA
PAV 
(5V )  18.75mW
2
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Power Supply Currents Versus
Frequency
10 KHz
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100 KHz
1 MHz
10 MHz
100 MHz
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Propagation Delay Time
Vin
Vout
A measure of how
long it takes for a
gate to change state.
Ideally, should be as
short as possible.
tPHL - the time it takes
the output to go from
a high to a low
Average Propagation Delay Time
tp =
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t pHL  t pLH
2
tPLH - the time it takes
the output to go from
a low to a high
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Modeling Propagation Delay
Model circuit as a first-order RC network
R
vout
C
vin
I R  IC
V  Vout (t )
dVout (t )
C
R
dt
V
Vout (t) = (1 – e–t/)V ,where  = RC
Time to reach 50% point is t = ln(2)  = 0.69 
0
t
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Modeling Propagation Delay
Driver Gate and Load Gate are modeled as a first-order
RC network
vout
R
vin
vS
C
+
-
Driver
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Load
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CMOS Inverter as an Example
VDD
+
pullup
network
in
out
out
in
Drain
pulldown
network
VSS
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Switch Models of a CMOS Inverter
VDD
VDD
Rp
Vout = 0
Vout = VDD
Rn
Vin = V DD
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Vin = 0
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CMOS Inverter: Transient Response
V DD
V DD
Rp
Capacitor Charging
Capacitor Discharging
V out
V out
CL
CL
Rn
V in
0
(a) Low-to-high output
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V in
V DD
(b) High-to-low output
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CMOS Inverter Propagation Delay
• Propagation delay is proportional to the time-constant
of the network formed by the pull-down resistor or
pull-up resistor and the load capacitance.
tpHL = ln(2) Rn CL = 0.69 Rn CL
tpLH = ln(2) Rp CL = 0.69 Rp CL
tp = (tpHL + tpLH)/2 = 0.69 CL(Rn + Rp)/2
• To equalize propagation delay times make the onresistance of the NMOS and PMOS approximately
equal.
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PSPICE Simulation of Delay with RC Models (Switch)
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The Effect of Fan-Out on Propagation Delay
x
f
V out
To inputs of
x
n other inverters
To inputs of
n other inverters
Cn
(a) Inverter that drives n other inverters
(b) Equivalent circuit for timing purposes
V out for n =1
VDD
V out for n = 4
Gnd
0
Time
(c) Propagation times for different values of n
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Measurement of the Average
Propagation Delay Time under
Loaded Conditions
Ring Oscillator Circuit
T = Period
10
10
01
N = odd number of gates, tp = T/2N
Typically, CL= 15pf or 50pF
Note: This is a self-oscillating circuit that requires no signal input.
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Maximum Frequency of Operation
FMAX
1

2 max(tPLH , tPHL )
TTL Ex: tPLH = 22ns, tPHL = 15ns
Fmax = 22 MHz
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Propagation Delay and Power
Dissipation (for a single gate)
TTL
{
CMOS {
TTL
Comparison
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Propagation Delay Versus Power
Dissipation (for a single gate)
Ideal
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Power Dissipation and Propagation Delay
• Propagation delay and the power consumption of a gate are
related
• Propagation delay is (mostly) determined by the speed at
which a given amount of energy can be stored on the gate
– the faster the energy transfer (higher power dissipation) the faster the
gate
• For a given technology, the product of the power consumption
and the propagation delay is a constant and is used as a benchmark to compare digital technologies.
– Power-Delay Product (PDP) specifies the energy consumed by the gate
per switching event
• An ideal gate is one that is fast and consumes little energy and
therefore the PDP should be as small as possible.
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Power-Delay Product Calculation
 The average power dissipation (mW) by a single gate
is multiplied by the average propagation delay time
(ns) of a single gate to get the total energy (pJ)
dissipated by a gate for a given technology.
 Compare the PDP of 4000 CMOS, ALS TTL, and ECL
CMOS:
PDP = (1mW) (100ns) = 100pJ
ALS TTL: PDP = (1.5mW)( 4ns) = 6pJ
ECL:
PDP = (40mW) ( 1ns) = 40pJ
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Common IC Packages
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Comparison of Microprocessors in 2001
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How much space does a single MOS
transistor occupy on a silicon chip?
Example: 100 million transistors on a 1cm x 1cm Si die.
108 transistors N  N
1 transistor 1 transistor

or

2
2
-8
2
1cm
1cm
10 cm
1m m 2
OR
1 cm
N
N
1 cm
104 transistors
N
cm
Source
or
1 transistor 1 transistor

-4
10 cm
1m m
L<0.1 mm
Gate
Drain
~1 mm
~1 mm
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Moore’s Law in Microprocessors
Transistors on lead microprocessors double every 2 years
1000
2X growth in 1.96 years!
Transistors (MT)
100
Pentium® IV
10
486
1
386
286
0.1
0.01
8086
8080
8008
4004
8085
0.001
1970
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Pentium® proc
1980
1990
Year
Courtesy, Intel
2000
2010
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Intel 4004 Microprocessor
1971
2300 transistors
1 MHz clock
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Intel Pentium (IV) Microprocessor
2001
42 million transistors
2 GHz clock
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Evolution in DRAM Chip Capacity
human memory
human DNA
100000000
10000000
64,000,000
4X growth every 3 years!
16,000,000
Kbit capacity/chip
4,000,000
1000000
1,000,000
book
100000
256,000
64,000
16,000
10000
4,000
1000
1,000
256
100
64
10
1980
0.07 mm
0.1 mm
0.13 mm
0.18-0.25 mm
0.35-0.4 mm
0.5-0.6 mm
0.7-0.8 mm
1.0-1.2 mm
encyclopedia
2 hrs CD audio
30 sec HDTV
1.6-2.4 mm
page
1983
1986
1989
1992
1995
Year
1998
2001
2004
2007
2010
Clock Frequency
Lead microprocessors frequency doubles every 2 years
10000
2X every 2 years
Frequency (Mhz)
1000
100
Pentium® proc
486
10
8085
1
0.1
1970
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Pentium® IV
8086 286
386
8080
8008
4004
1980
1990
Year
Courtesy, Intel
2000
2010
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Power Dissipation
Lead Microprocessors power continues to increase
100
Pentium® IV
Power (Watts)
Pentium® proc
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
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Power Density
Power Density (W/cm2)
10000
Rocket
Nozzle
1000
Nuclear
Reactor
100
8086
Pentium® IV
10 4004
Hot Plate
8008 8085
Pentium® proc
386
286
486
8080
1
1970
1980
1990
Year
2000
2010
Power density too high to keep junctions at low temp
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Power Dissipation (Battery Lifetime) is
an issue in all types of handhelds
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What is the Current Power Dissipation in
Processors?
• For a group exercise in class, select a processor manufactured
during the last three years and provide information on its power
dissipation by specifying the thermal design power (TDP).
• TDP is the maximum amount of heat generated by the
processor which the cooling system is required to dissipate.
• Look at processors manufactured by Intel, AMD, or Freescale
and use the manufacturers’ web sites to obtain the information.
Provide the website where you found your information.
Specify the properties of the processors such as clock
frequency, die size, number of transistors, and the number of
cores (if applicable).
• Use the excel spreadsheet form at the course website to report
your findings and email this file to your instructor when
completed.
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