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Logic Families
Logic Family : A collection of different IC’s that
have similar circuit characteristics
The circuit design of the basic gate of each logic
family is the same
The most important parameters for evaluating and
comparing logic families include :
Logic Levels
Power Dissipation
Propagation delay
Noise margin
Fan-out ( loading )
1
Example Logic Families
General comparison or three commonly available logic
families.
the most important to understand
2
Implementing Logic Circuits
There are several varieties of transistors – the
building blocks of logic gates – the most important
are:
BJT (bipolar junction transistors)
one of the first to be invented
FET (field effect transistors)
especially Metal-Oxide Semiconductor types (MOSFET’s)
MOSFET’s are of two types: NMOS and PMOS
3
Transistor Size Scaling
Performance improves as size is decreased: shorter switching time, lower power consumption.
2 orders of magnitude reduction in transistor size in 30 years.
4
Moore’s Law
In 1965, Gordon Moore predicted that the number of
transistors that can be integrated on a die would
double every 18 to 14 months
i.e., grow exponentially with time
Considered a visionary – million transistor/chip
barrier was crossed in the 1980’s
2300 transistors, 1 MHz clock (Intel 4004/4040) - 1971
42 Million transistors, 2 GHz clock (Intel P4) - 2001
140 Million transistors, (HP PA-8500)
5
Moore’s Law and Intel
From Intel’s 4040 (2300 transistors) to Pentium II (7,500,000 transistors) and beyond
6
TTL and CMOS
Connecting BJT’s together gives rise to a family of logic gates
known as TTL
Connecting NMOS and PMOS transistors together gives rise
to the CMOS family of logic gates
BJT
transistor types
MOSFET
(NMOS, PMOS)
TTL
logic gate families
CMOS
7
Electrical Parameters And
Interpretation Of Data Sheets
Voltages and Currents
Noise Margin
Power Dissipation
Propagation Delay
Speed-Power Product
Fan-In, Fan-Out
Comparison of Logic Families
Interpretation of Data Sheets
8
Electrical Characteristics
TTL
faster (some versions)
strong drive capability
rugged
CMOS
lower power consumption
simpler to make
greater packing density
better noise immunity
• Complex IC’s contain many millions of transistors
• If constructed entirely from TTL type gates would melt
• A combination of technologies (families) may be used
• CMOS has become most popular and has had greatest development
9
Voltage & Current
For a High-state gate driving a second gate, we define:
VOH (min), high-level output voltage, the minimum voltage level that a logic
gate will produce as a logic 1 output.
VIH (min), high-level input voltage, the minimum voltage level that a logic
gate will recognize as a logic 1 input. Voltage below this level will not be
accepted as high.
IOH, high-level output current, current that flows from an output in the logic
1 state under specified load conditions.
IIH, high-level input current, current that flows into an input when a logic 1
voltage is applied to that input.
I OH
Test setup for
measuring
values
VOH
Ground
I IH
VIH
10
Voltage & Current
For a Low-state gate driving a second gate, we
define:
VOL (max), low-level output voltage, the maximum voltage level
that a logic gate will produce as a logic 0 output.
VIL (max), low-level input voltage, the maximum voltage level
that a logic gate will recognize as a logic 0 input. Voltage above
this value will not be accepted as low.
IOL , low-level output current, current that flows from an output
in the logic 0 state under specified load conditions.
IIL , low-level input current, current that flows into an input when
a logic 0 voltage is applied to that input.
I
Inputs are
connected to Vcc
instead of
Ground
Ground
OL
I IL
V OL
V IL
11
Electrical Characteristics
logic 1
indeterminate
input voltage
logic 0
Important characteristics are:
VOHmin min value of output recognized as a ‘1’
VIHmin min value input recognized as a ‘1’
VILmax max value of input recognized as a ‘0’
VOLmax max value of output recognized as a ‘0’
Values outside the given range are not allowed.
12
Logic Level & Voltage Range
Typical acceptable voltage ranges for positive logic 1 and
logic 0 are shown below
A logic gate with an input at a voltage level within the
‘indeterminate’ range will produce an unpredictable output
level.
5.0V
5.0V
Logic 1
Logic 1
3.5V
2.5V
Indeterminate
Indeterminate
0.8V
1.5V
Logic 0
Logic 0
0V
0V
TTL
CMOS
13
Noise Margin
If noise in the circuit is high enough
it can push a logic 0 up or drop a
logic 1 down into the indeterminate
or “illegal” region
The magnitude of the voltage
required to reach this level is the
noise margin
Noise margin for logic high is:
NMH = VOHmin – VIHmin
logic 1
VOHmin
VIHmin
indeterminate
input voltage
logic 0
VILmax
VOLmax
14
Noise Margin
Difference between the worst case output voltage of
one stage and worst case input voltage of next stage
Greater the difference, the more unwanted signal that
can be added without causing incorrect gate
operation
NMhigh = VOHmin - VIHmin
NMlow = VILmax - VOLmax
15
Worked Example
Given the following parameters, calculate the
noise margin of 74LS series.
Parameter
VIH (min)
VIL(max )
VOH (min)
VOL(max )
74LS
2V
0.8V
2.7V
0.4V
Solution:
High Level Noise Margin, VNH = VOH (min) - VIH (min)=2.7V-2.0V=0.7V
Low Level Noise Margin, VNL = VIL (max) - VOL (max)=0.8V-0.4V=0.4V
16
Noise Margin & Noise Immunity
Noise immunity of a logic circuit refers to the circuit’s ability
to tolerate noise voltages on its inputs.
A quantitative measure of noise immunity is called noise
margin
High Level Noise Margin, VNH = VOH (min) - VIH (min)
Low Level Noise Margin, VNL = VIL (max) - VOL (max)
Logic 1
VOH (min)
Logic 1
VNH
VIH (min)
VIL (max)
VNL
VOL (max)
Logic 0
Output Voltage Ranges
Logic 0
Input Voltage Ranges
17
Further Important Characteristics
The propagation delay (tpd) which is the time
taken for a change at the input to appear at the
output
The fan-out, which is the maximum number of
inputs that can be driven successfully to either
logic level before the output becomes invalid
18
Speed: Rise & Fall Times
Rise Time
Time from 10% to 90% of signal, Low to High
Fall Time
Time from 90% to 10% of signal, High to Low
rise time
10%
fall time
90%
90%
10%
19
Speed: Propagation Delay
A logic gate always takes some time to change states
tPLH is the delay time before output changes from low to high
tPHL is the delay time before output changes from high to low
both tPLH & tPHL are measured between the 50% points on the
input and output transitions
Input
50%
0
Output
0
tPHL
tPLH
20
Power Dissipation
Static
I2R losses due to passive components, no input signal
Dynamic
I2R losses due to charging and discharging capacitances
through resistances, due to input signal
21
Speed-Power Product
Speed (propagation delay) and power consumption
are the two most important performance parameters
of a digital IC.
A simple means for measuring and comparing the
overall performance of an IC family is the speedpower product (the smaller, the better).
For example, an IC has
an average propagation delay of 10 ns
an average power dissipation of 5 mW
the speed-power product = (10 ns) x (5 mW)
= 50 picoJoules (pJ)
22
Logic Family Tradeoffs
Looking for the best
speed/power product
tp and Pd are normally
included in the data
sheet for each device
Older logic families
are the worst
CMOS is one of the
best
FPGAs use CMOS
23
Comparison of Logic Families
24
TTL - Example SN74LS00
Recommended operating conditions
Vcc supply voltage
input voltages
5V ± 0.5 V
VIH = 2V
VIL = 0.8V
Electrical Characteristics
output voltage
(worst case)
max input currents
propagation delay
noise margins
Fan-out
5 Volt
Input
Range
for 1
VOH = 2.7V
VOL = 0.5V
IIH = 20µA
IIL = -0.4mA
tpd = 15 nS
for a logic 0 = 0.3V
for a logic 1 = 0.7V
20 TTL loads
Output
Range
for 1
2.7
2.0
Input
Range
for 0
0.8
0.5
0 Volt
Output
Range
for 0
25
Fan-In
Number of input signals to a gate
Not an electrical property
Function of the manufacturing process
NAND gate with a
Fan-in of 8
26
Fan-Out
A measure of the ability of the output of one gate to
drive the input(s) of subsequent gates
Usually specified as standard loads within a single
family
e.g., an input to an inverter in the same family
May have to compute based on current drive
requirements when mixing families
Although mixing families is not usually recommended
27
Current Sourcing and Sinking
Current-source : the driving gate produces a
outgoing current
VOH
Low
IIH
Current-sinking : the driving gate receives an
incoming current
VOL
High
IIL
28
Fan-Out
An illustration of fan-out and the associated source
and sink currents
29
Worked Example
How many 74LS00 NAND gate inputs can be driven
by a 74LS00 NAND gate outputs ?
Solution:
Refer to data sheet of 74LS00, the maximum values of
IOH = 0.4mA, IOL = 8mA, IIH = 20uA, and IIL = 0.4mA
Hence,
fan-out(high) = IOH(max) / IIH (max)=0.4mA/20uA=20
fan-out(low) = IOL(max) / IIL(max)=8mA/0.4mA=20,
the overall fan-out = fan-out(high) or fan-out(low) whichever is lower.
Hence, overall fan-out = 20
30
Gate Drive Capability: Fan-Out
A logic gate can supply a maximum output current
IOH(max), in the high state or
IOL(max), in the low state
A logic gate requires a maximum input current
IIH(max), in the high state or
IIL(max), in the low state
Ratio of output and input current decide how many logic
gates can be driven by a logic gate
fan-out(high) = IOH(max) / IIH (max)
fan-out(low) = IOL(max) / IIL(max)
overall fan-out = fan-out(high) or fan-out(low) whichever is lower
A typical figure of fan-out is ten (10)
31
Wired-AND
Open collector outputs connected together to a common pullup resistor
Any collector can pull the signal line low
Logically an AND gate
32
Tri-State Logic
Both output transistors of totem-pole output are turned off
Usually used to bus multiple signals on the same wire
Gates not enabled present high-Z to bus and therefore do
not interfere with other gates putting signals on the bus
33
Tri-State Logic
Tri-state logic includes a switch at the output
In the figure below, the three states are illustrated:
a) Logic High output
b) Logic Low output
c) High impedance (Hi-Z) output
34
Electronic Combinational Logic
Within each of these families there is a large variety of different devices
We can break these into groups based on the number gates per device
Acronym
Description
No Gates
Example
SSI
Small-scale integration
<12
4 NAND gates
MSI
Medium-scale integration
12 – 100
Adder
LSI
Large-scale integration
100 – 1000
6800
VLSI
Very large-scale integration
1000 – 1M
68000
ULSI
Ultra large scale integration > 1M
80486/80586
35
SSI Devices
Each package contains a code identifying the package
N74LS00
Manufacturers Code
N = National Semiconductors
SN = Signetics
Specification
Family
L
LS
H
Member
00 = Quad 2 input NAND
02 = Quad 2 input Nor
04 = Hex Invertors
20 = Dual 4 Input NAND
36
7400 Series History
1960s space program drove
development of 7400 series
Consumed all available devices for
internal flight computer
$1000 / device (1960 dollars)
10:1 integration improvement over
discrete transistors
1963 Minuteman missile forced
7400 into mass production
Drove pricing down to $25 / circuit
(1963 dollars)
37
7400 Series Evolution
BJT storage time reduction by using a BC Schottky diode.
Schottky diode has a Vfw=0.25V. When BC junction becomes forward
biased Schottky diode will bypass base current.
C
B
38
Characteristics: TTL and MOS
Remember:
TTL stands for Transistor-Transistor Logic
uses BJTs
MOS stands for Metal Oxide Semiconductor
uses FETs
MOS can be classified into three sub-families:
PMOS (P-channel)
NMOS (N-channel)
CMOS (Complementary MOS, most common)
39
TTL Circuit Operation
+Vcc
4K
R1
1.6K
R2
130
R3
Q
Q
2
A
B
D
4
D3
Y O/P
1
Q
1
D2
I CQ1
Q
A
B
I
CQ1
Q
1
Q
2
Q
3
Q
4
Y O/P
0
0
+
ON
OFF
OFF
ON
1
0
1
+
ON
OFF
OFF
ON
1
1
0
+
ON
OFF
OFF
ON
1
1
1
－
OFF
ON
ON
OFF
0
3
1K
R4
Table explaining the operation of the
TTL NAND gate circuit
A standard TTL NAND gate circuit
40
Transistor-Transistor Logic Families
Transistor-Transistor Logic Families:
74L
Low power
74H
High speed
74S
Schottky
74LS
Low power Schottky
74AS
Advanced Schottky
74ALS
Advance Low power Schottky
41
MOS Circuit Operation
+VDD
S
Q1
D
O/P
I/P
Q1
Q2
O/P
0
ON
OFF
1
1
OFF
ON
0
D
I/P
Q2
S
A CMOS inverter circuit
Table explaining the operation of
the CMOS inverter circuit
42
CMOS Logic Families
CMOS Logic Families
40xx/45xx
74C
74HC
74ACT
Metal-gate CMOS
TTL-compatible CMOS
High speed CMOS
Advanced CMOS -TTL compatible
43
CMOS Family Evolution
CMOS Logic Trend: Reduction of dynamic losses
(cross-conduction, capacitive charge/discharge cycles)
by decreasing supply voltages:
12V→5V →3.3V →2.5V → 1.8V → 1.5V …
Reduction of IC power dissipation is the key to:
lower cost (packaging)
higher integration
improved reliability
44
Comparison of Logic Families
vo
vi
45