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EET 1131 Unit 9
Logic Families

Read Kleitz, Chapter 9.
Homework #9 and Lab #9 due next
week.

Quiz next week.

Two Main Goals of Unit 9
1. Make sense of the most important numbers in a
datasheet.
2. Understand the three kinds of output pins that
you’re likely to encounter:
•
Totem-pole outputs
•
Open-collector outputs
•
Three-state outputs
Two Kinds of Transistors
•In Electronic Devices & Circuits (EET 2201) you’ll
study two major classes of transistors:
•Bipolar Junction Transistors (BJTs)
•Metal-Oxide Semiconductor Field Effect Transistor
(MOSFETs)
Logic Families
•Three major logic families since the 1960’s:
•TTL (Transistor-Transistor Logic), based on bipolar
junction transistors
•CMOS (Complementary Metal Oxide
Semiconductor), based on MOSFETs
•ECL (Emitter-Coupled Logic), based on bipolar
junction transistors
•Originally, TTL chips were fast but used lots of power,
and CMOS chips used little power but were slow.
•CMOS chips are sensitive to static discharge, and
must be handled carefully.
•ECL chips are the fastest and most power-hungry.
7400 Series and 4000 Series
•A popular series of TTL chips is the 7400
series that you’ve used in this course:
Wikipedia's list
•A popular series of CMOS chips is the 4000
series: Wikipedia's list
Some Subfamilies within the TTL
Family
Description
Designator
Example
Standard TTL
None
7404
Low-Power Schottky
LS
74LS04
Advanced Low-Power Schottky
ALS
74ALS04
Fast
F
74F04
•The 7404, the 74LS04, the 74ALS04, and
the 74F04 all perform the same logic
functions.
•But they differ in terms of their performance
characteristics (primarily speed and power
consumption).
Some Subfamilies within the
CMOS Family
Description
Designator
Example
Standard CMOS
4069
Pin-compatible with 7400 series C
74C04
High-Speed CMOS
HC
74HC04
Advanced High-Speed CMOS
AHC
74AHC04
Low-Voltage CMOS (3.3 V)
LVC
74LVC04
Advanced Low-Voltage CMOS
(2.5 V)
ALVC
74ALVC04
Review: Voltage and Current
A wire is like a water pipe. The amount of
electricity per second flowing through a wire is called
current, which is measured in amperes.
A voltage source is like a
water pump. Its voltage
rating (in volts)
tells you how strong it is.
Power (measured in watts) is
the rate at which energy is used.
Power = Voltage  Current
The voltage
(pressure)
at this point
is greater than
the voltage
at this point.
Resistors are like partial blockages
in the pipe. They restrict the amount
of current that flows through the circuit.
Some Electrical Quantities and Units

Quantity
Symbol
Unit
Symbol for
the Unit
Current
I
ampere
A
Voltage
V
volt
V
Power
P
watt
W
Resistance
R
ohm

For example,


To say that a voltage is 5 volts, we write
V=5V
To say that a current is 30 milliamperes,
we write
I = 30 mA
Basic Operational Characteristics
and Parameters

Consult datasheets for







DC supply voltage
Input/output current and fan-out
Input/output voltages and noise margin
Propagation delay
Power dissipation
Speed-power product
Example datasheets:


7404 TTL inverter
74HC04 High-Speed CMOS inverter
DC Supply Voltages


TTL chips are optimized for 5 V supply,
and cannot tolerate voltages far above
or below 5 V.
CMOS chips may be optimized for 5 V,
3.3 V, 2.5 V, or 1.8 V supplies. Most
CMOS chips can tolerate a much wider
range of supply voltages than TTL
chips.
Current-Sourcing and CurrentSinking

For TTL:


A HIGH output sources current
A LOW output sinks current.
Input and Output Currents

Four key current parameters:





IIH = the current flowing through an input
pin when it’s HIGH.
IIL = the current flowing through an input
pin when it’s LOW.
IOH = the current flowing through an output
pin when it’s HIGH.
IOL = the current flowing through an output
pin when it’s LOW.
In all cases, a negative sign on the
current value means current is leaving
the gate through the pin. No negative
sign means current is entering.
Fan-out



Fan-out means the
number of load inputs
that a given output can
drive.
With TTL, current is the
limiting factor in
determining fan-out.
With CMOS, capacitance
is the limiting factor.
How many
more gates
can I connect?
Example: Calculating TTL Fan-out

For a standard TTL gate:



Also:




A LOW input sources up to 1.6 mA.
A LOW output can sink up to 16 mA.
A HIGH input sinks up to 40 A.
A HIGH output can source up to 400 A.
Thus, standard TTL has a fan-out of
10.
See Wisconsin Online’s Fan-out Lesson
Logic Levels





Four key voltage parameters when
you’re interfacing logic:
VIH(min) = the minimum voltage that an
input pin will recognize as a HIGH.
VIL(max) = the maximum voltage an input
pin will recognize as a LOW.
VOH(min) = the minimum voltage that can
appear on an output pin when it’s
HIGH .
VOL(max) = the maximum voltage that can
appear on an output pin when it’s
LOW.
Logic levels for TTL
Noise Margin

The noise margin is the room for
error between the voltage that an
output pin produces and the voltage
that an input pin expects.
VNH = VOH(min) − VIH(min)
 VNL = VIL(max) − VOL(max)

Propagation Delay



Data sheets specify propagation delays
for low-to-high output transitions (tPLH)
and high-to-low output transitions
(tPHL).
A device with a smaller propagation
delay can run faster (at a higher
frequency) than a device with a higher
propagation delay.
Next slide shows example waveforms.
Figure 9.10
(Continued) (b) propagation delay times.
Digital Electronics: A Practical Approach with VHDL, 9th Edition
William Kleitz
Copyright ©2012 by Pearson Education, Inc.
All rights reserved.
Power Dissipation



Recall that power equals current times
voltage (P=IV).
So a chip’s power dissipation is given
by its supply voltage (VCC) times its
supply current (ICC). (Note that we
don’t use the voltages and currents on
input or output pins.)
A lower-power device wastes less
energy, generates less heat, and costs
less to run than a higher-power device.
Speed-Power Product


A useful overall measure of a device’s
performance is its speed-power
product, found by multiplying its
average power dissipation times its
average propagation delay.
The lower the speed-power product,
the better.
Comparing Logic Families
Interfacing Logic Families


We’ve seen that different logic families
have different voltage and current
specifications.
These differences can cause problems when
you connect a gate from one family to a
gate from another family.
Voltage-Related Interfacing
Problems


In some interfacing situations, a
HIGH output pin may produce a
voltage that is too low to be
recognized as a HIGH by the input
pin it’s connected to.
Example: Suppose you want to
connect a 74TTL device to a 4000B
CMOS device. Using the textbook’s
Table 9-4, determine whether this
may result in any voltage-related
interfacing problems.
Voltage-Related Interfacing
Problem Example: TTL to CMOS



A TTL HIGH output may be as low as 2.4 V.
But a 4000B CMOS input expects HIGHs to
be at least 3.33 V. That’s a problem!
Solution: use a pull-up resistor, as shown
in Figure 9-27.
Current-Related Interfacing
Problems


In some interfacing situations, either
a HIGH output pin may not source
enough current to drive the input pin
it’s connected to, or a LOW output
pin may not sink enough current to
drive the input pin it’s connected to.
Continuing the previous example,
should we be concerned about any
current-related interfacing problems
when connecting 74TTL to 4000B
CMOS?
Current-Related Interfacing
Problem Example: CMOS to TTL



A 4000B CMOS LOW output can only sink
0.51 mA.
But as much as 1.6 mA may flow out of a
TTL LOW input. That’s a problem!
Solution: use a CMOS buffer, as shown in
Figure 9-29.
Two Main Goals of Unit 9
1. Make sense of the most important numbers in a
datasheet.
2. Understand three kinds of output pins that you’re
likely to encounter:
•
Totem-pole outputs
•
Open-collector outputs
•
Three-state outputs
Three Kinds of Outputs

TTL chips can have three kinds of
outputs:



Totem-pole (the most common)
Open-collector (discussed in Chapter 9)
Three-state (discussed in Chapter 13)
Totem-Pole Output

Most chips you’ve used up to now
have had totem-pole outputs.
A standard TTL inverter circuit.
Digital Fundamentals, Tenth Edition
Thomas L. Floyd
Copyright ©2009 by Pearson Higher Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Open-Collector Output


Missing a transistor internally, so
you must provide an external
pull-up resistor.
Two advantages:
1.
2.
Allows for the use of higher-thanusual voltages and currents.
Allows a trick called “wired-AND,”
which means you can AND the outputs
of two chips by tying them directly
together. (Never tie totem-pole
outputs together.)
TTL inverter with open-collector output.
Digital Fundamentals, Tenth Edition
Thomas L. Floyd
Copyright ©2009 by Pearson Higher Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Open-collector symbol in an inverter.
Digital Fundamentals, Tenth Edition
Thomas L. Floyd
Copyright ©2009 by Pearson Higher Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Some Open-Collector Chips

7405 (Hex Inverters with Open-Collector
Outputs)


7409 (Quad 2-Input AND with OpenCollector Outputs)
7412 (Triple 3-Input NAND with OpenCollector Outputs)
Three-State Output (also called tri-state output)



In addition to the two usual output
states (HIGH and LOW), has a third
output state called high-impedance
(“high-Z”).
In the high-Z state, the output is
disconnected from the external
circuit.
Useful when the outputs of many
chips are tied to the same bus: at
any time, only one of them should
be connected to the bus.
The three states of a tristate circuit.
Digital Fundamentals, Tenth Edition
Thomas L. Floyd
Copyright ©2009 by Pearson Higher Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Basic tristate inverter circuit.
Digital Fundamentals, Tenth Edition
Thomas L. Floyd
Copyright ©2009 by Pearson Higher Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Some Three-State Chips

74251 (Data Selectors/Multiplexers with 3State Outputs)

74LS348 (8-Line To 3-Line Priority
Encoders With 3-State Outputs)
A CMOS inverter circuit.
Digital Fundamentals, Tenth Edition
Thomas L. Floyd
Copyright ©2009 by Pearson Higher Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Three Kinds of CMOS Outputs

Like TTL chips, CMOS chips can
have two kinds of special-purpose
outputs instead of the usual
outputs:

Open-drain
Similar to open-collector in TTL
 Requires an external pull-up resistor


Three-state
Other Logic Families
•TTL and CMOS are by far the two most
common logic families, but you may also
encounter chips from other families:
•ECL (Emitter-Coupled Logic): The fastest logic
family
•PMOS (p-Channel Metal Oxide Semiconductor)
•NMOS (n-Channel Metal Oxide Semiconductor)
•E2CMOS (Electrically Erasable CMOS)