Chapter 15
Introduction to Digital Electronics
Microelectronic Circuit Design
Richard C. Jaeger
Travis N. Blalock
3 Microelettronica – Elettronica digitale 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Chapter Goals
• Introduce binary digital logic concepts
• Explore the voltage transfer characteristics of ideal and
nonideal inverters
• Define logic levels and logic states of logic gates
• Introduce the concept of noise margin
• Present measures of dynamic performance of logic devices
• Review of Boolean algebra
• Investigate simple transistor, diode, and diode-transistor
implementations of the inverter and other logic circuits
• Explore basic design techniques of logic circuits
3 Microelettronica – Elettronica digitale 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Brief History of Digital Electronics
• Digital electronics can be found in many applications in
the form of microprocessors, microcontrollers, PCs, DSPs,
and an uncountable number of other systems.
• The design of digital circuits has progressed from resistortransistor logic (RTL) and diode-transistor logic (DTL) to
transistor-transistor logic (TTL) and emitter-coupled logic
(ECL) to complementary MOS (CMOS)
• The density and number of transistors in microprocessors
has increased from 2300 in the 1971 4-bit 4004
microprocessor to 25 million in the more recent IA-64 chip
and it is projected to reach over one billion transistors by
2010
3 Microelettronica – Elettronica digitale 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Ideal Logic Gates
• Binary logic gates are the most common style of
digital logic
• The output will consist of either a 0 (low) or a 1 (high)
• The most basic digital building block is the inverter
3 Microelettronica – Elettronica digitale 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
The Ideal Inverter
The ideal inverter has the following voltage transfer characteristic
(VTC) and is described by the following symbol
V+ and V- are the supply rails, and VH and VL describe
the high and low logic levels at the output
3 Microelettronica – Elettronica digitale 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Logic Level Definitions
An inverter operating with power supplies at V+ and
0 V can be implemented using a switch with a
resistive load
3 Microelettronica – Elettronica digitale 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Logic Voltage Level Definitions
• VL – The nominal voltage corresponding to a low-logic
state at the input of a logic gate for vi = VH
• VH – The nominal voltage corresponding to a high-logic
state at the output of a logic gate for vi = VL
• VIL – The maximum input voltage that will be recognized
as a low input logic level
• VIH – The maximum input voltage that will be recognized
as a high input logic level
• VOH – The output voltage corresponding to an input
voltage of VIL
• VOL – The output voltage corresponding to an input
voltage of VIH
3 Microelettronica – Elettronica digitale 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Logic Voltage Level Definitions (cont.)
Note that for the VTC of the nonideal inverter, there is now an
undefined logic state
3 Microelettronica – Elettronica digitale 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Noise Margins
• Noise margins represent “safety margins” that
prevent the circuit from producing erroneous
outputs in the presence of noisy inputs
• Noise margins are defined for low and high input
levels using the following equations:
NML = VIL – VOL
NMH = VOH – VIH
3 Microelettronica – Elettronica digitale 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl
Noise Margins (cont.)
• Graphical representation of
where noise margins are
defined
Continua…
3 Microelettronica – Elettronica digitale 2/ed
Richard C. Jaeger, Travis N. Blalock
Copyright © 2005 – The McGraw-Hill Companies srl