Digital System Design

1. Digital System Design
Introduction
Digital System
• A digital signal is discrete-time
as well as discrete-valued signal
• A system that works with digital
signal is digital system
• Digital doesn’t always mean
binary
Why Digital?
• Perfect
Reconstruction/Regeneration
• Error control (ARQ / FCS)
• Encryption/Decryption

2. Advantages of Digital
System
• Easy to store and retrieve digital
information
• Easy to process digital data
• Digital is less error prone
• Error can be controlled
• Noise does not accumulate from
one logic stage to next as it does
in analog system (Regeneration)
• The ease of large scale
fabrication
• Can be processed by a general
purpose processor
Analog vs. Digital

3. design
• Digital design need to handle
with only few specified (mostly
two) logic levels
– It is less important to precisely
simulate resistive,
capacitive and inductive parameters
– Design simulation can be based on
the logic only,
tools can therefore follow the truth
tables
Analog vs. Digital
design
• Analog design requires precise
voltage and current

4. characteristics of the devices
– Parasitic capacitance, resistance,
inductance are
very important – Circuit elements are
non-linear – Design verifications
require complex simulations
involving solutions of differential
equations
Integrated Circuits
Integrated Circuit (IC)
• A set of electronic circuits on one
small plate ("chip") of semiconductor
material, usually silicon
• Much smaller than a discrete circuit

5. made from independent electronic
components
• ICs can be made very compact,
having up to several billion
transistors and other electronic
components in a very small area
• The width of each conducting line in
a circuit is being smaller and smaller
as the technology advances (from
micrometer to nanometer)
• In 2008 it was dropped below 100
nanometers, and has now been

6. reduced to tens of nanometers
Why ICs
• Let a recent PC is more than
1000 times faster than that in
1982 then:
– If one small table was required to
set a PC in 1982,
now does it requires more than 1000
tables to set a PC? – If a PC in 1982
consumed one unit of power, now
does it need to supply more than
1000 unit of power?
Why ICs
• 80286 microprocessor
– released on February 1982 – with

7. 134,000 number of transistors
• Pentium Pro microprocessor –
released on November 1995 – with
5,500,000 number of transistors
Why ICs
• Electronic systems are
backbone of many systems
today, and it is a continuous
trend towards digital
electronics solutions in all
areas of our life – Car electronics

8. which includes music system, engine
and dashboard controls –
Electronics in aircraft, Washing
Machines, Computing and
Computers etc. – Communication,
Phones, entertainment, TVs,
measurements (instrumentation)
etc.
Properties of IC
• Size:
– Integrated circuits are much
smaller, both

9. transistors and wires are shrink to
micrometer (or even nanometer)
sizes within IC – Smaller size leads
to advantages in speed and
power consumption, because smaller
components have smaller parasitic
capacitances, resistances
Properties of IC
• Speed: – Signals can be switched
between logic 0 and logic1 much
quicker within a chip –
Communications within a chip can
occur hundreds of times faster than
communication between chips –

10. The high speed within chip is
possible due to their small size. The
smaller components and wires
have smaller parasitic
capacitances to slow down the
signal
Properties of IC
• Power consumption:
– Logic operations within a chip takes
much less
power – Due to small size of circuits
on the chip-smaller parasitic
capacitances and resistances

11. requires less power to drive them
Advantages of ICs
• Small physical size
• High speed
• Low power consumption
• Reduced cost
• High reliability
• Good performance
Properties of Digital
Circuits
Properties of Digital
Circuits
• The binary output signal must

12. be a prescribed function of the
binary input or inputs
• Quantization of amplitudes
within the normal range of
operating voltage is required,
which implies strong
nonlinearities in circuit operation
Properties of Digital
Circuits
• Amplitude levels are
regenerated in passing from the
input to the output of a digital
circuit
• Changes in an output level

13. should not appear at any
unchanging input of the same
circuit; there must be an explicit,
unilateral cause- effect
relationship between input(s) and
output(s)
Properties of Digital
Circuits
• The output of one circuit must
be capable of driving more than
one input of similar circuits
• The number that can be driven

14. is termed as fan-out of the
circuit
• The digital circuits must be
capable of accepting more than
one input, the number of
independent input nodes is
termed as fan-in
Modern Electronics
Development
Modern Electronics

15. Development
• The invention of transistor at Bell
Telephone Laboratories in 1947
• The introduction of bipolar transistor
by Schockley in 1949
• 1956 the first bipolar digital logic
gate by Harris
• The first truly successful IC logic
family, TTL (transistor transistor
logic) was pioneered in 1962
• Other logic families were devised
for higher performance i.e. ECL
(emitter coupled logic) – first

16. subnanosecond digital gates
Modern Electronics
Development
• TTL had the advantage of offering
higher integration density and was
the basis of first integrated circuit
revolution
• Ultimately, in bipolar digital logic,
the large power consumption per
gate puts an upper limit on the
number of gates that can be

17. reliably integrated on a single die
or package
• The attempts were made to develop
high integration density, low-power
bipolar families (such as I2L –
Integrated Injection Logic)
Modern Electronics
Development
• The torch was gradually passed
to the MOS digital integrated
circuit approach

18. • The basic principle behind the
MOSFET transistor was
proposed in a patent by J.
Lilienfeld (Canada) as early as
1925, and independently by
O. Heil in England in 1935
• Insufficient knowledge of
materials and gate stability
problems delayed the practical
usability of the device for long

19. time
Modern Electronics
Development
• The next age of the digital
integrated circuit revolution was
inaugurated with the introduction of
the first microprocessors by Intel in
1972 the 4004 and in 1974 the 8080
• MOS technology enabled the
realization of the first high-density
semiconductor memories
• The large majority of the current
integrated circuits are implemented in
the MOS technology
Moore’s Law

20. • Gordon Moore: co-founder of Intel
• In 1960, he predicted that number
of transistors per chip would grow
exponentially (double every 18
months)
• This prediction later called Moore’s
law, has proven to be amazingly
visionary
• Integration complexity doubles
approximately every 1 to 2 years, as
a result, memory density has
increased by more than a

21. thousand-fold since 1970
Evolution in
Complexity
Evolution in
Transistor Count
Evolution in
Speed/Performance
10 μm – 1971 6 μm –
1974 3 μm – 1977 1.5 μm
– 1982 1 μm – 1985 800
nm – 1989 600 nm –

22. 1994 350 nm – 1995 250
nm – 1997 180 nm –
1999
130 nm – 2001 90 nm –
2004 65 nm – 2006 45
nm – 2008 32 nm – 2010
22 nm – 2012 14 nm –
2014 10 nm – 2016–2017
7 nm – 2017–2018 5 nm
– 2020–2021
Hierarchical Design
• Divide and conquer approach

23. • Design complexity is reduced
by breaking it down into
manageable pieces
• Example: – A 32 bit ripple carry
counter design is specified as 32 flip
flops – Each flip flop is specified as
connection of several gates – Each
gate as connection of transistors
Design Abstraction
• Multiple levels of design
abstraction are given for the ICs

24. • Design abstraction goes from
the specifications to layout (or
the physical design)
Design Abstraction for
the ICs
Specifications
Behavior
RTL specs
Logic
Circuit
Layout Natural language
Complexity HDL
Finite State Machine
List of gates
Abstraction

25. Transistor level circuit
With EDA tools, design abstraction
can be high Physical realization
and lower level can be synthesized
Design Abstraction
Levels
MODULE
+
GATE
CIRCUIT
DEVICE
G
S
n+ n+
SYSTEM
D
Design Techniques
• Two contradicting issues
– increasing complexity of the
designs – reducing time for design

26. • Only possible solutions are
– increase more and more
automation – multiple levels of
abstraction for design to eliminate
unnecessary details
Electronic Design
Automation (EDA)
Electronic Design
Automation (EDA)
• Automation of the Electronic
Design
• A category of software tools for
designing electronic systems

27. i.e. printed circuit boards and
integrated circuits
• The tools work together in a
design flow that chip designers
use to design and analyze
entire semiconductor chips
• Also referred to as "Electronic
Computer- Aided Design"
(ECAD)
EDA

28. • Transistor simulation: low-level
transistor- simulation of a
schematic/layout's behavior at
device-level
• Logic simulation: digital-
simulation of an RTL or gate's
digital (Boolean 0/1) behavior, at
Boolean-level
• Behavioral Simulation: high-
level simulation of a design's
architectural operation
EDA
• Issues
– Devices: transistors, gates, flip
flops, digital building blocks, memory,

29. processors etc. – Levels: Chip,
Board, System, Concept – Simplicity:
Custom Design vs. reuse of design
• EDA focus is mainly on digital
circuit design though analog is
getting in
EDA
• With high speed designs,
various other issues are also
important – Interconnect resistance
and capacitance – RF interference,
induced currents etc.
• EDA need to take care of this

30. using various simulation
• Before EDA, integrated circuits
were designed by hand, and
manually laid out – time
consuming and difficult
EDA Examples
• EAGLE for PCB Design by
CadSoft Computer
• National Instruments
Electronics Workbench Group
– NI Multisim - schematic capture
and simulation – NI Ultiboard -

31. printed circuit board layout editor
• Advanced Design System for
high frequency and high speed
design by Keysight Technologies
Digital System and
Characteristics
Requirements
• Highly reliable
• Low power consumption
• Small size
• High speed
• Low cost and
• High performance
But there is trade-offs between requirements

32. Choices
• Logic family
– What are the permissible tradeoffs ?
• Level of integration
– How many functions?
• Programmable versus fixed
function circuits
– What is the context?
Logic families
• Various logic families are:
– Diode Logic – Diode Transistor
Logic (DTL) – Resistor Transistor
Logic (RTL) – Transistor Transistor
Logic (TTL) – Emitter coupled logic
(ECL) – Integrated Injection Logic

33. (I2L) – MOS:- NMOS, PMOS and
CMOS
Level of integration
• Small-scale integration (SSI):
no of gates less than 10
– Simple logic functions, basic storage
elements etc.
• Medium-scale integration
(MSI): 10–100 gates
– 4-bit adders, 8-to-1 multiplexers,
counters, etc.
• Large-scale integration (LSI): 100 to a
few 1000 gates
– Programmable logic array (PLA), PROM,
small processors, etc.
• Very large-scale integration (VLSI):

34. several thousand to millions of gates
– Processors, large memory array, etc.
• Ultra large-scale integration (ULSI): no of
gates > 10 million
– Advanced processors, system chips, etc.
An ideal logic element
• Operates from a single power
source
• Draws a minimum of power, ideally
zero
• The two binary output levels are 0
and vDD
Vcc

35. inputs
output
Gnd Out
VDD
VDD/2 In
contd...
• The output impedance is low, so
that large current may be
delivered to external loads
without altering the output
voltage levels
• The transition between states
at output occurs abruptly for an
input level equal to one half the
supply voltage
contd...

36. • There is no time delay between
the input transition and the
consequent output transition
• Virtually any numbers of inputs
are available; the input
impedance is high so that the
circuit imposes little loading on
the driving signal
Noise and noise margin
• Noise:-
– Any unwanted signal added to vary
voltage or
currents at logic nodes – If the
magnitude of noise is high it will cause
logic

37. errors
contd...
• Noise margin:-
– The maximum noise voltage level
that a logic
signal can tolerate without any error –
If the noise amplitude at input at any
logic is
smaller than noise margin (a specified
critical magnitude) the noise
amplitude will be attenuated – High-
level noise margin: NMH=VOH - VIH –
Low-level noise margin: NML=VIL -
VOL

38. Vcc(5 V)
Vec(5 V)
H
VOH (2.4)
High noise
margin
VIH (2.0)
VIL (0.8)
VOL (0.4)
Low noise margin
GND (0)
GND (0)
Output voltage
range
Input voltage
range
Mutual inductance with another logic node

39. --Inductive and
resistive drops
-Capacitive coupling from another node
3
Radio frequency signals from external sources
Transient
characteristics
• Rise time – Time taken by the
signal to switch from 10% to 90% of
its normal value
• Fall time – Time taken by the
signal to switch from 90% to 10% of
its normal value
• High-low and low-high

40. transition times at the output of
a gate are defined as tHL and
tLH
Transient
characteristics
• Propagation delay
– It takes certain amount of time to
propagate from
input to the output – Propagation
delay is the average transition delay
for a signal to propagate from input to
output when the binary signals
change in value – tPHL, tPLH and tPD
• tPHL:- the time required for the

41. output signal to reach 50% of
its normal value on the H-to-L
transition after the input signal
reached 50% of its normal
signal • tPLH:- the time required
for the output signal to reach
50% of its normal value on the
L-to-H transition after the input
signal reached 50% of its

42. normal signal • tPD:- It is the
mean of the above two and is
generally used for hand
calculations. And in early days
max of the two quantities was
considered
Propagation delay for
an Inverter
High-to-low (tPHL) and low-to-high (tPLH) propagation
delays are measured on output transitions
Power

43. • Power supply (VCC)
– Power required by the circuit to
operate
• Power dissipation
– Power consumed by the circuit –
Each gate can be at any of these
three states, high,
transition and low states – And the
current flowing at high stage may be
Icch,
that at transition is Icct and at low is Iccl
• In MOS
– The transient current is maximum –
Pavg=Vcc*Icct
• In TTL and older families
– Icct<<Icch and Icct <<Iccl –

44. Pavg=Vcc*(Icch + Iccl)/2
Power delay product
• Power delay product PDP is
the product of power
consumption (avg) and average
propagation delay
o PDP=Pd(avg)*tpd
Fan-in and Fan-out
• Fan-in:- number of inputs
available to a gate
• Fan-out:- number of outputs the
gate can drive
Logic Families
Logic families

45. • Various logic families are:
– Diode Logic – Resistor Transistor
Logic (RTL) – Diode Transistor Logic
(DTL) – Transistor Transistor Logic
(TTL) – Emitter coupled logic (ECL) –
Integrated Injection logic (IIL) – MOS:-
NMOS, PMOS and CMOS
contd....
• For the comparison of various
logic families the following
parameters should be
considered
– Speed (propagation delay time) –
Noise immunity – Fan in and fan out –
Power supply requirements – Power
dissipation per gate

46. contd....
– Number of functions available –
Maximum frequency at which
sequential circuits
operate – Components packing
density – Cost
Diode logic
• In the early days (by 1950)
diode logic were used
• Diodes were cheaper than
transistors
• However the diode logic were
followed by transistors to
eliminate some disadvantages
• With the advent of SSI in 1960,

47. the cost of diodes and
transistors were equalized
Diode OR logic
Diode AND logic
Simple Diode Logic
• Advantages
– Diodes were cheap ⇒ cheap Diode Logic
– Diode logic is simple
• Disadvantages
– They degrade digital signals (there is no
signal reconstruction) – The output terribly
follows the inputs – NOT gate can not be
performed
• So diode logic were replaced by
advanced logic
Resistor transistor logic
• The first form of digital ICs to

48. receive general usage
• Was invented in 1962
• Each input was associated with
one resistor and the output stage
had transistors
• If the input is low then the
transistor will be cut-off and the
output will be high
• If the input is high enough to
drive the transistor to saturation
then the output is low
RTL NOT gate
RTL NAND
RTL NOR
• They are simple and remain
inexpensive
• They do draw a significant

49. amount of current from the
power supply for each gate
• The fan-out of RTL gate is
limited by the value of the
output voltage when high
RTL characteristics
(typical)
• Supply voltage = 3.6 V
• Fan out = 5
• Propagation delay tPD = 25 ns
• Power dissipation/gate = 12

50. mW
• VOH/VOL= 1.2V/0.2V
• VIH/VIL = 0.8/0.7V
• NMH/NML = 0.4/0.5V
Diode transistor logic
• Each input is associated with
one diode
• Input stage has diode(s) and
the output has transistor(s)
• NAND and NOR functions,
which are easy to build in DTL,
are logically complete
• Any logical function can be
expressed

51. • This logic family uses lower
voltages, less power