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3-1Logic System Design I
Digital Circuits
ECGR2181Chapter 3 Notes
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Data
A-data
B-datahere
new-A
new-B
A
B
A
B
S
VCC
Z
Logic System Design I 3-2
What is a digital system?
It is a organized collection of digital elements which is designed to perform specified operations on a set of digital inputs and to generate a set of digital responses.
A digital system can be as simple as a block of combinational logic or as complex as a microprocessor.
Logic System Design I 3-3
Characteristics of Digital Systems
What are the characteristics of a digital system?
• Data processing and storage.
• Coordinate and sequence its internal operations.
• Cooperate in transferring data to & from itself.
• Sequences operations of external entities.
Logic System Design I 3-4
Overview of a digital system
Digital SystemInputs Outputs
SequencingProcessing
Storage
DataControl
DataControl
Logic System Design I 3-5
Input & Output Signals
Data:•Multi-bit: “values”
•Single-bit: decision-making / information
Control: {generally single-bit signals}
•Sequencing operations of system
•Coordinating operations with external units
Logic System Design I 3-6
Introduction to Digital Systems
Nomenclature: (Terms to know.)
Word: A group of binary bits. Typically represents some element of data. The number of bits in a word is indeterminate unless specified. [Example: “24-bit word”]
Byte: An 8-bit word.
Nibble: A 4-bit word.
Logic System Design I 3-7
Introduction to Digital Systems
Structure of digital systems: “system” vs. “module”
• For small systems which can be conveniently designedmonolithically the terms “system” and “module” may beused interchangeably.
• A digital system can be created as a monolithic structure.
• Complex systems often need to be partitioned into somenumber of subsystems -- “modules”
Logic System Design I 3-8
Introduction to Digital Systems
Single module system:
moduledata in
controldata out
control
System
Logic System Design I 3-9
Introduction to Digital Systems
Multiple module system:
module
data in
control
data out
control
System
module
module
data in
control
Logic System Design I 3-10
Examples of digital systems
• Data Selector: Route input data to one of two outputs.
• Data Converter: Inputs a 32-bit data word and outputs it as 4 bytes.
• Message Generator: Outputs a fixed message when a “start” command is received
• Communications Buffer: Receives and stores a “block”of data. When the block is complete, it resends thestored data.
• Microprocessor: “Does everything!”
Logic System Design I 3-11
A first look at the design process
1. Understand the functional specification.
2. Create a block diagram from the external viewpoint.
3. Fill in the major internal components.
4. Determine the sequence of operations which must occur within the module
Logic System Design I 3-12
Data Selector
Route input data to one of two outputs.
Specification: When a new data word arrives at the input, the module inspects the state of the most significant bit and routes the data to output A if the bit is true and to B if the bit is false. The last value sent to either output is retained until replaced.
Data
A-data
B-datahere
new-A
new-B
A
B
Logic System Design I 3-13
Data Converter
Inputs 32-bit data word and outputs it as 4 bytes.
Specification: When a new data word arrives at the input, the module accepts it and then outputs the word as 4 bytes.
IN
ready
OUT
new
REG
Logic System Design I 3-14
Message Generator
Outputs a fixed message when a “start” command is received.
Specification: When a “start” command is received, the module retrieves the bytes of a message stored in an internal ROM and outputs them sequentially.
startDOUT
newROM
Memory
Logic System Design I 3-15
Communications Buffer
Receives and stores a “block” of data. When the block is complete, it resends the data.
Specification: The module receives a series of data bytes and stores them in an internal memory. Intake of data stops when a byte of all 1’s is received. Then it resends the message with pairs of bytes packed in 16-bit words.
IN
here
OUT
newRAM
Memory
Logic System Design I 3-16
Digital Logic
Binary system -- 0 & 1, LOW & HIGH, negated and asserted.Basic building blocks -- AND, OR, NOT
(c)
X NOT X
10 1
0
NOT XX(a)
X
0
1
X AND YY
0
1
10
10
0001
X AND YXY (b)
X
0
1
X OR YY
0
1
10
10
0111
X OR YX′ X • Y X + Y
XY
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Logic System Design I 3-17
NAND and NOR
(a)
X
0
1
X NAND YY
0
1
10
10
1110
X NAND YXY (b)
X
0
1
X NOR YY
0
1
10
10
1000
X NOR Y(X • Y)′ (X + Y)′
XY
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Logic System Design I 3-18
Truth TablesXY
Z
F
X • Y
X′ • Y′ • Z
X′
Y′ X • Y + X′ • Y′ • Z
X Y Z FXY X’ Y’ X’+Y’+Z
Logic System Design I 3-19
More Practice
X Y Z F
Logic System Design I 3-20
Many representations of digital logic
Transistor-levelcircuit diagrams
A
B
S
VCC
Z
Logic System Design I 3-21
Truth tables
Logic diagrams
A
S
B
Z
SNASN
SB
Table 1 -1Truth table for the multiplexer function.
S A B Z
0 0 0 0
0 0 1 0
0 1 0 1
0 1 1 1
1 0 0 0
1 0 1 1
1 1 0 0
1 1 1 1
Logic System Design I 3-22
Logic levels
Switching threshold varies with voltage, temp, process, etc.• need “noise margin”
Operating closer to the tolerances requires an increase in attention to “analog” behavior.
Logic voltage levels decreasing with process• 5 -> 3.3 -> 2.5 -> 1.8 V
5.0 V
3.5 V
1.5 V
0.0 V
Logic 1 (HIGH)
Logic 0 (LOW)
undefinedlogic level
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Logic System Design I 3-23
MOS Transistors
VIN
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
gatedrain
source
Voltage-controlled resistance:increase Vgs ==> decrease Rds
Note: normally, Vgs ≥0� Vgs
+
−Copyright © 2000 by Prentice Hall, Inc.
Digital Design Principles and Practices, 3/e
gate drain
sourceVoltage-controlled resistance:decrease Vgs ==> decrease Rds
Note: normally, Vgs ≤ 0
Vgs+
−
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
NMOS
PMOS
Voltage-controlled resistance
Logic System Design I 3-24
CMOS Inverter
VIN
VDD = +5.0 V
VOUT
Q2(p-channel)
Q1(n-channel)
0.05.0
VIN
(L)(H)
(H)(L)
Q1
offon
Q2
onoff
5.00.0
VOUT(b)
(c)
(a)
IN OUT
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Logic System Design I 3-25
Alternate transistor symbols
Q2(p-channel)
VIN
VDD = +5.0 V
VOUT
Q1(n-channel)
on whenVIN is low
on whenVIN is high
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Logic System Design I 3-26
Switch model
VDD = +5.0 V
VOUT = HVIN = L
(a)VDD = +5.0 V
VOUT = LVIN = H
(b)
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Logic System Design I 3-27
CMOS NAND Gates
Use 2n transistors for n-input gateVDD
A
B
Z
Q1
Q3
Q2 Q4A
LLHH
B
LHLH
Q1
offoffonon
Q2
ononoffoff
Q3
offonoffon
Q4
onoffonoff
Z
HHHL
AB
Z
(a)
(b)
(c)
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Logic System Design I 3-28
CMOS NAND -- switch model
VDD
A = L
Z = H
(a)
B = L
VDD
A = H
Z = H
(b)
B = L
VDD
A = H
Z = L
(c)
B = H
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
VDD
A = L
Z = H
(a)
B = L
VDD
A = H
Z = H
(b)
B = L
VDD
A = H
Z = L
(c)
B = H
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
VDD
A = L
Z = H
(a)
B = L
VDD
A = H
Z = H
(b)
B = L
VDD
A = H
Z = L
(c)
B = H
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Logic System Design I 3-29
CMOS NAND -- more inputs (3)
A
LLLLHHHH
ABC
Z
B
LLHHLLHH
C
LHLHLHLH
Q1
offoffoffoffonononon
Q3
offoffononoffoffonon
Q5
offonoffonoffonoffon
Q6
onoffonoffonoffonoff
Q4
ononoffoffononoffoff
Q2
ononononoffoffoffoff
Z
HHHHHHHL
VDD
B
C
Z
Q3
A Q1
Q5
Q2 Q4 Q6
(a) (b)
(c)
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Logic System Design I 3-30
CMOS NOR Gates
Like NAND -- 2n transistors for n-input gate
A
LLHH
B
LHLH
Q1
offoffonon
Q2
ononoffoff
Q3
offonoffon
Q4
onoffonoff
Z
HLLL
AB
Z
VDD
A
B
Z
Q2
Q4
Q1 Q3
(a)
(b)
(c)
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Logic System Design I 3-31
NAND vs. NOR
PMOS transistors have higher “on” resistance than NMOS transistors.
VDD
A
B
Z
Q1
Q3
Q2 Q4A
LLHH
B
LHLH
Q1
offoffonon
Q2
ononoffoff
Q3
offonoffon
Q4
onoffonoff
Z
HHHL
AB
Z
(a)
(b)
(c)
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
NAND
A
LLHH
B
LHLH
Q1
offoffonon
Q2
ononoffoff
Q3
offonoffon
Q4
onoffonoff
Z
HLLL
AB
Z
VDD
A
B
Z
Q2
Q4
Q1 Q3
(a)
(b)
(c)
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
NOR
Result: NAND gates are preferred in CMOS.
Logic System Design I 3-32
(a)
(b)
(c)
tr tf
tr tf
HIGH
LOW
VIHmin
VILmax
Copyright © 2000 by Prentice Hall, Inc. Digital Design Principles and Practices, 3/e
Additional Terms• Sinking/Sourcing Current – (sect. 3.5.2, p. 106) current entering/leaving the output of
a device.
• Fanout – (sect. 3.5.4) how many gate inputs can a particular device drive and still maintain digital logic characteristics.
• Unused Inputs – (sect. 3.5.6) always connect unused inputs to either power supply rail (Vcc or Gnd.) – Static conditions – may appear to be stable,
– Dynamic conditions – could be unstable
– Makes circuit behavior unpredictable.
• ESD – Electro-Static Discharge. (sect. 3.5.7) ESD involves the discharge of static electricity and is the deadly enemy of electronic circuits, especially, CMOS devices. ESD damage can be avoided with the use of ESD straps and rubber mats.
• Transition Time – (sect. 3.6.1) time required for signal to transit the abnormal region. The time to transit the abnormal region may be different for traversing the region in different directions.
Logic System Design I 3-33
More Terms …• Propagation Delay – (sect. 3.6.2) time required for a change on the input to produce a
change on the output.
• Current Spikes – (sect. 3.6.4) typically seen on the power rails. Produced when many outputs change at the same time. Switching Power Supplies often produce these effects. {t/s note: check frequency to help find source}
• Decoupling Capacitors – (sect. 3.6.4) distributes the filtering on the board and aids in the reduction of noise on the power rails.
• Ground Bounce – (sect. 3.6.6) read the text. {describe how it looks on an o’scope}
• Three-state Outputs – (sect. 3.7.3) devices whose outputs are one of three states, high, low and high impedance. Used to drive an output from multiple, mutually exclusive sources (or devices.)
• Different types of Logic Families:– TTL – Transitor-Transitor Logic
– CMOS – Complementary Metal Oxide Semiconductor
– ECL – Emitter-Coupled Logic
• Take extreme care when interfacing TTL and CMOS logic devices