engr 303 introduction to logic design lecture 2 · 2020. 8. 13. · engr 303 –introduction to...
TRANSCRIPT
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ENGR 303 – Introduction to Logic Design Lecture 2
Dr. Chuck BrownEngineering and Computer Information Science
Folsom Lake College
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• Binary Arithmetic
• Two’s Complement
• Logic Levels
• CMOS Transistors
• Intro Logic Circuits
Outline for Todays Lecture
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37345168+
10110011+
• Decimal
• Binary
Addition
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37345168+
8902
carries 11
10110011+
1110
11 carries
• Decimal
• Binary
Addition
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10010101+
10110110+
• Add the following
4-bit binary
numbers
• Add the following
4-bit binary
numbers
Binary Addition Examples
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10010101+
1110
1
10110110+
10001
111
• Add the following
4-bit binary
numbers
• Add the following
4-bit binary
numbers
Overflow!
Binary Addition Examples
ENGR 303
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• Digital systems operate on a fixed number of
bits
• Overflow: when result is too big to fit in the
available number of bits
• See previous example of 11 + 6
Overflow
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• Sign/Magnitude Numbers
• Two’s Complement Numbers
Signed Binary Numbers
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• 1 sign bit, N-1 magnitude bits
• Sign bit is the most significant (left-most) bit
– Positive number: sign bit = 0
– Negative number: sign bit = 1
• Example, 4-bit sign/mag representations of ± 6:+6 =
- 6 =
• Range of an N-bit sign/magnitude number:
1
1 2 2 1 0
2
0
: , , , ,
( 1) 2n
N N
na i
i
i
A a a a a a
A a
L
Sign/Magnitude Numbers
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• 1 sign bit, N-1 magnitude bits
• Sign bit is the most significant (left-most) bit
– Positive number: sign bit = 0
– Negative number: sign bit = 1
• Example, 4-bit sign/mag representations of ± 6:+6 = 0110
- 6 = 1110
• Range of an N-bit sign/magnitude number:[-(2N-1-1), 2N-1-1]
1
1 2 2 1 0
2
0
: , , , ,
( 1) 2n
N N
na i
i
i
A a a a a a
A a
L
Sign/Magnitude Numbers agnitude Numbers
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• Problems:
– Addition doesn’t work, for example -6 + 6:
1110
+ 0110
10100 (wrong!)
– Two representations of 0 (± 0):
1000
0000
Sign/Magnitude Numbers
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• Don’t have same problems as sign/magnitude
numbers:– Addition works
– Single representation for 0
Two’s Complement Numbers
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2
1
1
0
2 2n
n i
n i
i
A a a
• Msb has value of -2N-1
• Most positive 4-bit number:
• Most negative 4-bit number:
• The most significant bit still indicates the sign
(1 = negative, 0 = positive)
• Range of an N-bit two’s comp number:
Two’s Complement Numbers
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2
1
1
0
2 2n
n i
n i
i
A a a
• Msb has value of -2N-1
• Most positive 4-bit number: 0111 (+7)
• Most negative 4-bit number: 1000 (-8)
• The most significant bit still indicates the sign
(1 = negative, 0 = positive)
• Range of an N-bit two’s comp number:
[-(2N-1), 2N-1-1]
Two’s Complement Numbers
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-8
1000 1001
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1010 1011 1100 1101 1110 1111 0000 0001 0010 0011 0100 0101 0110 0111 Two's Complement
10001001101010111100110111101111
00000001 0010 0011 0100 0101 0110 0111
1000 1001 1010 1011 1100 1101 1110 11110000 0001 0010 0011 0100 0101 0110 0111
Sign/Magnitude
Unsigned
Number System Range
Unsigned [0, 2N-1]
Sign/Magnitude [-(2N-1-1), 2N-1-1]
Two’s Complement [-2N-1, 2N-1-1]
For example, 4-bit representation:
Number System Comparison
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• Flip the sign of a two’s complement number
• Method:1. Invert the bits
2. Add 1
• Example: Flip the sign of 310 = 00112
“Taking the Two’s Complement”
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• Flip the sign of a two’s complement number
• Method:1. Invert the bits
2. Add 1
• Example: Flip the sign of 310 = 00112
1. 1100
2. + 1
1101 = -310
“Taking the Two’s Complement”
ENGR 303
Alternative Two’s Complement Method: starting with the LSB write the same digits until you reach the first 1, keep the 1 and invert all other bits to its left
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• Take the two’s complement of 610 = 01102
• What is the decimal value of 10012?
Two’s Complement Examples
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• Take the two’s complement of 610 = 01102
1. 1001
2. + 1
10102 = -610
• What is the decimal value of the two’s
complement number 10012?1. 0110
2. + 1
01112 = 710, so 10012 = -710
Two’s Complement Examples
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+01101010
+11100011
• Add 6 + (-6) using two’s complement
numbers
• Add -2 + 3 using two’s complement numbers
Two’s Complement Addition
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+01101010
10000
111
+11100011
10001
111
• Add 6 + (-6) using two’s complement
numbers
• Add -2 + 3 using two’s complement numbers
Two’s Complement Addition
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• Discrete voltages represent 1 and 0
• For example:
0 = ground (GND) or 0 volts
1 = VCC or 5 volts [TTL: VCC CMOS: VDD ]
• What about 4.99 volts? Is that a 0 or a 1?
• What about 3.2 volts?
Logic Levels
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• Range of voltages for 1 and 0
• Different ranges for inputs and outputs to allow for noise
Logic Levels
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• Anything that degrades the signal
– E.g., resistance, power supply noise, coupling to neighboring wires, etc.
• Example: a gate (driver) outputs 5 V but, because of resistance in a long wire, receiver gets 4.5 V
Driver ReceiverNoise
5 V 4.5 V
What is Noise?
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• With logically valid inputs, every circuit element must produce logically valid outputs
• Use limited ranges of voltages to represent discrete values
The Static Discipline
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Driver Receiver
Forbidden
Zone
NML
NMH
Input CharacteristicsOutput Characteristics
VO H
VDD
VO L
GND
VIH
VIL
Logic High
Input Range
Logic Low
Input Range
Logic High
Output Range
Logic Low
Output Range
Logic Levels
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Driver Receiver
Forbidden
Zone
NML
NMH
Input CharacteristicsOutput Characteristics
VO H
VDD
VO L
GND
VIH
VIL
Logic High
Input Range
Logic Low
Input Range
Logic High
Output Range
Logic Low
Output Range
NMH = VOH – VIH
NML = VIL – VOL
Noise Margins
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VDD
V(A)
V(Y)
VOH
VDD
VOL
VIL, V
IH
0
A Y
VDD
V(A)
V(Y)
VOH
VDD
VOL
VIL
VIH
Unity Gain
Points
Slope = 1
0V
DD / 2
Ideal Buffer: Real Buffer:
NMH = NML = VDD/2 NMH , NML < VDD/2
DC Transfer Characteristics
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Forbidden
Zone
NML
NMH
Input CharacteristicsOutput CharacteristicsV
DD
VO L
GND
VIH
VIL
VO H
A Y
VDD
V(A)
V(Y)
VOH
VDD
VOL
VIL
VIH
Unity Gain
Points
Slope = 1
0
DC Transfer Characteristics
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Logic Family VCC or VDD VIL VIH VOL VOH
TTL 5 (4.75 - 5.25) 0.8 2.0 0.4 2.4
CMOS 5 (4.5 - 6) 1.35 3.15 0.33 3.84
LVTTL 3.3 (3 - 3.6) 0.8 2.0 0.4 2.4
LVCMOS 3.3 (3 - 3.6) 0.9 1.8 0.36 2.7
Logic Family Examples
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g
s
d
g = 0
s
d
g = 1
s
d
OFF ON
• Logic gates built from transistors
• 3-ported voltage-controlled switch– 2 ports connected depending on voltage of 3rd
– d and s are connected (ON) when g is 1
Transistors
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Silicon Lattice
Si SiSi
Si SiSi
Si SiSi
As SiSi
Si SiSi
Si SiSi
B SiSi
Si SiSi
Si SiSi
-
+
+
-
Free electron Free hole
n-Type p-Type
• Transistors built from silicon, a semiconductor
• Pure silicon is a poor conductor (no free charges)
• Doped silicon is a good conductor (free charges)
– n-type (free negative charges, electrons)
– p-type (free positive charges, holes)
Silicon
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n
p
gatesource drain
substrate
SiO2
nMOS
Polysilicon
n
gate
source drain
• Metal oxide silicon (MOS) transistors:
– Polysilicon (used to be metal) gate
– Oxide (silicon dioxide) insulator
– Doped silicon
MOS Transistors
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n
p
gate
source drain
substrate
n n
p
gatesource drain
substrate
n
GND
GND
VDD
GND
+++++++
- - - - - - -
channel
Gate = 0
OFF (no connection
between source and
drain)
Gate = 1
ON (channel between
source and drain)
Transistors: nMOS
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• pMOS transistor is opposite– ON when Gate = 0
– OFF when Gate = 1
SiO2
n
gatesource drain
Polysilicon
p p
gate
source drain
substrate
Transistors: pMOS
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g
s
d
g = 0
s
d
g = 1
s
d
g
d
s
d
s
d
s
nMOS
pMOS
OFFON
ONOFF
Transistor Function
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• nMOS: pass good 0’s, so connect source to GND
• pMOS: pass good 1’s, so connect source to VDD
pMOS
pull-up
network
output
inputs
nMOS
pull-down
network
Transistor Function
ENGR 303
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VDD
A Y
GND
N1
P1
NOT
Y = A
A Y0 1
1 0
A Y
A P1 N1 Y
0
1
CMOS Gates: NOT Gate
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VDD
A Y
GND
N1
P1
NOT
Y = A
A Y0 1
1 0
A Y
A P1 N1 Y
0 ON OFF 1
1 OFF ON 0
CMOS Gates: NOT Gate
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A
B
Y
N2
N1
P2 P1
NAND
Y = AB
A B Y0 0 1
0 1 1
1 0 1
1 1 0
AB
Y
A B P1 P2 N1 N2 Y
0 0
0 1
1 0
1 1
CMOS Gates: NAND Gate
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A
B
Y
N2
N1
P2 P1
NAND
Y = AB
A B Y0 0 1
0 1 1
1 0 1
1 1 0
AB
Y
A B P1 P2 N1 N2 Y
0 0 ON ON OFF OFF 1
0 1 ON OFF OFF ON 1
1 0 OFF ON ON OFF 1
1 1 OFF OFF ON ON 0
CMOS Gates: NAND Gate
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pMOS
pull-up
network
output
inputs
nMOS
pull-down
network
CMOS Gate Structure
ENGR 303
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How do you build a three-input NOR gate?
NOR Gate
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B
CY
A
NOR3 Gate
ENGR 303
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A logic circuit is composed of:
• Inputs
• Outputs
• Functional specification
• Timing specification
inputs outputsfunctional spec
timing spec
Introduction
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• Nodes– Inputs: A, B, C
– Outputs: Y, Z
– Internal: n1
• Circuit elements– E1, E2, E3
– Each a circuit
A E1
E2
E3B
C
n1
Y
Z
Circuits
ENGR 303
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• Combinational Logic
– Memoryless
– Outputs determined by current values of inputs
• Sequential Logic
– Has memory
– Outputs determined by previous and current values of
inputs
inputs outputsfunctional spec
timing spec
Types of Logic Circuits
ENGR 303