eet 1131 unit 6 exclusive-or and exclusive-nor gates
DESCRIPTION
EET 1131 Unit 6 Exclusive-OR and Exclusive-NOR Gates. Read Kleitz , Chapter 6. Do Unit 6 e-Lesson. Homework #6 and Lab #6 due next week. Quiz next week. - PowerPoint PPT PresentationTRANSCRIPT
EET 1131 Unit 6Exclusive-OR and Exclusive-NOR Gates
Read Kleitz, Chapter 6. Do Unit 6 e-Lesson. Homework #6 and Lab #6 due next
week. Quiz next week.
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
The XOR gate produces a HIGH output only when the inputs are at opposite logic levels. The truth table is
The XOR Gate
InputsA B X
Output
0 00 11 01 1
01 10
AB
X AB
X= 1
The XOR operation is written as X = AB + AB. Alternatively, it can be written with a circled plus sign between the variables as X = A + B.
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Example waveforms:
A
XNotice that the XOR gate will produce a HIGH only when exactly one input is HIGH.
The XOR Gate
B
AB
X AB
X= 1
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Some common gate configurations are shown.
Fixed Function Logic
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'00
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'04
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'08
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND' 02
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'10
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'11
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'20
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'21
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'27
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'32
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'86
14
1
8
7
9
6
10
5
11
4
12
3
13
2
VCC
GND'30
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
The XNOR gate produces a HIGH output only when the inputs are at the same logic level. The truth table is
The XNOR Gate
InputsA B X
Output
0 00 11 01 1
10 01
AB
X AB
X
The XNOR operation can be written as X = AB + AB or as
X = A + B.
= 1
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Example waveforms:
A
XNotice that the XNOR gate will produce a HIGH when both inputs are the same. This makes it useful for comparison functions.
The XNOR Gate
B
AB
X AB
X= 1
Applications of XOR and XNOR Gates
Three common applications:1. Comparators2. Controlled inverters3. Parity generation and checking
Convention for Multi-Bit Strings
When dealing with multi-bit binary strings, we use subscripts to refer to the individual bits in the string.
The least significant bit (LSB) always gets the smallest subscript, which may be either 1 or 0.
Example: In a four-bit string A, the bits may be labeled either A4A3A2A1 or A3A2A1A0
Application #1: Comparator
A comparator compares two strings of bits to see whether they are equal to each other: Example: if string A = 0101 and string B = 0100,
then A≠ B. Next slide shows how to build a 4-bit
comparator from XNOR gates.
Comparator Circuit (Book’s Fig. 6-14)
Application #2: Controlled Inverter
A controlled inverter takes an input string and, depending on the logic level on a control line, either Leaves the string unchanged or Inverts each bit in the string
Next slide shows how to build an 8-bit controlled inverter from XOR gates.
Controlled Inverter (Book’s Fig. 6-15)
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Application #3: Parity CheckingParity checking is a method of error detection for simple transmission errors involving one bit. A parity bit is an “extra” bit attached to a group of bits to force the total number of 1’s to be either even (even parity) or odd (odd parity).
The ASCII character for “a” is 1100001 and for “A” is 1000001. What is the correct bit to append to make both of these have odd parity?The ASCII “a” has an odd number of bits that are equal to 1; therefore the parity bit is 0. The ASCII “A” has an even number of bits that are equal to 1; therefore the parity bit is 1.
Parity Generators
To implement parity checking, we need circuitry on the sending end that generates the parity bit for each group of bits being sent. This circuitry is called a parity generator.
Next slide shows how to build 4-bit even or odd parity generators.
Parity Generators (Book’s Fig. 6-9)
Parity Checkers
On the receiving end, we need circuitry that checks the data bits and parity bit as they’re received to decide whether an error has occurred during transmission. This circuitry is called a parity checker.
Next slide shows how to build a 4-bit-plus-parity even parity checker.
Parity Checker (Book’s Fig. 6-11)
A Parity Generator/Checker Chip
74280 Nine-bit Parity Generator/Checker Most chips we’ve studied have been SSI
(small-scale integration) chips containing fewer than 10 gates that are not connected to each other.
The 74280 is an MSI (medium-scale integration) chip. Instead of containing a few disconnected gates, it contains about 45 gates connected internally on the chip to perform a specific function.
Parity System (Book’s Fig. 6-13)
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved
Parity Generator/Checker ChipThe 74280 can be used to generate a parity bit or to check an incoming data stream for even or odd parity. Checker: The 74280 can test codes with up to 9 bits. The even output will be HIGH if the data lines have even parity; otherwise it will be LOW. Likewise, the odd output will be HIGH if the data lines have odd parity; otherwise it will be LOW.
Generator: To generate even parity, the parity bit is taken from the odd parity output. To generate odd parity, the output is taken from the even parity output.
(5)(6)
(13)(1)
(11)(12)
(2)
(10)(9)(8)
FG
DE
H
CBA
(4)I
74280
Data inputs
S EvenS Odd
Printing from Our Oscilloscopes
You can print the oscilloscope screen by pressing the PRINT button.
There’s a delay of about 40 seconds before the page will print, so be patient.
Only one oscilloscope can print at a time, or else the printer gets confused and prints hundreds of pages.
Please shout “Printing!” before you press the PRINT button, and make sure that you don’t print while someone else is waiting for their page to print.