# Chapter # 1: digital circuits

Chapter 1: Digital Systems and Binary Numbers A digital system is a system that manipulates discrete elements of information represented internally in binary form. Digital computers general purposes many scientific, industrial and commercial applications Digital systems

telephone switching exchanges digital camera electronic calculators, PDA's digital TV Signal An information variable represented by physical quantity For digital systems, the variable takes on discrete values Two level, or binary values are the most prevalent values

Binary values are represented abstractly by: digits 0 and 1 words (symbols) False (F) and True (T) words (symbols) Low (L) and High (H) and words On and Off. Binary values are represented by values or ranges of

values of physical quantities Binary Numbers Decimal number Base or radix aj a5a4a3a2a1.a1a2a3 Decimal point

Power 105 a5 104 a4 103 a3 102 a2 101 a1 100 a0 10 1 a 1 10 2 a 2 10 3 a 3 Example: 7,329 7 103 3 102 2 101 9 100 General form of base-r system an r n an 1 r n 1 a2 r 2 a1 r1 a0 a 1 r 1 a 2 r 2 a m r m Coefficient: aj = 0 to r 1

Binary Numbers Example: Base-2 number (11010.11) 2 (26.75)10 1 24 1 23 0 22 1 21 0 20 1 2 1 1 2 2 Example: Base-5 number (4021.2)5 4 53 0 52 2 51 1 50 2 5 1 (511.5)10 Example: Base-8 number (127.4)8

1 83 2 82 1 81 7 80 4 8 1 (87.5)10 Example: Base-16 number (B65 F)16 11 163 6 162 5 161 15 160 (46,687)10 Binary Numbers Example: Base-2 number (110101)2 32 16 4 1 (53)10 Special Powers of 2 210 (1024) is Kilo, denoted "K"

220 (1,048,576) is Mega, denoted "M" 230 (1,073, 741,824)is Giga, denoted "G" Powers of two Table 1.1 Arithmetic operation Arithmetic operations with numbers in base r follow the same rules as decimal numbers. Binary Arithmetic

Single Bit Addition with Carry Multiple Bit Addition Single Bit Subtraction with Borrow Multiple Bit Subtraction Multiplication BCD Addition

Binary Arithmetic Subtraction Addition Augend: 101101 Minuend: 101101

Addend: +100111 Subtrahend: 100111 Sum: Difference: 1010100 Multiplication

000110 Number-Base Conversions Name Radix Digits Binary

2 0,1 Octal 8 0,1,2,3,4,5,6,7 Decimal

10 0,1,2,3,4,5,6,7,8,9 Hexadecimal 16 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F The six letters (in addition to the 10 integers) in hexadecimal represent: 10, 11, 12, 13, 14, and 15,

respectively. Number-Base Conversions Example1.1 Convert decimal 41 to binary. The process is continued until the integer quotient becomes 0. Number-Base Conversions The arithmetic process can be manipulated more conveniently as follows: Number-Base Conversions Example 1.2

Convert decimal 153 to octal. The required base r is 8. Example1.3 Convert (0.6875)10 to binary. The process is continued until the fraction becomes 0 or until the number of digits has sufficient accuracy. Number-Base Conversions Example1.3 To convert a decimal fraction to a number expressed in base r, a similar procedure is used. However, multiplication is by r instead of 2,

and the coefficients found from the integers may range in value from 0 to r 1 instead of 0 and 1. Number-Base Conversions Example1.4 Convert (0.513)10 to octal. From Examples 1.1 and 1.3: (41.6875)10 = (101001.1011)2 From Examples 1.2 and 1.4:

(153.513)10 = (231.406517)8 Octal and Hexadecimal Numbers Numbers with different bases: Table 1.2. Octal and Hexadecimal Numbers Conversion from binary to octal can be done by positioning the binary number into groups of three digits each, starting from the binary point and proceeding to the left and to the right. (10 110 001 101 011

2 6 1 5 3

111 100 000 7 4 0

110) 2 = (26153.7406)8 6 Conversion from binary to hexadecimal is similar, except that the binary number is d ivided into groups of four digits: Conversion from octal or hexadecimal to binary is done by reversing the preceding procedure. Complements There are two types of complements for each base-r system: the radix complement and diminished radix complement.

the r's complement and the second as the (r 1)'s complement. Diminished Radix Complement Example: For binary numbers, r = 2 and r 1 = 1, so the 1's complement of N is (2n 1) N. Example: Complements Radix Complement The r's complement of an n-digit number N in base r is defined as rn N for N 0 an d as 0 for N = 0. Comparing with the (r 1) 's complement, we note that the r's compl

ement is obtained by adding 1 to the (r 1) 's complement, since rn N = [(rn 1) N] + 1. Example: Base-10 The 10's complement of 012398 is 987602 The 10's complement of 246700 is 753300 Example: Base-2 The 2's complement of 1101100 is 0010100 The 2's complement of 0110111 is 1001001 Complements Subtraction with Complements The subtraction of two n-digit unsigned numbers M N in base r can be done as follows:

Complements Example 1.5 Using 10's complement, subtract 72532 3250. Example 1.6 Using 10's complement, subtract 3250 72532 There is no end carry. Therefore, the answer is (10's complement of 30718) = 69282.

Complements Example 1.7 Given the two binary numbers X = 1010100 and Y = 1000011, perform the subtraction (a) X Y and (b) Y X by using 2's complement. There is no end carry. Therefore, the answer is Y X = (2's complement of 1101111) = 0010001. Complements Subtraction of unsigned numbers can also be done by means of the (r 1)'s complem

ent. Remember that the (r 1) 's complement is one less then the r's complement. Example 1.8 Repeat Example 1.7, but this time using 1's complement. There is no end carry, Therefore, the answer is Y X = (1's complement of 1101110) = 0010001. Signed Binary Numbers To represent negative integers, we need a notation for negative values. It is customary to represent the sign with a bit placed in the leftmost position of the

number. The convention is to make the sign bit 0 for positive and 1 for negative. Example: Table 1.3 lists all possible four-bit signed binary numbers in the three representations. Signed Binary Numbers Signed Binary Numbers Arithmetic Addition The addition of two numbers in the signed-magnitude system follows the rules of ordinary arithmetic. If the signs are the same, we add the two magnitudes and give

the sum the common sign. If the signs are different, we subtract the smaller magnitude from the larger and give the difference the sign of the larger magnitude. The addition of two signed binary numbers with negative numbers represented in signed-2's-complement form is obtained from the addition of the two numbers, including their sign bits. A carry out of the sign-bit position is discarded. Example: Binary Codes BCD Code A number with k decimal digits will

require 4k bits in BCD. Decimal 396 is represented in BCD with 12bits as 0011 1001 0110, with each group of 4 bits representing one decimal digit. A decimal number in BCD is the same as its equivalent binary number only when the number is between 0 and 9. A BCD number greater than 10 looks different from its equivalent binary number, even though both contain 1's and 0's. Moreover, the binary combinations 1010 through

1111 are not used and have no meaning in BCD. Signed Binary Numbers Arithmetic Subtraction In 2s-complement form: 1. 2. Take the 2s complement of the subtrahend (including the sign bit) and add it to the minuend (including sign bit). A carry out of sign-bit position is discarded.

( A) ( B ) ( A) ( B ) ( A) ( B ) ( A) ( B ) Example: ( 6) ( 13) (11111010 11110011) (11111010 + 00001101) 00000111 (+ 7) Binary Codes Example:

Consider decimal 185 and its corresponding value in BCD and binary: BCD Addition Binary Codes Example: Consider the addition of 184 + 576 = 760 in BCD: Decimal Arithmetic Binary Codes Other Decimal Codes

Binary Codes Gray Code Binary Codes ASCII Character Code Binary Codes ASCII Character Code ASCII Character Codes American Standard Code for Information Interchange

(Refer Table 1.7) A popular code usedtoto represent information sent as character-based data. It uses 7-bits to represent: 94 Graphic printing characters. 34 Non-printing characters

Some non-printing characters are used for text format (e.g. BS = Backspace, CR = carriage return) Other non-printing characters are used for record marking and flow control (e.g. STX and ETX start and end text areas). ASCII Properties ASCII has some interesting properties: Digits 0 to 9 span Hexadecimal values 3016 to 3916 . Upper case A - Z span 4116 to 5A16 . Lower case a - z span 6116 to 7A16 . Lower to upper case translation (and vice versa)

occurs by flipping bit 6. Delete (DEL) is all bits set, a carryover from when punched paper tape was used to store messages. Punching all holes in a row erased a mistake! Binary Codes Error-Detecting Code To detect errors in data communication and processing, an eighth bit is sometimes added to the ASCII character to indicate its parity. A parity bit is an extra bit included with a message to make the total number of 1's either even or odd. Example:

Consider the following two characters and their even and odd parity: Binary Codes Error-Detecting Code Redundancy (e.g. extra information), in the form of extra bits, can be incorporated into binary code words to detect and correct errors. A simple form of redundancy is parity, an extra bit appended onto the code word to make the number of 1s odd or even. Parity can detect all single-bit errors and some multiple-bit errors.

A code word has even parity if the number of 1s in the code word is even. A code word has odd parity if the number of 1s in the code word is odd. Binary Storage and Registers Registers A binary cell is a device that possesses two stable states and is capable of storing one of the two states. A register is a group of binary cells. A register with n cells can store any discrete qua ntity of information that contains n bits. n cells

2n possible states A binary cell two stable state store one bit of information examples: flip-flop circuits, ferrite cores, capacitor A register a group of binary cells AX in x86 CPU

Register Transfer a transfer of the information stored in one register to another one of the major operations in digital system an example Transfer of information The other major component of a digital system circuit elements to manipulate individual bits of information Binary Logic Definition of Binary Logic

Binary logic consists of binary variables and a set of logical operations. The variables are designated by letters of the alphabet, such as A, B, C, x, y, z, etc, with each variable having two and only two distinct possible values: 1 and 0, There are three basic logical operations: AND, OR, and NOT. Binary Logic The truth tables for AND, OR, and NOT are given in Table 1.8. Binary Logic Logic gates Example of binary signals

Binary Logic Logic gates Graphic Symbols and Input-Output Signals for Logic gates: Fig. 1.4 Symbols for digital logic circuits Fig. 1.5 Input-Output signals for gates Binary Logic Logic gates

Graphic Symbols and Input-Output Signals for Logic gates: Fig. 1.6 Gates with multiple inputs

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