Digital principles and computer organization in Digital components (unit II)

1. DIGITAL PRINCIPLES & COMPUTER
ORGANIZATION
DIGITAL COMPONENTS
Unit II
Submitted by
K.Lalithambiga.,Msc(cs),
Nadar Saraswathi College of Arts & Science,
Theni.

2. BasicsOf Digital Components
 INTEGRATED CICUITS
 DECODERS
 ENCODERS
 MULTIPLEXERS
 REGISTERS
 SHIFT REGISTERS
 BINARY COUNTERS
 MEMORY UNIT

3. INTEGRATED CICUITS (IC)
 An integrated circuit (IC) is a small silicon
semiconductor crystal, called a chip, containing the
electronic components for the digital gates.
 The various gates are interconnected inside the chip to
form the required circuit.
 The chip is mounted in a ceramic or plastic container,
and connections are welded by thin gold wires to external
pins to form the IC.
 The number of pins range from 14 to 100.
 Each IC has a numeric designation.

4. CATEGORIES
1. SSI(Small Scale Integration Device)
2. MSI(Medium Scale Integration Device)
3. LSI(Large Scale Integration Device)
4. VLSI(Very Large Scale Integration
Device)
5. ULSI(Ultra Large Scale Integration
Device)

5. SSI(Small Scale Integration Device)
 It contains several independent gates in a single package.
 The inputs and outputs of gates are connected directly to
the pins in the package.
 The number of gates is usually less than 10.
MSI(Medium Scale Integration Device)
 It contains 10 to 200 gates in a single package.
 They perform elementary digital functions such as
decoders, adders, registers.

6. LSI(Large Scale Integration Device)
 It contains gates between 200 to few thousand in a single
package.
 They include digital systems such as processors, memory
chips etc.
VLSI(Very Large Scale Integration Device)
 It contains thousands of gates within a single package
such as microcomputer chip.
ULSI(Ultra Large Scale Integration Device)
 It contains hundred of thousands of gates within a single
package such as microcomputer chip.

7. Digital Logic Family
 IC's are also classified by the specific circuit technology
to which they belong.
 The basic circuit in each technology
is NAND, NOR, NOT gates.
 The earliest logic family was Resistor-transistor
logic which used a resistor as input and a transistor as
switching device.
 Diode-transistor logic is a direct ancestor of the
Transistor-transistor logic, and used a diode for logic
functions while a transistor for amplifying functions.

8. TTL(Transistor Transistor Logic)
 It is the modified form of DTL(Diode Transistor Logic),
invented in 1961 by James L Buie.
 The diodes were replaced by transistor to improve the
circuit operation. It is called transistor-transistor logic
because transistor performs both the logic function and
the amplifying function.
Advantages:
 TTL circuits are fast.
 Low propagation delay.
 Power dissipation does not depend upon the frequency.

9. Two is inputs TTL NAND gate with one output

10. ECL(Emitter Coupled Logic)
 It provides highest speed digital circuits.
 It is used in systems such as supercomputers and signal
processors where high speed is required.
 ECL uses overdriven BJT(Bipolar junction Transistors)
in its circuit.
Advantages:
 ECL is the fastest logic family.
 Propagation delay is very less.
 Noise margin is low.

11. two input ECL gate

12. MOS(Metal Oxide Semiconductor)
PMOS NMOS
•MOS depends upon the flow of one type of
carriers (electrons or holes). It is basically of two
types:
A p-channel MOS is
called PMOS.
Transistors operate by
creating an inversion
layer in an n-type
transistor body.
A n-channel MOS is
called NMOS.
Transistors functions
by creating an n-
channel(inversion layer)
in a p-type transistor
body.

13.  FET(Field Effect Transistor) have 4 terminals:
Gate, Drain, Source and Substrate.
 They have four modes of operation:
 cut-off (or sub threshold),
 triode,
saturation (sometimes called active) and
velocity saturation.

14. CMOS(Complimentary MOS)
 CMOS used to construct Integrated Circuits.
 It uses both P and N channel MOS.
 It is also known as COS-MOS(Complementary-
symmetry metal oxide semiconductor)
 It uses complementary and symmetrical pairs of p-
type and n-type MOS for logic functions.
 It is used in systems which require low power
consumption.

15. Advantages:
 Noise margin is high.
 Power dissipation is low.
 Waste heat production is very less
as compared to other logic families.
 CMOS allows high density Integra
-tion of logic functions on a chip.
two input ECL gate

16. DECODERS
 A decoder is a combinational circuit that converts binary
information from n coded inputs to maximum 2n outputs.
 Commercial decoders include one or more enable (E)
inputs to control the operation of circuit.
 The decoder is enabled when E is equal to 1 and
disabled when E is equal to 0.
 Used in code converters.
 Used to Boolean functions.

17. ENCODERS
 An encoder is a digital circuit that performs the inverse
operation of a decoder.
 An encoder has 2n input lines and n output lines.
 It converts octal input to binary digits.
Types:
 Priority encoders.
 Decimal to BCD encoder.
 Octal to binary encoder.
 Hexadecimal to binary encoder.

18. MULTIPLEXERS
 A multiplexer is a combinational circuit that receives
binary information from one of the 2n input lines and
directs it to a single output line.
Advantages:
 It reduces number of wires.
 Circuit is less complex.
 Less costly.
 It is used to implement many circuits.

19. REGISTERS
 A register is a group of flip-flops with each flip-flop
capable of storing one bit of information.
 The register is mainly used for storing and shifting binary
data entered into it from an external sources.
 The register is a type of sequential circuit and an
important building block used in digital system like
multipliers, dividers, memories, microprocessors, etc.,

20. 4-bit register

21. SHIFT REGISTERS
 A shift register is capable of shifting its binary information
either to the right or left is called shift register.
 The shift register permits the stored data to move from a
particular location to another location within the register.
 There is two method shifting the data
 Serial shifting .
 Parallel shifting.
 The serial shifting method shifts one bit at a time for each
clock pulse in a serial fashion, beginning with either MSB or
LSB.
 The parallel shifting operation all the data (I/O)get shifted
simultaneously during a single clock pulse.

22. Serial Input Serial Output(siso)
 Let all the flip-flop be initially in the reset condition i.e. Q3 = Q2 =
Q1 = Q0 = 0.
 If an entry of a four bit binary number 1 1 1 1 is made into the
register, this number should be applied to Din bit with the LSB bit
applied first. The D input of FF-3 i.e. D3 is connected to serial data
input Din.
 Output of FF-3 i.e. Q3 is connected to the input of the next flip-flop
i.e. D2 and so on.

23. Serial Input Parallel Output(sipo)
 The data is entered serially and taken out in parallel fashion.
 Data is loaded bit by bit. The outputs are disabled as long as the
data is loading.
 As soon as the data loading gets completed, all the flip-flops
contain their required data.
 The outputs are enabled so that all the loaded data is made
available over all the output lines at the same time.
 4 clock cycles are required to load a four bit word.
 Hence the speed of operation of SIPO mode is same as that of
SISO mode.

24. Parallel Input Serial Output (PISO)
 Data bits are entered in parallel fashion.
 The circuit shown below is a four bit parallel input serial output
register.
 Output of previous Flip Flop is connected to the input of the next one
via a combinational circuit.
 The binary input word B0, B1, B2, B3 is applied though the same
combinational circuit.
 There are two modes in which this circuit can work namely - shift
mode or load mode.

25. Parallel Input Parallel Output (PIPO)
 In this mode, the 4 bit binary input B0, B1, B2, B3 is applied to
the data inputs D0, D1, D2, D3 respectively of the four flip-flops.
 As soon as a negative clock edge is applied, the input binary bits
will be loaded into the flip-flops simultaneously.
 The loaded bits will appear simultaneously to the output side.
Only clock pulse is essential to load all the bits.

26. counters
 Binary counter- A counter that follows the binary number
sequence is called a binary counter.
Counters are available in two categories
 Ripple Counters-A register that goes through a prescribed
sequence of states upon the application of input pulse is called a
counter.
 Synchronous counters- In this clock pulses are applied to the
inputs of all flip-flops.
 A common clock triggers all flip-flops simultaneously rather than
one at a time in succession as in a ripple counter.
 BCD Ripple Counter-A decimal counter follows a sequence of
ten states and returns to 0 after the count of 9.
 Up-Down Binary Counter-The two operations can be combined
in one circuit to form a counter capable of counting up or down.
 It has an up control input and down control input.

27.  Binary Counter with Parallel Load-A counter with parallel load
can be used to generate any desire count sequence.
 Is used to generate the BCD count. up control input and down
control input.
 Ring Counter- A ring counter is a circular shift register with only
one flip-flop being set at any particular time, all others are
cleared.
 The single bit is shifted from one flip-flop to the next to produce
the sequence
 Johnson Counter-Generate the timing signals with a combination
of a shift register and a decoder, which is called a Johnson
counter.
 The number of states can be double if the shift register is connect
as a switch-tail ring counter of timing signals.
 HDL for Registers and Counters-Registers and counters can be
describe in HDL at either the behavioral or the structural level.

28. MEMORY UNIT
 A memory unit is a collection of storage cells together with
associated circuits to transfer information in and out of storage .
 The memory stores binary data in groups of bits called words.
 A word can represent an instruction code or alphanumeric
characters.
 Each word in memory is assigned an address from 0 to 2k –1,
where k is the number of address lines.
 A decoder inside the memory accepts an address opens the
paths needed to select the bits of the specified word.
 The memory capacity is stated as the total number of bytes that
can be stored.

29.  In random-access memory (RAM) the memory cells can
be accessed for information from any desired random
location.
 The process of locating a word in memory is the same and
requires an equal amount of time no matter where the cells
are located physically in memory.
 Communication between memory and its environment is
achieved via data input and output lines, address selections
lines, and control lines.

30.  The n data input lines provide the information to be
stored in memory.
 The n data output lines supply the information
coming out of memory.
 The k address lines provide a binary number of k
bits that specify a specific word or location .
 The two control lines specify the direction of
transfer – either read or write.

31. Block diagram of random access memory(RAM)

32. Steps to write to memory:
Apply the binary
address of the desired
word into the address
lines.
Apply the data bits that
are to be stored in
memory on the data
lines.
Activate the write input.
Steps to read from memory:
Apply the binary address of
the desired word into the
address lines.
 Activate the read input
A read-only memory (ROM)
is a memory unit that
performs the read operation
only – there is no write
capability .
The binary information
stored in a ROM is permanent
during the hardware
production.

33.  RAM is a general-purpose device whose contents can be
altered.
 The information in ROM forms the required
interconnection pattern.
 ROMs come with special internal electronic fuses that
can be programmed for a specific configuration.
 An m x n ROM is an array of binary cells organized into
m words of n bits each.

34.  A ROM has k address lines to select one of m words
in memory and n output lines, one for each bit of the
word.
 May have one or more enable inputs for expansion.
 The outputs are a function of only the present input
(the address), so it is a combinational circuit
constructed of decoders and OR gates.

35. Block diagram of read only memory (ROM)

36.  When used as a memory unit, it stores fixed programs
that are not to be altered and for tables of constants that
will not change .
 When used in the design of control units for digital
computers, it stores coded information that represents the
sequence of internal control variables to enable the
various operations .
 A control unit that utilizes a ROM is called a micro
programmed control unit .

37. The required paths may be programmed in three different
ways :
 Mask programming is done by the semiconductor
company based upon a truth table provided by the
manufacturer.
 Programmable read-only memory (PROM) is more
economical. PROM units contain all fuses intact and are
blown by users.
 Erasable PROM (EPROM) can be altered using a
special ultraviolet light .
 Electrical erasable PROM (EEPROM) can be erased
with electrical signals .