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Computer Science & Engineering – 15CS31T

Digital logic and data presentation

Introduction • Generally output from most of sensors are in analog form, when

microprocessor are used as a part of control system this analog

signal has to be converted to digital form before it is supplied to

microprocessor.

• Similarly most of the actuators operates with analog inputs so

digital output from microprocessor has to be converted to an

Analog form before it is supplied to actuator


Binary coded decimal system ( BCD)

This system widely used in computer and electronics device. each

decimal digit is coded with separate binary form i.e. 0 and 1

For example the decimal number 15 in BCD system becomes

00010101

This code is useful for outputs from microprocessor based systems

where the output has to derive decimal displays

Each decimal digit in the display being supplied by the microprocessor

with its own binary code


Relationship between

decimal, binary and

BCD system

Some examples of binary to decimal

& decimal to binary


Analog and digital signals

• Analog signals: An analog signal is any continuous signal for

which the time-varying feature of the signal is a representation of

some other time-varying quantity, i.e., analogous to another time-

varying signal. For example, in an analog audio signal, the

instantaneous voltage of the signal varies continuously with

the pressure of the sound waves.

• The term analog usually refers to electrical signals

however,mechanical,pneumatic,hydralic,human speech also

considered as analog signals

• Any information may be conveyed by analog signals often such

signals is a measured response to change in physical phenomena,

such as sound,light,temprature,position,pressure

• These variables converted to analog signals by transducer

https://en.wikipedia.org/wiki/Continuous_signal

https://en.wikipedia.org/wiki/Audio_signal

https://en.wikipedia.org/wiki/Voltage

https://en.wikipedia.org/wiki/Sound_pressure


Digital signal

• A Digital signal is a signal that is being used to represent data as a

sequence of discrete values; at any given time it can only take on, at

most, one of a finite number of values.


Difference between Analog and Digital signals

Applications of Digital signals

PCs, PDAs

Mobile phone


Analog to Digital conversion (A D C)

1. Supply the regular time signal pulses produced by the clock to the ADC

2. Evert time ADC receive a pulse ,it samples the analog signal

3. the results of sampling is a series of narrow pulses

4. A sample and hold unit is then used to hold each sampled value until the next pulse occurs

5. Converter requires finite amount of time ,termed as conversion time to convert A to D


Sampled and held input and output for ADC


• A successive approximation ADC is a type of ADC converter that

coverts a continues waveform in to a discrete digital

representation via a binary search through all possible

quantization level before finally converging upon digital output

for each conversion


SAR It consists following 4 sub circuits

1. A sample and hold circuit to acquire input voltage

2. An analog voltage comparator that compares Vin to the output of the internal DAC

and outputs the results of comparison to SAR

3 A SAR sub circuit designed to supply an approximate digital code

4. An internal reference DAC that ,for comparison with Vref ,supplies the comparator

with an analog voltage equal to digital code output of the SAR input

SAR ADC requires

1.An input voltage Vin

2.A reference voltage Vref

3.A DAC to convert ith approximation xi to voltage

4. A comparator to perform the function s(xi-x)by comparing DAC’s voltage with i/p

voltage

5 A register to store o/p of comparator and apply xi-1-s(xi-1-x)2i

Initial approximation x0 = 0 , ith approximation xi =s(xi-1-x)/2i


Digital to analog conversion (DAC)

• For DAC an input is a binary word ,the output is an analog signal

that represent weighted sum of the non zero bits represented by

word

• Each additional increase in bit increases output voltage by 1V

Analog output in volts


Weighted resistor DAC

• It uses summing amplifier to form weighted sum of all the non zero bits in the

input word

• The reference voltage connected to resistors by means of electronic switches

which respond to binary 1

• The value of input resistance depends on which bit in the word a switch is

responding to the values of resistor for successive bits from LSB being halved

• Sum of voltages is a weighted sum of the digits in the word

• Accurate resistance have to be used for each resistor

• Limited to 4 bit conversion


• Only two values required 0 and 1


Logic gates

• Logic gates are circuits with one or more input

with only one output


Output of an exclusive-or gate

Only goes “HIGH” when its two

input terminals are at “Different”

logic levels with respect to each

other

Then this type of gate gives

and output “1” when its

BOTH inputs are “logically

equal” or “equivalent


Application of logic gates

• 1 parity generators

• 2 digital comparator

• 3 encoder for traffic signal

• 4 code converter

• 5 electronic switches

• 6 robotics

• 7 timer and control units

• 8 flip flops

• 9 printer ic

• 10 speed control units


Coder (digital coded signal to set traffic lights)

Consists of 3 and gates and two not gates

*These gates are so arranged so that code determines which light red,

green or amber that one will turned on

*For getting red light we should use transmitted signal as a=0 and b=0

*For getting amber light we should use transmitted signal as a=0 & b=1

*For getting red light we should use transmitted signal as a=1 and b=0

Traffic lights


Decoder with 7 segment display


When HIGH appears on one of decimal digit

input lines the appropriate level occurs on the 4

BCD output lines for eg input line 9 is HIGH

On output A0 and A3 and lows on output

a1and A2

A3=8+9

A2=4+5+6+7

A1=2+3+6+7

A0=1+3+5+7+9


Sequential logic system

Sometimes system requires an output which depends on earlier

input values such system known as sequential logic system

Main difference b/w a combination logic system and sequential

logic system is SLS have some form of memory

Combination logic gates receive signal from external input and

from memory and produces output after processing them

Output is a function of both external i/p and i/p from memory


Flip flop

The flip flop is a basic memory elements and it is made up of

assembly of logic gates 3

There are 3 number of flip flops

1.SR flip flop

2 JK flip flop

3 D flip flop


SR flip flop

Consists of NOR gates

Initially we have s=0 and R=0 the output will be in previous state know

as No change condition

If we set s=1 and R=0 the output from NOR gate 2 will become 0

This will then results in both input to NOT gate 1 becomes 0 and so its

output becomes 1

This feed back acts as an input to NOR gate 2 which then has both its

input as 1 and results in no further change

Now if S changes from 1 to 0 the output from NOR gate 2 remains 0

Therefore there is no further change in output when S changes from 1to 0

S R Q Q

0 0 NO CHANGE

0 1 0 1

1 0 1 0

1 1 NOT ALLOWED

Mechanical Engineering – 15ME54T

Sequential logic system

Truth table

In this case JK and output Q Qbar are same as SRQ Qbar in SR flip

flop only additional is there is a CLK(clocked input) input is given with

negative triggering( )

• When both J & K ar 0 ,q and Qbar retains their previous state i.e. Qn

• When j=0 and k=1 with clocked input resets flip flop with Q=0 and

Qbar=1

• When J =1 and K=0 with clocked input sets flip flop with Q=1 and

Qbar=0

• When both j and k equals to 1 flip flop will toggle with clocked input

i.e. as per clocked input there is continuously Q=1 and Qbar=0 and

Q=0 and Qbar=1 Q=1 and Qbar=0 and so on


D flip flop

D flip flop is basically a clocked SR or JK flip flop with D input being

connected directly S or J input via NOT gate to the Ror K input

D flip flop needs only one data input

When D=0 then Q=0 and Qbar =1

When D=1 with clock pulse then Q=1 and Qbar =0


Register

A Register is a set of memory elements which is made up of

group of flip flops which store the information until it is needed

Each flip flop stores binary signal i.e. 0 and 1

Fig shows 4 bit register consists of d flip flop

When the loaded signal is 0 no clock input occurs to the D flip

flops and so no change occurs to the states of flip flop

When the load signal is 1 then input can change the states of

flip flop

As long as load signal is 0 flip flop will hold their old states

values

2ways of of input the data in to a register

Similarly 2 way to transfer data out of register

1. Serial in serial out (SISO)

2. Serial in parallel out (SIPO)

3. Parallel in serial out (PISO)

4. Parallel in parallel out(PIPO)


Display

• Electronic device used for visual representation of data or images

• Many display system use light indicator s to indicate on/off status or

give alphanumeric displays

Seven segment display

Light emitting diodes (LED) • The led require low voltage and low currents and are cheaper

hence has widely used in display

• commonly used LED materials are gallium arsenide, gallium

phosphide and alloys of gallium arsenide.

• Most commonly used LED can give RED ,Yellow and Green

colors

Decoder with seven segment display Most often seven-segment displays are used to display the digits in digital watches, calculators, clocks, measuring instruments and digital counters, etc. ... A display decoder is used to convert a BCD or a binary code into a 7 segment code. It generally has 4 input lines and 7 output lines. A BCD to Seven Segment decoder is a combinational logic circuit that accepts a decimal digit in BCD (input) and generates appropriate outputs for the segments to display the input decimal digit.

Liquid crystal display (LCD)

LCD do not produce light by own but works on polarization Consist of liquid crystal which is sandwiched between two sheets of polymer When plane polarized light is incident on the liquid crystal material its plane of polarization is rotated as it passes through material The rotation allows the light to be transmitted and so the material appears light. How ever if an electric field is applied across material ,the molecules become aligned with field and light passing through the top polarizer is not rotated and cannot pass through lower polarizer but become absorbed the material then appears dark


Data presentation system

Data presentation system generally classified as two groups

1.Indicators :give instant visual indication of sensed variable

2.Records:record the output signal over the period of time.

Data presentation elements • Analogue and digital meter

• Analogue chart recorder

• Cathode ray oscilloscope

• Visual display unit

• Printers

• Magnetic recorders

• Optical recorder

• Display LED & LCD

Cathode ray Oscilloscope It is voltage measuring device capable of displaying extremely high frequency signal up to 10MHz visual displays unit An electronic visual display, informally a screen, is a display device for presentation of images, text, or video transmitted electronically, without producing a permanent record. Electronic visual displays include television sets, computer monitors, and digital signage.


Printers

Data presenting element record permanently on paper

• Types of printer

• Dot matrix

• Ink jet

• Laser

• Daisy wheel

• Line

• plotter

Dot matrix

Has photosensitive drum coated with selenium based light sensitive material Able to produce 1200 dots per inch

Data acquisition system (DAQ)

Selection criteria for DAQ

1.Type of computer software used i.e. windows, MacOs

2.Type of connector board to be plugged i.e PCMCIA for

laptop NuBus for MacOS

3.Analog input and its range

4.Digital input

5.Resolution

6.Minimum sampling rate

7.Timing or counting signal required

Elements of DAQ

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