DigitalLogicDesignWeekNo.01

MuhammadNouman Durrani

DigitalDigital

The term digital is derived from the way computersg y pperform operations, by counting digits.

Today, digital technology is applied in a wide rangeof areas such as computers, television,communications systems, radar, navigation andy , , gguidance systems, military systems, medicalinstrumentation, industrial process control.

ANALOGQUANTITIES An analog quantity is one having continuous values.

For example, the air temperature changes over a continuous range of values.

During a given day, the temperature does not go from, say, 70F to 71 F instantaneously; it takes on all the infinite values in between.

If we graphed the temperature we would have a smooth, continuous curve.

Other examples of analog quantities are time, pressure, distance, and sound.

Digital and Analog QuantitiesDigitalandAnalogQuantities

Analog quantities haveAnalogquantitieshavecontinuousvalues

Digital quantitiesDigital quantities havehaveDigitalquantitiesDigitalquantities havehavediscretesetsofvaluesdiscretesetsofvalues

DIGITAL QUANTITIESDIGITALQUANTITIES A digital quantity is one having a discrete set of values.

Digital data can be processed and transmitted moreefficiently and reliably than analog dataefficiently and reliably than ,analog data.

Also, digital data has a great advantage when storage isnecessary For example music when converted to digitalnecessary. For example. music when converted to digitalform can be stored more compactly and reproduced withgreater accuracy and clarity than is possible when it is inanalog form.analog form.

Noise(unwantedvoltagefluctuations)doesnotaffectdigital data nearly as much as it does analog signalsdigitaldatanearlyasmuchasitdoesanalogsignals.

An Analog Electronic SystemAnAnalogElectronicSystemA public address system, used to amplify sound so that it canbe heard by a large audience is an example of analogbe heard by a large audience, is an example of analogelectronics.

Sound waves, which are analog in nature, are picked up by a, g , p p ymicrophone and converted to a small analog voltage calledthe audio signal.

This voltage varies continuously as the volume and frequencyThis voltage varies continuously as the volume and frequencyof the sound changes and is applied to the input of a linearamplifier.

The output of the amplifier, which is an increasedreproduction of input voltage, goes to the speaker(s). Thespeaker changes the amplified audio signal back to soundwaves that have a much greater volume than the originalg gsound waves picked up by the microphone.

The compact disk (CD) player is an example of a systemp ( ) p y p yin which both digital and analog circuits are used.

Music in digital form is stored on the compact disk A Music in digital form is stored on the compact disk. Alaser diode optical system picks up the digital data fromthe rotating disk and transfers it to the digital-to-analog

t (DAC)converter (DAC).

The DAC changes the digital data into an analog signalg g g gthat is an electrical reproduction of the original music.This signal is amplified and sent to the speaker.

Digital electronics involves circuits and systems inwhich there are only two possible states. These stateswhich there are only two possible states. These statesare represented by two different voltage levels: A HIGHand a LOW.

In digital systems, combinations of the two states,called codes, are used to represent numbers, symbols,alphabetic characters, and other types of information.

The two-state number system is called binary, and itstwo digits are 0 and 1. A binary digit is called a bit.

1 is represented by the higher voltage, and a 0 isrepresented by the lower voltage level.HIGH = 1 and LOW = 0HIGH 1 and LOW 0

LogicLevels:

The voltages used to represent a 1 and a 0 are calledlogic levels. Ideally, one voltage level represents a HIGHand another voltage level represents a LOW.

In a practical digital circuit, a HIGH can be any voltagebetween a specified minimum value and a specifiedmaximum value. Likewise, a LOW can be any voltagemaximum value. Likewise, a LOW can be any voltagebetween a specified minimum and a specifiedmaximum.

There can be no overlap between the accepted range ofHIGH levels and the accepted range of LOW levels.

Forexample,theHIGHvaluesforacertaintypeofdigitalcircuitcalledCMOSmayrangefrom2Vto3.3VandtheLOWvaluesmayrangefromoVto0.8V.Forexample,ifavoltageof2.5Visapplied,thecircuitwillacceptitasaHIGHorbinary 1binary1.Ifavoltageof0.5Visapplied,thecircuitwillacceptitasaLOWorbinaryO.

Forthistypeofcircuit,voltagesbetween0.8Vand2Vareunacceptable.

DigitalWaveforms Digital waveforms consist of voltage levels that are

changing back and forth between the HIGH and LOW levelsor states.

Figure l6(a) shows that a single positivegoing pulse isgenerated when the voltage (or current) goes from itsnormally LOW level to its HIGH level and then back to itsLOW level.

The negativegoing pulse in Figure 16(b) is generated whenThe negative going pulse in Figure 1 6(b) is generated whenthe voltage goes from its normally HIGH level to its LOWlevel and back to its HIGH level. A digital waveform is madeup of a series of pulses.p p

AsindicatedinFigure16,apulsehastwoedges:aleadingedgethatoccursf d l d h lfirstattimetoandatrailingedgethatoccurslastattimet1.Forapositivegoingpulse,theleadingedgeisarisingedge,andthetrailingedgeisafallingedge.g g

Waveform CharacteristicsWaveformCharacteristics MostWaveformsencounteredindigitalsystemsarecomposedof

i f l ti ll d l t i d b l ifi dseriesofpulses,sometimescalledpulsetrains,andcanbeclassifiedaseitherperiodicornonperiodic.

A periodic pulse waveform is one that repeats itself at a fixed Aperiodicpulsewaveformisonethatrepeatsitselfatafixedinterval,calledaperiod(T). The frequency (f) is the rate at which it repeats itself and is measuredThefrequency(f)istherateatwhichitrepeatsitselfandismeasured

inhertz(Hz).

A nonperiodic pulse waveform, does not repeat itself at fixedp p , pintervals and may be composed of pulses of randomly differingpulse widths and/or randomly differing time intervals between thepulses.

TheClock:Indigitalsystems,allwaveformsaresynchronizedwithabasictimingwaveformcalledtheclock.

Timing DiagramsTimingDiagrams A timing diagram is a graph of digital waveforms showing theg g g p g g

actual time relationship of two or more waveforms and howeach waveform changes in relation to the others.

By looking at a timing diagram, we can determine the states(HIGH or LOW) of all the waveforms at any specified point in( ) y p ptime and the exact time that a waveform changes staterelative to the other waveforms.

Figure 1.11 is an example of a timing diagram made up of fourwaveforms.waveforms.

Datareferstogroupsofbitsthatconveysometypeofinformation.

Binarydata,whicharerepresentedbydigitalwaveforms,mustbetransferredfromonecircuittoanotherwithinadigitalsystemorfromonesystemtoanotherinordertoaccomplishagivenpurpose.

Whenbitsaretransferredinserialformfromonepointtoanother,theyaresentonebitatatimealongasingleline,asillustratedinFigure112(a)forthecaseofacomputerto modemtransfer. Duringthetimeintervalfromtoto t"thefirstbitistransferred.

Duringthetimeintervalfromt,tot2,thesecondbitistransferred,andsoon.To transfer eight bits in series it takes eight time intervals Totransfereightbitsinseries,ittakeseighttimeintervals.

When bits are transferred in parallel form, all thee b ts a e t a s e ed pa a e o , a t ebits in a group are sent out on separate lines atthe same time. There is one line for each bit, ash (b) f h l f hshown in Figure 112(b) for the example of eightbits being transferred from a computer to aprinterprinter.

To transfer eight bits in parallel it takes one time To transfer eight bits in parallel, it takes one timeinterval compared to eight time intervals for theserial transfer.