key board interfacing

8/7/2019 Key Board Interfacing

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Key Board Interfacing:

Keyboards are organized in a matrix of rows and columnsThe CPU accesses both rows and columns through portsTherefore, with two 8-bit ports, an 8 x 8 matrix of keys can be connected to a microprocessorWhen a key is pressed, a row and a column make a contact

Otherwise, there is no connection between rows and columnsIn IBM PC keyboards, a single microcontroller takes care of hardware and software interfacing

A 4x4 matrix connected to two ports

The rows are connected to an output port and the columns are connected to an input port

It is the function of the microcontroller to scan the keyboard continuously to detect and identify the key pressed

To detect a pressed key, the microcontroller grounds all rows by providing 0 to the output latch, then it reads thecolumns

If the data read from columns is D3 D0 =1111, no key has been pressed and the process continues till key pressis detected

If one of the column bits has a zero, this means that a key press has occurredFor example, if D3 D0 = 1101, this means that a key in the D1 column has been pressedAfter detecting a key press, microcontroller will go through the process of identifying the key

Starting with the top row, the microcontroller grounds it by providing a low to row D0 only

It reads the columns, if the data read is all 1s, no key in that row is activated and the process is moved to the nextrow

It grounds the next row, reads the columns, and checks for any zeroThis process continues until the row is identifiedAfter identification of the row in which the key has been pressedFind out which column the pressed key belongs to

Program 12-4 for detection and identification of key activation goes through the following stages:

1. To make sure that the preceding key has been released, 0s are output to all rows at once, and the columns are read

and checked repeatedly until all the columns are high

When all columns are found to be high, the program waits for a short amount of time before it goes to the next

stage of waiting for a key to be pressed2. To see if any key is pressed, the columns are scanned over and over in an infinite loop until one of them has a 0

on it

Remember that the output latches connected to rows still have their initial zeros (provided in stage 1), makingthem grounded

After the key press detection, it waits 20 ms for the bounce and then scans the columns again(a) it ensures that the first key press detection was not an erroneous one due a spike noise

(b) the key press. If after the 20-ms delay the key is still pressed, it goes back into the loop to detect a real key press


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3. To detect which row key press belongs to, it grounds one row at a time, reading the columns each time

If it finds that all columns are high, this means that the key press cannot belong to that row Therefore, it groundsthe next row and continues until it finds the row the key press belongs to

Upon finding the row that the key press belongs to, it sets up the starting address for the look-up table holding thescan codes (or ASCII) for that row4. To identify the key press, it rotates the column bits, one bit at a time, into the carry flag and checks to see if it is

low

Upon finding the zero, it pulls out the ASCII code for that key from the look-up tableotherwise, it increments the pointer to point to the next element of the look-up table

Program 12-4: Keyboard Program;keyboard subroutine. This program sends the ASCII;code for pressed key to P0.1 ;P1.0-P1.3 connected to rows,

P2.0-P2.3 to column

MOV P2,#0FFH ; make P2 an input port

K1: MOV P1,#0 ; ground all rows at onceMOV A,P2 ; read all col

;(ensure keys open)

ANL A,00001111B ; masked unused bits

CJNE A,#00001111B,K1 ; till all keys release

K2: ACALL DELAY ; call 20 msec delayMOV A,P2 ; see if any key is pressed

ANL A,00001111B ; mask unused bits

CJNE A,#00001111B,OVER; key pressed, find row

SJMP K2 ; check till key pressed

OVER: ACALL DELAY ; wait 20 msec debounce time

MOV A,P2 ; check key closureANL A,00001111B ; mask unused bits

CJNE A,#00001111B,OVER1; key pressed, find row

SJMP K2 ; if none, keep polling

OVER1: MOV P1, #11111110B ; ground row 0

MOV A,P2 ; read all columnsANL A,#00001111B ; mask unused bits

CJNE A,#00001111B,ROW_0 ; key row 0, find col.

MOV P1,#11111101B ; ground row 1

MOV A,P2 ; read all columns

ANL A,#00001111B ; mask unused bits


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MOV P1,#11111011B ; ground row 2

MOV A,P2 ; read all columns

ANL A,#00001111B ; mask unused bits

CJNE A,#00001111B,ROW_2 ; key row 2, find col.MOV P1,#11110111B ; ground row 3MOV A,P2 ; read all columnsANL A,#00001111B ; mask unused bits


LJMP K2 ; if none, false input,

;repeat

ROW_0: MOV DPTR,#KCODE0 ; set DPTR=start of row 0

SJMP FIND ; find col. Key belongs to

ROW_1: MOV DPTR,#KCODE1 ; set DPTR=start of row

SJMP FIND ; find col. Key belongs to

ROW_2: MOV DPTR,#KCODE2 ; set DPTR=start of row 2SJMP FIND ; find col. Key belongs to

ROW_3: MOV DPTR,#KCODE3 ; set DPTR=start of row 3FIND: RRC A ; see if any CY bit lowJNC MATCH ; if zero, get ASCII code

INC DPTR; point to next col. addrSJMP FIND ; keep searching

MATCH: CLR A ; set A=0 (match is found)

MOVC A,@A+DPTR; get ASCII from table

MOV P0,A ; display pressed key

LJMP K1

;ASCII LOOK-UP TABLE FOR EACH ROW

ORG 300HKCODE0: DB 0,1,2,3 ;ROW 0KCODE1: DB 4,5,6,7 ;ROW 1KCODE2: DB 8,9,A,B ;ROW 2

KCODE3: DB C,D,E,F ;ROW 3

END

ADC interfacing with Microcontrollers: Introduction

Introduction

In our daily life, anything we deal like sound, prassure, voltage or any measurable quantity, are usually in analog form Sowhat if we want to interface any analog sensor with our digital controllers? There must be something that translate theanalog inputs to digital output, and so Analog to digital convertors come to play. Usually we call them ADC (Analog to digitalconvertor). Before going to learn how to interface an ADC with a controller we first take a look at basic methods of analogto digital conversion.

This is a sample of the large number of analog-to-digital conversion methods. The basic principle of operation is to use the

comparator principle to determine whether or not to turn on a particular bit of the binary number output. It is typical for anADC to use a digital-to-analog converter (DAC) to determine one of the inputs to the comparator.

Following are the most used converion methods:

Digital-Ramp ADC Successive Approximation ADC Flash ADC


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Digital-Ramp ADC

Conversion from analog to digital form inherently involves comparator action where the value of the analog voltage at somepoint in time is compared with some standard. A common way to do that is to apply the analog voltage to one terminal of acomparator and trigger a binary counter which drives a DAC. The output of the DAC is applied to the other terminal of thecomparator. Since the output of the DAC is increasing with the counter, it will trigger the comparator at some point when itsvoltage exceeds the analog input. The transition of the comparator stops the binary counter, which at that point holds thedigital value corresponding to the analog voltage.

Successive Approximation ADC

Illustration of 4-bit SAC with 1 volt step size

The successive approximation ADC is much faster than the digital ramp ADC because it uses digital logic to converge on thevalue closest to the input voltage. A comparator and a DAC are used in the process. A flowchart explaning the working isshown in the figure below.


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Flash ADC

Illustrated is a 3-bit flash ADC with resolution 1 volt (after Tocci). The resistor net andcomparators provide an input to the combinational logic circuit, so the conversion time is just the propagation delay throughthe network - it is not limited by the clock rate or some convergence sequence. It is the fastest type of ADC available, but

requires a comparator for each value of output (63 for 6-bit, 255 for 8-bit, etc.) Such ADCs are available in IC form up to 8-bit and 10-bit flash ADCs (1023 comparators) are planned. The encoder logic executes a truth table to convert the ladder ofinputs to the binary number output.


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ADC interfacing with Microcontrollers: Interfacing ADC0804

ADC0804 Pinoutand TypicalConnections

As shown in the typica circuit, ADC0804 can be interfaced with any microcontroller. You need a minimum of 11 pins tointerface ADC0804, eight for data pins and 3 for control pins. As shown in the typical circuit the chip select pin can be madelow if you are not using the microcontroller port for any other peripheral (multiplexing).

There is a universal rule to find out how to use an IC. All you need is the datasheet of the IC you are working with and takea look at the timing diagram of the IC which shows how to send the data, which signal to assert and at what time the signalshould be made high or low etc.

Note: Keep this in mind that whenever you are working with an IC and you want to know how to communicate with thatIC, then simply look into the timing diagram of that IC from its datasheet. It gives you complete information that you needregarding the communication of IC.


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The above timing diagrams are from ADC0804 datasheet. The first diagram (FIGURE 10A) shows how to start a conversion.Also you can see which signals are to be asserted and at what time to start a conversion. So looking into the timing diagram

FIGURE 10A. We note down the steps or say the order in which signals are to be asserted to start a conversion of ADC. Aswe have decided to make Chip select pin as low so we need not to bother about the CS signal in the timing diagram. Belowsteps are for starting an ADC conversion. I am also including CS signal to give you a clear picture. While programming wewill not use this signal.

1. Make chip select (CS) signal low.2. Make write (WR) signal low.3. Make chip select (CS) high.

4. Wait for INTR pin to go low (means conversion ends).

Once the conversion in ADC is done, the data is available in the output latch of the ADC. Looking at the FIGURE 10B whichshows the timing diagram of how to read the converted value from the output latch of the ADC. Data of the new conversionis only avalable for reading after ADC0804 made INTR pin low or say when the conversion is over. Below are the stepts toread output from the ADC0804.

1. Make chip select (CS) pin low.2. Make read (RD) signal low.3. Read the data from port where ADC is connected.4. Make read (RD) signal high.

5. Make chip select (CS) high.


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DIGITAL TO ANALOG.

Digital To Analog Converter.


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Introduction

Thepurposeofthischapteristoprovide basicinformationaboutmicrocontrollersthatoneneedsto know

inorderto beabletousethemsuccessfully inpractice. Thisiswhy thischapterdoesn'tcontainany super

interestingprogramordeviceschematicwithamazingsolutions.Instead,thefollowingexamplesare

betterproofthatprogramwritingisneitheraprivilegenoratalentissue, buttheability ofsimply putting

puzzlepiecestogetherusingdirectives.Restassuredthatdesignanddevelopmentofdevicesmainly

consistsofthefollowingmethod test-correct-repeat.Ofcourse,themore youareinit,themore

complicatedit becomessincethepuzzlepiecesareputtogetherby bothchildrenandfirst-class

architects...

6.1 Basic connecting

Asseeninthefigureabove,inordertoenablethemicrocontrollertooperateproperly itisnecessary to

provide:

y Powersupply:

y Resetsignal:and

y Clock signal.

Clearly,itisaboutvery simplecircuits, butitdoesnothaveto bealwayslikethat.Ifthetargetdeviceis

usedforcontrollingexpensivemachinesormaintainingvitalfunctions,everythinggetsincreasingly

complicated. However,thissolutionissufficientforthetime being...

Power supply

Eventhoughthismicrocontrollercanoperateatdifferentpowersupply voltages,why totest Murphys

low?! A 5V DCismostcommonly used. Thecircuit,showninthefigure,usesacheapintegratedthree-

terminalpositiveregulatorLM7805,andprovideshigh-quality voltagestability andquiteenoughcurrentto

enablethemicrocontrollerandperipheralelectronicstooperatenormally (enoughcurrentinthiscase

means 1Amp).

Reset signal

Inorderthatthemucrocontrollercanoperateproperly,alogic0 (0V)must beappliedtotheresetpinRS.

Thepush buttonconnectingtheresetpinRStopowersupply VCCisnotnecessary. However,itisalmost

alwaysprovided becauseitenablesthemicrocontrollersafereturntonormaloperatingconditionsif

somethinggoeswrong. 5V is broughttothispin,themicrocontrollerisresetandprogramstartsexecution

fromthe beginning.

Clock signal


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Eventhoughthemicrocontrollerhasa built-inoscillator,itcannotoperatewithouttwoexternalcapacitors

andquartzcrystalwhichstabilizeitsoperationanddeterminesitsfrequency (operatingspeedofthe

microcontroller).

Ofcourse,itisnotalwayspossibletoapply thissolutionsothattherearealwaysalternativeones. Oneof

themistoprovideclock signalfromaspecialsourcethroughinvertor.Seethefigureontheleft.

6.2 Additional components

Regardlessofthefactthatthemicrocontrollerisaproductofmoderntechnology,itisofnousewithout

beingconnectedtoadditionalcomponents.Simply put,theappearanceofvoltageonitspinsmeans

nothingifnotusedforperformingcertainoperations (turnsomethingon/off,shift,display etc.).

Switches and Push buttons

Therearenosimplerdevicesthanswitchesandpush-buttons. Thisisthesimplestway ofdetecting

appearanceofavoltageonthemicrocontrollerinputpin.

Nevertheless,itisnotsosimpleinpractice...Itisaboutcontact bounce-acommonproblemwithmech

anicalswitches. Whenthecontactsstriketogether,theirmomentumandelasticity acttogetherto

cause bounce. Theresultisarapidly pulsedelectricalcurrentinsteadofacleantransitionfromzerotofull

current.Itmostly occursduetovibrations,slightroughspotsanddirt betweencontacts. Thiseffectis

usually unnoticeablewhenusingthesecomponentsineveryday life becausethe bouncehappenstoo

quickly.Inotherwords,thewholethisprocessdoesnotlastlong (afewmicro-ormiliseconds), butitis

longenoughto beregistered by themicrocontroller. Whenusingonly apush-buttonasapulsecounter,

errorsoccurinalmost 100%ofcases!

ThesimplestsolutiontothisproblemistoconnectasimpleRCcircuittosuppressquick voltagechanges.

Sincethe bounceperiodisnotdefined,thevaluesofcomponentsarenotprecisely determined.Inmost

cases,itisrecomendedtousethevaluesshowninfigure below.

Ifcompletestability isneededthenradicalmeasuresshould betaken. Theoutputofthecircuit,shownin

figure (RSflip-flop),willchangeitslogicstateonly afterdetectingthefirstpulsetriggered by contact

bounce. Thissolutionisexpensive (SPDT switch), buteffecient,theproblemisdefinitely solved.Since

thecapacitorisnotused,very shortpulsescanalso beregisteredinthisway.


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Inadditiontothesehardwaresolutions,thereisalsoasimplesoftwaresolution. Whenaprogramtests

thestateofaninputpinanddetectsachange,thecheck should bedoneonemoretimeafteracertain

delay.Ifthechangeisconfirmed,itmeansthataswitchorpush buttonhaschangeditsposition. The

advantagesofsuchsolutionareobvious:itisfreeofcharge,effectsofnoisesareeliminatedanditcan be

appliedtothepoorerquality contactsaswell.DisadvantageisthesameaswhenusingRCfilter,i.e.

pulsesshorterthanprogramdelay cannot beregistered.

Optocoupler

Anoptocouplerisadevicecommonly usedtogalvanically separatemicrocontrollerselectronicsfromany

potentially dangerouscurrentorvoltageinitssurroundings. Optocouplersusually haveone,twoorfour

lightsources (LEDdiodes)ontheirinputwhileontheiroutput,oppositetodiodes,thereisthesame

numberofelementssensitivetolight (phototransistors,photo-thyristorsorphoto-triacs). Thepointisthat

anoptocouplerusesashortopticaltransmissionpathtotransferasignal betweentheelementsofcircuit,

while keepingthemelectrically isolated. Thisisolationmakessenseonly ifdiodesandphoto-sensitive

elementsareseparately powered.Inthisway,themicrocontrollerandexpensiveadditionalelectronics

arecompletely protectedfromhighvoltageandnoiseswhicharethemostcommoncauseofdestroying,

damagingorunstableoperationofelectronicdevicesinpractice. Themostfrequently usedoptocouplers

arethosewithphototransistorsontheiroutputs. Whenusingtheoptocouplerwithinternal base-to-pin 6

connection (therearealsooptocouplerswithoutit),the basecan beleftunconnected. Anoptional

connectionwhichlessenstheeffectsofnoises by eliminatingvery shortpulsesispresented by the

brokenlineinthefigure.

Relay

A relaysisanelectricalswitchthatopensandclosesundercontrolofanotherelectricalcircuit.Itis

thereforeconnectedtoouputpinsofthemicrocontrollerandusedtoturnon/offhigh-powerdevicessuch

asmotors,transformers,heaters, bulbs,antennasystemsetc. Thesearealmostalwaysplacedaway

fromthe boardsensitivecomponents. Therearevarioustypesofrelays butallofthemoperateinthesameway. Whenacurrentflowsthroughthecoil,therelay isoperated by anelectromagnettoopenor

closeoneormany setsofcontacts.Similartooptocouplers,thereisnogalvanicconnection (electrical

contact) betweeninputandoutputcircuits.Relaysusually demand bothhighervoltageandcurrentto

startoperation, buttherearealsominiatureoneswhichcan beactivated by alowcurrentdirectly

obtainedfromamicrocontrollerpin.


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Thefigureshowsthesolutionspecifictothe8051 microcontroller. A darlingtontransistorisusedhereto

activaterelays becauseofitshighcurrentgain. Thisisnotinaccordancewith rules, butisnecessary in

theeventthatlogiconeactivationisappliedsincetheoutputcurrentisthenvery low (pinactsasan

input).

Inordertopreventtheappearanceofself-inductionhighvoltage,caused by asuddenstopofcurrentflow

throughthecoil,aninvertedpolarizeddiodeisconnectedinparalleltothecoil. Thepurposeofthisdiode

isto cutoffthevoltagepeak.

Light-emitting diode (LED)

Light-emittingdiodesareelementsforlightsignalizationinelectronics. They aremanufacturedindifferent

shapes,colorsandsizes.Fortheirlowprice,lowpowerconsumptionandsimpleuse,they havealmost

completely pushedasideotherlightsources, bulbsatfirstplace. They performsimilartocommondiodes

withthedifferencethatthey emitlightwhencurrentflowsthroughthem.

Itisimportanttolimittheircurrent,otherwisethey will bepermanently destroyed.Forthisreason,a

conductormust beconnectedinparalleltoan LED.Inordertodeterminevalueofthisconductor,itis

necessary to knowdiodesvoltagedropinforwarddirection,whichdependsonwhatmaterialadiodeis

madefromandwhatcolouritis. Typicalvaluesofthemostfrequently useddiodesareshownintable

below. Asseen,therearethreemaintypesofLEDs.Standardonesgetful brightnessatcurrentof

20mA.Low Currentdiodesgetful brightnessattentimeslowercurrentwhileSuper Brightdiodesproduce

moreintensivelightthanStandardones.

COLOR T YPE T YP I C AL C U R RENT ID

( M A ) M A X I M AL C U R REN T I F

( M A ) VOL T AGE D

( V )

Infrared - 30 50 1.4

Red Standard 20 30 1.7

RedSuperBright

20 30 1.85

Red Low Current 2 30 1.7

Orange - 10 30 2.0

Green Low Current 2 20 2.1

Yellow - 20 30 2.1

Blue - 20 30 4.5

White - 25 35 4.4


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Sincethe8051 microcontrollercanprovideonly lowoutputcurrentandsinceitspinsareconfiguredas

outputswhenvoltageprovidedonthemis0V,directconnectingto LEDsisperformedasshowninfigure

ontheright (Low currentLED,cathodeisconnectedtotheoutputpin).

LED displays

Basically,an LEDdisplay isnothingmorethanseveral LEDsmouldedinthesameplasticcase. Thereare

many typesofdisplayscomposedofseveraldozensofbuiltindiodeswhichcandisplay different

symbols.

Mostcommonly usedisasocalled 7-segmentdisplay.Itiscomposedof8 LEDs, 7 segmentsare

arrangedasarectangleforsymboldisplayingandthereisanadditionalsegmentfordecimalpoint

displaying.Inordertosimplify connecting,anodesandcatodesofalldiodesareconnectedtothe

commonpinsothattherearecommonanodedisplaysandcommoncatodedisplays,respectively.

Segmentsaremarkedwiththelattersfrom A to G,plusdp,asshowninthefigureontheleft. On

connecting,eachdiodeistreatedsepartely,whichmeansthateachmusthaveitsowncurrentlimiting

resistor.

Displaysconnectedtothemicrocontrollerusually occupy alargenumberofvaluableI/O pins,whichcan

bea bigproblemespecially ifitisneededtodisplay multy digitnumbers. Theproblemismorethan

obviousif,forexample,itisneededtodisplay two 6-digitnumbers (asimplecalculationshowsthat 96

outputpinsareneededinthiscase). Thesolutiontothisproblemiscalled MULTIPLEXING. Thisishow

anopticalillusion basedonthesameoperatingprincipleasafilmcameraismade. Only onedigitis

activeatatime, butthey changetheirstatesoquickly makingimpressionthatalldigitsofanumberare

simultaneously active.

Hereisanexplanationonthefigureabove.Firsta byterepresentingunitsisappliedonamicrocontroller

portandatransistorT1 isactivatedatthesametime. Afterawhile,thetransistorT1 isturnedoff,a byte

representingtensisappliedonaportandatransistorT2 isactivated. Thisprocessis beingcyclicallyrepeatedathighspeedforalldigitsandcorrespondingtransistors.

Thefactthatthemicrocontrolleris justa kindofminiaturecomputerdesignedtounderstandonly the

languageofzerosandonesisfully expressedwhendisplayingany digit.Namely,themicrocontroller

doesn't knowwhatunits,tensorhundredsare,norwhattendigitsweareusedtolook like. Therefore,

eachnumberto bedisplayedmust bepreparedinthefollowingway:


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Firstofall,amulty digitnumbermust besplitintounits,tensetc.inaparticularsubroutine. Theneachof

thesedigitsmust bestoredinspecial bytes.Digitsgetfamiliarformat by performing masking.Inother

words,a binary formatofeachdigitisreplaced by adifferentcombinationofbitsinasimplesubroutine.

Forexample,thedigit8 (0000 1000)isreplaced by the binary number0111 111 inordertoactivateall

LEDsdisplayingdigit8. Theonly dioderemaininginactiveinthiscaseisreservedforthedecimalpoint.If

amicrocontrollerportisconnectedtothedisplay insuchaway that bit0activatessegment a, bit 1

activatessegment b, bit 2 segment cetc.,thenthetable belowshowsthe maskforeachdigit.

D I G I T S TO D I SPL AY D I SP LAY SEG MEN T S

dp a b c d e

0 1 0 0 0 0 0

1 1 0 0 1 1 1

2 1 0 0 1 0 0

3 1 0 0 0 0 1

4 1 1 0 0 1 1

5 1 0 1 0 0 1

6 1 0 1 0 0 0

7 1 0 0 0 1 1

8 1 0 0 0 0 0

9 1 0 0 0 0 1

Inadditiontodigitsfrom0to 9,somelettersofalphabet- A,C,E, J,F, U, H, L, b,c,d,o,r,t-canalso be

displayed by performingappropriatemasking.

Iftheeventthatcommonchatodedisplaysareusedallunitsinthetableshould bereplaced by zerosand

viceversa. Additionally,NPNtransistorsshould beusedasdriversaswell.

Liquid Crystal Displays (LCD)

An LCDdisplay isspecifically manufacturedto beusedwithmicrocontrollers,whichmeansthatitcannot

beactivated by standardICcircuits.Itisusedfordisplayingdifferentmessagesonaminiatureliquid

crysaldisplay.


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Themodeldescribedhereisforitslowpriceandgreatcapabilitiesmostfrequently usedinpractice.Itis

basedonthe HD44780microcontroller(Hitachi)andcandisplay messagesintwolineswith 16

characterseach.Itdisplaysallthelettersofalphabet, Greek letters,punctuationmarks,mathematical

symbolsetc.Inaddition,itispossibletodisplay symbolsmadeup by theuser. Otherusefulfeatures

includeautomaticmessageshift (leftandright),cursorappearance, LED backlightetc.

LCDPins

Therearepinsalongonesideofasmallprinted board. Theseareusedforconnectingtothe

microcontroller. Thereareintotalof14pinsmarkedwithnumbers (16 ifithas backlight). Theirfunctionis

describedinthetable bellow:

F UNC T ION P IN NU M BER NAME LOG I C S T A TE DES C R IPT ION

Ground 1 Vss - 0V

Power supply 2 Vdd - +5V

Contrast 3 Vee - 0 - Vdd

Control of operating

4 RS01

D0 D7 are interpreted as coD0 D7 are interpreted as

5 R/W01

Write data (from controller Read data (from LCD to con

6 E

0

1From 1 to 0

Access to LCD disable

Normal operatingData/commands are transferr

Data / commands

7 D0 0/1 Bit 0 LSB

8 D1 0/1 Bit 1

9 D2 0/1 Bit 2

10 D3 0/1 Bit 3

11 D4 0/1 Bit 4

12 D5 0/1 Bit 5

13 D6 0/1 Bit 6

14 D7 0/1 Bit 7 MSB


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LCD screen

An LCDscreenconsistsoftwolineseachcontaining 16 characters.Eachcharacterconsistsof5x8or

5x11 dotmatrix. This book coversthemostcommonly useddisplay,i.e.the 5x8characterdisplay.

Display contrastdependsonthepowersupply voltageandwhethermessagesaredisplayedinoneor

twolines.Forthisreason,varyingvoltage0-Vddisappliedonthepinmarkedas Vee. Trimmer

potentiometerisusually usedforthatpurpose.Some LCDdisplayshave built-in backlight (blueorgreen

LEDs). Whenusedduringoperation,acurrentlimitingresistorshould beserially connectedtooneofthe

pinsforbacklightpowersupply (similarto LEDs).


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Iftherearenocharactersdisplayedorifallofthemaredimmedwhenthedisplay ison,thefirstthingthat

should bedoneistocheck thepotentiometerforcontrastregulation.Isitproperly adjusted? Thesame

appliesifthemodeofoperationhas beenchanged (writinginoneortwolines).

LCD Memory

The LCD display contains three memory blocks:

y DDRAM Display DataRAM;

y CGRAM CharacterGeneratorRAM; and

y CGROM CharacterGeneratorROM.

DDRAM Memory

DDRAM memory isusedforstoringcharactersto bedisplayed. Thesizeofthismemory issufficientfor

storing80characters.Somememory locationsaredirectly connectedtothecharactersondisplay.


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TheaddressesofCGROM memory locationsmatchthecharactersofASCII.Iftheprogram being

currently executedencountersacommand sendcharacterP toport,thenthe binary value0101 0000

appearsontheport. Thisvalueisthe ASCIIequivalenttothecharacterP.Itisthenwrittento LCD,which

resultsindisplayingthesymbolfrom0101 0000locationofCGROM.Inotherwords,thecharacterPis

displayed. Thisappliestoalllettersofalphabet (capitalsandsmall), butnottonumbers.

Asseenontheprevious map,addressesofalldigitsarepushedforward by 48relativetotheirvalues

(digit0addressis48,digit 1 addressis49,digit 2 addressis 50etc.). Accordingly,inordertodisplay

digitscorrectly,eachofthemneedsto beaddedadecimalnumber48priorto besentto LCD.

Fromtheirinceptiontilltoday,computerscanrecognizeonly numbers, butnotletters.Itmeansthatall

dataacomputerswapswithaperipheraldevicehasa binary format,eventhoughthesameisrecognized

by themanasletters (keyboardisanexcellentexample).Every charactermatchestheunique

combinationofzeroesandones. ASCIIischaracterencoding basedontheEnglishalphabet. ASCIIcode

specifiescorrespondancebetweenstandardcharactersymbolsandtheirnumericalequivalents.

CGRAM memory

Apartfromstandardcharacters,the LCDdisplay canalsodisplay symbolsdefined by theuseritself.It

can beany symbolinthesizeof5x8pixels.RAM memory calledCGRAM inthesizeof64 bytesenables

it.

Memory registersare8 bitswide, butonly 5 lowerbitsareused. Logicone (1)inevery registerrepresents

adimmeddot,while8locationsgroupedtogetherrepresentonecharacter.Itis bestillustratedinfigure

below:


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Symbolsareusually definedatthe beginnigoftheprogram by simply writingzerosandonestoregisters

ofCGRAM memory sothatthey formdesiredshapes.Inordertodisplay themitissufficienttospecify

theiraddress. Pay attentiontothefirstcoloumnintheCGROM mapofcharacters.Itdoesn'tcontainRAM

memory addresses, butsymbols beingdiscussedhere.Inthisexample, display 0means-display ,

display 1means-display etc.

/&'%DVLF&RPPDQGV

Alldatatransferredto LCDthroughtheoutputsD0-D7 will beinterpretedasacommandoradata,which

dependsonthepinRSlogicstate:

RS = 1-BitsD0-D7 areaddressesofthecharactersto bedisplayed. LCDprocessoraddressesone

characterfromthecharactermapanddisplaysit. TheDDRAM addressspecifiesthelocationonwhich

thecharacteristo bedisplayed. Thisaddressisdefined beforethecharacteristransferredortheaddress

ofpreviously transferredcharacterisautomatically incremented.

RS = 0-BitsD0-D7 arecommandswhichdeterminethedisplay mode. Thecommandsrecognized by

the LCDaregiveninthetable below:

COM M AND R S R W D7 D 6 D5 D4 D3 D 2 D 1 D 0 EXEC U T

Clear display 0 0 0 0 0 0 0 0 0 1 1.6

Cursor home 0 0 0 0 0 0 0 0 1 x 1.6

Entry mode set 0 0 0 0 0 0 0 1 I/D S 40

Display on/off control 0 0 0 0 0 0 1 D U B 40

Cursor/Display Shift 0 0 0 0 0 1 D/C R/L x x 40

Function set 0 0 0 0 1 DL N F x x 40

Set CGRAM address 0 0 0 1 CGRAM address 40

Set DDRAM address 0 0 1 DDRAM address 40

Read BUSY flag (BF) 0 1 BF DDRAM address

Write to CGRAM or DDRAM 1 0 D7 D6 D5 D4 D3 D2 D1 D0 40

Read from CGRAM or DDRAM 1 1 D7 D6 D5 D4 D3 D2 D1 D0 40

I/D 1 = Increment (by 1) R/L 1 = Shift right

0 = Decrement (by 1) 0 = Shift left

S 1 = Display shift on DL 1 = 8-bit interface

0 = Display shift off 0 = 4-bit interface

D 1 = Display on N 1 = Display in two lines

0 = Display off 0 = Display in one line


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U 1 = Cursor on F 1 = Character format 5x10 dots

0 = Cursor off 0 = Character format 5x7 dots

B 1 = Cursor blink on D/C 1 = Display shift0 = Cursor blink off 0 = Cursor shift

What is the Busy flag?

Comparedtothemicrocontroller,the LCDisanextremely slowcomponent.Becauseofthis,itwas

necessary toprovideasignalwhichwill,uponcommandexecution,indicatethatthedisplay isready to

receiveanewdata. Thatsignal,calledthe busy flag,can bereadfromlineD7. WhentheBF bitiscleared

(BF=0),thedisplay isready toreceiveanewdata.

LCD Connection

Dependingonhowmany linesareusedforconnectingthe LCDtothemicrocontroller,thereare8-bitand

4-bit LCDmodes. Theappropriatemodeisselectedatthe beginningoftheoperation. Thisprocessis

called initialization.8-bit LCDmodeusesoutputsD0-D7 totransferdataintheway explainedonthe

previouspage. Themainpurposeof4-bit LEDmodeistosavevaluableI/O pinsofthemicrocontroller.

Only 4higherbits (D4-D7)areusedforcommunication,whileothermay beleftunconnected.Eachdata

issenttothe LCDintwosteps:fourhigherbitsaresentfirst (normally throughthelinesD4-D7),thenfour

lowerbits.Initializationenablesthe LCDtolink andinterpretreceived bitscorrectly.Dataisrarely read

fromthe LCD (itismainly transferredfromthemicrocontrollerto LCD)sothatitisoftenpossibletosave

anextraI/O pin by simpleconnectingR/W pintoground.Suchsavinghasitsprice. Messageswill be

normally displayed, butitwillnot bepossibletoreadthe busy flagsinceitisnotpossibletoreadthe

display either.


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Fortunately,thereisasimplesolution. Aftersendingacharacteroracommanditisimportanttogivethe

LCDenoughtimetodoits job. Owingtothefactthatexecutionoftheslowestcommandlastsfor

approximately 1.64mS,itwill besufficienttowaitapproximately 2mSforLCD.

LCD Initialization

The LCDisautomatically clearedwhenpoweredup.Itlastsforapproximately 15mS. Afterthat,the

display isready foroperation. Themodeofoperationisset by default.Itmeansthat:

1. Display iscleared

2. Mode

o DL= 1 Communicationthrough8-bitinterface

o N=0 Messagesaredisplayedinoneline

o F=0Characterfont 5 x8dots


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3. Display/Cursoron/off

o D=0Display off

o U=0Cursoroff

o B=0Cursor blink off

4. Characterentry

o ID= 1 Displayedaddressesareautomatically incremented by 1

o S=0Display shiftoff

Automaticresetisinmostcasesperformedwithoutany problems.Inmostcases, butnotalways!Iffor

any reasonthepowersupply voltagedoesnotreachfulvaluewithin 10mS,thedisplay willstartto

performcompletely unpredictably.Ifthevoltagesupply unitisnotabletomeetthisconditionorifitis

neededtoprovidecompletely safeoperation,theprocessofinitializationisapplied.Initialization,among

otherthings,causesanewresetenablingdisplay tooperatenormally.

Refertothefigure belowfortheprocedureon8-bitinitialization:


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6.3Examples

Theschematic belowisusedintheseveralfollowingexamples:

Apartfromcomponentsnecessary fortheoperationofthemicrocontrollersuchasoscillatorwith

capacitorsandthesimplestresetcircuit,therearealsoseveral LEDsandonepush button. Theseare

usedtoindicatetheoperationoftheprogram.

All LEDsarepolarizedinsuchaway thatthey areactivated by drivingamicrocontrollerpinlow (logic0).

LED Blinking


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ThepurposeofthisexampleisnottodemonstratetheoperationofLEDs, buttheoperatingspeedofthe

microcontroller.Simply put,inordertoenable LED blinkingto bevisible,itisnecessary toprovide

sufficientamountoftimetopass betweenon/offstatesofLEDs.Inthisexampletimedelay isprovided by

executingasubroutinecalledDelay.Itisatripleloopinwhichtheprogramremainsforapproximately 0.5

secondsanddecrementsvaluesstoredinregistersR0,R1 orR2. Afterreturningfromthesubroutine,the

pinstateisinvertedandthesameprocedureisrepeated...

;************************************************************************

;* PROGRAM NAME : Delay.ASM

;* DESCRIPTION: Program turns on/off LED on the pin P1.0

;* Software delay is used (Delay).

;************************************************************************

;BASIC DIRECTIVES

$MOD53

$TITLE(DELAY.ASM)

$PAGEWIDTH(132)

$DEBUG

$OBJECT

$NOPAGING

;STACK

DSEG AT 03FH

STACK_START: DS 040H


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;RESET VECTORS

CSEG AT 0

JMP XRESET ;Reset vector

ORG 100H

XRESET: MOV SP,#STACK_START ;Define Stack pointer

MOV P1,#0FFh ;All pins are configured as inputs

LOOP:

CPL P1.0 ;Pin P1.0 state is inverted

LCALL Delay ;Time delay

SJMP LOOP

Delay:

MOV R2,#20 ;500 ms time delay

F02: MOV R1,#50 ;25 ms

F01: MOV R0,#230

DJNZ R0,$


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DJNZ R1,F01

DJNZ R2,F02

END ;End of program

Using Watch-dog Timer

Thisexampledescribeshowthewatch-dogtimershouldnotoperate. Thewatch-dogtimerisproperly

adjusted (nominaltimeforcountingis 1024mS), butinstructionusedtoresetitisintentionally leftoutso

thatthistimeralways "wins". Asaresult,themicrocontrollerisreset (stateinregistersremains

unchanged),programstartsexecutionfromthe beginningandthenumberinregisterR3isincremented

by 1 andthencopiedtoport P1.

LEDsdisplay thisnumberin binary format...

;************************************************************************

;* PROGRAM NAME : WatchDog.ASM

;* DESCRIPTION : After watch-dog reset, program increments number in

;* register R3 and shows it on port P1 in binary format.

;************************************************************************

;BASIC DIRECTIVES

$MOD53

$TITLE(WATCHDOG.ASM)

$PAGEWIDTH(132)


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$DEBUG

$OBJECT

$NOPAGING

;DECLARATION OF VARIABLES

;STACK

DSEG AT 03FH


;RESET VECTORS

CSEG AT 0

JMP XRESET ; Reset vector

ORG 00BH

JMP TIM0_ISR ; Timer T0 reset vector

ORG 100H


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Anotherinterruptisgeneratedupon TimerT1 reset.Routine TIM1_ISRinwhichthe bitstateDIRECTION

invertsisexecuted.Sincethis bitdeterminesdirectionofbitrotationthenthemovingdirectionofLEDis

alsochanged.

Ifyoupressapush button T1 atsomepoint,alogiczero (0)onthe P3.2 outputwilldisable TimerT1.

;************************************************************************

;* PROGRAM NAME : Split.ASM

;* DESCRIPTION: Timer TL0 rotates bit on port P1, while TL1 determines

;* the rotation direction. Both timers operate in mode

;* 3. Logic zero (0) on output P3.2 disables rotation on port P1.

;************************************************************************

;BASIC DIRECTIVES

$MOD53

$TITLE(SPLIT.ASM)

$PAGEWIDTH(132)

$DEBUG

$OBJECT

$NOPAGING

;DECLARATION OF VARIABLES


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ORG 01BH

JMP TIM1_ISR ; Timer T1 reset vector

ORG 100H

XRESET: MOV SP,#STACK_START ; Define Stack pointer

MOV TMOD,#00001011B ; Define MOD3

MOV A,#0FFH

MOV P1,#0FFH

MOV R0,#30D

SETB TR0 ; TL0 is turned on

SETB TR1 ; TL1 is turned on

MOV IE,#08AH ; Interrupt enabled

CLR C

CLR DIRECTION ; Rotate to the right

LOOP1: SJMP LOOP1 ; Remain here

TIM0_ISR:

DJNZ R0,LAB3 ; Slow down rotation by 256 times

JB DIRECTION,LAB1


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RRC A ; Rotate contents of Accumulator

to the right through

; Carry flag

SJMP LAB2

LAB1: RLC A ; Rotate contents of Accumulator

to the left through

; Carry flag

LAB2: MOV P1,A ; Contents of Accumulator is moved

to port P1

LAB3: RETI ; Return from interrupt

TIM1_ISR:

DJNZ R1,LAB4 ; Slow down direction of rotationby 256 times

DJNZ R2,LAB4 ; When time expires, change

rotation direction

CPL SMER

MOV R2,#30D

LAB4: RETI

END ; End of program


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Simultaneous use of timers T0 and T1

Thisprogramcan beconsideredascontinuationofthepreviousone. They sharethesameidea, butin

thiscasetruetimers T0and T1 areused.Inordertodemonstratetheoperationofbothtimersonthe

sameportatthesametime,timerT0resetisusedtoshiftlogiczero (0)ontheport,while TimerT1 reset

isusedtochangerotationdirection. ThisprogramspendsmostofitstimeintheloopLOOP1waitingfor

aninterruptto becaused by reset.By checkingtheDIRECTION bit,informationonrotationdirectionof

both bitsinaccumulatoraswellasofmovingport LEDisobtained.

;************************************************************************

;* PROGRAM NAME : Tim0Tim1.ASM

;* DESCRIPTION: Timer TO rotates bit on port P1 while Timer1

;* changes rotation direction. Both timers are configured to operate in mode

1.

;************************************************************************

;BASIC DIRECTIVES

$MOD53

$TITLE(TIM0TIM1.ASM)

$PAGEWIDTH(132)

$DEBUG

$OBJECT

$NOPAGING


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ORG 01BH ; Timer 1 Reset vector

JMP TIM1_ISR

ORG 100H


MOV TMOD,#11H ; Select MOD1 for both timers

MOV A,#0FFH

MOV P1,#0FFH

MOV R0,#30D ; R0 is initialized

SETB TR0 ; TIMER0 is turned on

SETB TR1 ; TIMER1 is turned on

MOV IE,#08AH ; Timer0 and Timer1 Interrupt

enabled

CLR C

CLR DIRECTION ; Rotate to the right



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TIM0_ISR:

JB DIRECTION,LAB1

RRC A ; Rotate contents of accumulator


; Carry flag

SJMP LAB2

LAB1: RLC A ; Rotate contents of Accumulator

to the left through

; Carry flag

LAB2: MOV P1,A ; Contents of Accumulator is moved

to port P1

RETI ; Return from interrupt

TIM1_ISR:

DJNZ R0,LAB3 ; When time expires, change

rotation direction

CPL DIRECTION

MOV R0,#30D ; Initialize R0

LAB3:

RETI


Using Timer T2


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ThisexampledescribestheuseofTimerT2 configuredtooperateinAuto-Reloadmode.Inthisvery

case, LEDsareconnectedtoport P3whilethepush buttonusedforforcedtimerreset (T2EX)is

connectedtothe P1.1 pin.

Programexecutionissimilartothepreviousexamples. Whentimerendscounting,aninterruptisenabled

andsubroutineTIM2_ISRisexecuted,thusrotatingalogiczero (0)inaccumulatorandmovingthe

contentsofaccumulatortothe P3pin. Atlast,flagswhichcausedaninterruptareclearedandprogram

returnstotheloop LOOP1 whereitremainsuntilanewinterruptrequestarrives...

Ifpush button T2EXispressed,timeristemporarily reset. Thispush buttonresetstimer,whilepush

buttonRESET resetsthemicrocontroller.

;************************************************************************

;* PROGRAM NAME : Timer2.ASM

;* DESCRIPTION: Program rotates log. "0" on port P3. Timer2 determines

;* the speed of rotation and operates in auto-reload mode


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TIM2_ISR: RRC A ; Rotate contents of Accumulator


; Carry flag

MOV P3,A ; Move the contents of Accumulator

A to PORT3

CLR TF2 ; Clear timer T2 flag TF2

CLR EXF2 ; Clear timer T2 flag EXF2

RETI ; Return from interrupt


Using External Interrupt

Hereisanotherexampleofinterruptexecution. Anexternaliterruptisgeneratedwhenalogiczero (0)is

presentonpin P3.2 orP3.3.Dependingonwhichinputisactive,oneoftworoutineswill beexecuted:

A logiczero (0)onthe P3.2 pininitiatesexecutionofinterruptroutineIsr_Int0,thusincrementingnumber

inregisterR0andcopyingittoport P0. Logiczeroonthe P3.3pininitiatesexecutionofsubroutine

Isr_Int1,numberinregisterR1 isincremented by 1 andthencopiedtoport P1.

Inshort,eachpressonpush buttonsINT0andINT1 will becountedandimmediately shownin binary

formatonappropriateport (LEDwhichemittslightrepresentsalogiczero (0)).


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;************************************************************************

;* PROGRAM NAME : Int.ASM

;* DESCRIPTION : Program counts interrupts INT0 generated by appearance of

high-to-low

;* transition signal on pin P3.2 Result appears on port P0. Interrupts INT1

are also

;* counted up at the same time. They are generated byappearing high-to-low

transition

;* signal on pin P3. The result appears on port P1.


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ORG 100H

XRESET:

MOV TCON,#00000101B ; Interrupt INT0 is generated by

appearing

; high-to-low transition signal on pin

P3.2

; Interrupt INT0 is generated by

appearing

; high-to-low transition signal on pin

P3.3

MOV IE,#10000101B ; Interrupt enabled

MOV R0,#00H ; Counter starting value

MOV R1,#00H

MOV P0,#00H ; Reset port P0

MOV P1,#00H ; Reset port P1

LOOP: SJMP LOOP ; Remain here

Isr_Int0:

INC R0 ; Increment value of interrupt INT0

counter

MOV P0,R0


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RETI

Isr_Int1:

INC R1 ; Increment value of interrupt INT1

counter

MOV P1,R1

RETI


Using LED display

ThefollowingexamplesdescribetheuseofLEDdisplays.Commonchatodedisplaysareusedhere,

whichmeansthatall built-in LEDsarepolarizedinsuchaway thattheiranodesareconnectedtothe

microcontrollerpins.Sincethecommonway ofthinkingisthatlogicone (1)turnssomethingonandlogic

zero (0)turnssomethingof, LowCurrentdisplays (lowpowerconsumption)andtheirdiodes (segments)

areconnectedserially toresistorsofrelatively highresistance.

InordertosaveI/O pins,fourLEDdisplaysareconnectedtooperateinmultiplexmode.Itmeansthatall

segmentshavingthesamenameareconnectedtooneoutputporteachandonly onedisplay isactiveat

atime.

Tranzistorsandsegmenatsondisplaysarequickly activated,thusmakingimpressionthatalldigitsare

activesimultaneously.


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determinedcombinationofzeroesandonesappearsoneachofthesenewlocations (digit 1 mask,digit 2

mask...digit 9 mask). Whenthiscombinationistransferredtotheport,thedisplay showsdesireddigit.

;************************************************************************

;* PROGRAM NAME : 7Seg1.ASM

;* DESCRIPTION: Program displays number "3" on 7-segment LED display

;************************************************************************

;BASIC DIRECTIVES

$MOD53

$TITLE(7SEG1.ASM)

$PAGEWIDTH(132)

$DEBUG

$OBJECT

$NOPAGING

;STACK

DSEG AT 03FH



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DB 3FH ; Digit 0 mask

DB 06H ; Digit 1 mask

DB 5BH ; Digit 2 mask



DB 6DH ; Digit 5 mask






Writing and changing digits on LED display

Thisprogramisonly anextendedversonofthepreviousone. Thereisonly onedigitactive-thefirstone

ontheright,andthereisnouseofmultiplexing. Unlikethepreviousexample,alldecimalnumbersare

displayed (0-9).Inordertoenabledigitstochangeatreasonablepace,asoubroutine L2 whichcausesa

shorttimedelay isexecutedpriortoeachchangeoccurs.Basically,thewholeprocessisvery simpleand

takesplaceinthemainloopcalled LOOP whichlooksasfollows:

1. R3iscopiedto AccumulatorandsubroutineformaskingdigitsDispisexecuted;

2. Accumulatoriscopiedtotheportanddisplayed;

3. ThecontentsoftheR3registerisincremented;

4. Itischeckedwhether 10cyclesarecountedornot.Ifitis,registerR3isresetinorderto

enablecountingtostartfrom0; and

5. Instructionlabeledas L2 withinsubroutineisexecuted.

;************************************************************************


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;* PROGRAM NAME: 7Seg2.ASM

;* DESCRIPTION: Program writes numbers 0-9 on 7-segment LED display

;************************************************************************

;BASIC DIRECTIVES

$MOD53

$TITLE(7SEG2.ASM)

$PAGEWIDTH(132)

$DEBUG

$OBJECT

$NOPAGING

;STACK

DSEG AT 03FH


;RESET VECTORS

CSEG AT 0



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ORG 100H


MOV R3,#0 ; Counter initial value

MOV P1,#0 ; Turn off all display segments

MOV P3,#20h ; Activate display D4

LOOP:

MOV A,R3

LCALL Disp ; Perform appropriate masking for

number in

; Accumulator

MOV P1,A

INC R3 ; Increment number in register by

1

CJNE R3,#10,L2 ; Check whether the number 10 is

in R3

MOV R3,#0 ; If it is, reset counter

L2:

MOV R2,#20 ; 500 mS time delay

F02: MOV R1,#50 ; 25 mS


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Writing two-digit number on LED display

Itistimefortimemultiplexing! Thisisthesimplestexamplewhichdisplaysthenumber23ontwodisplays

insuchaway thatoneofthemdisplaysunits,whiletheotherdisplaystens. Themostimportantthingin

theprogramistimesynchronization. Otherwise,everythingisvery simple. TransistorT4enablesdisplay

D4andatthesametimea bitcombinationcorrespondingtothedigit3issetontheport. Afterthat,

transistorT4isdisabledandthewholeprocessisrepeatedusingtransistorT3anddisplay D3inorderto

display digit 2. Thisproceduremust becontinuosly repeatedinordertomakeimpressionthat both

displaysareactiveatthesametime.

;************************************************************************

;* PROGRAM NAME: 7Seg3.ASM

;* DESCRIPTION: Program displays number "23" on 7-segment LED display

;************************************************************************

;BASIC DIRECTIVES

$MOD53

$TITLE(7SEG3.ASM)

$PAGEWIDTH(132)

$DEBUG

$OBJECT


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MOV A,#02 ; Write digit 2 on display D3

LCALL Disp ; Find mask for that digit

MOV P1,A ; Put the mask on the port

SJMP LOOP ; Return to the label LOOP

Disp: ; Subroutine for writing digits

INC A

MOVC A,@A+PC

RET












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Using four digit LED display

Inthisexampleallfourdisplays,insteadoftwo,areactivesothatitispossibletowritenumbersfrom0to

9999. Here,thenumber1 234isdisplayed. Afterinitialization,theprogramremainsintheloop LOOP

wheredigitalmultiplexingisperformed. ThesubroutineDispisusedtoconvert binary numbersinto

correspondingcombinationsofbitsforthepurposeofactivatingdisplay lightingsegments.

;************************************************************************

;* PROGRAM NAME : 7Seg5.ASM

;* DESCRIPTION : Program displays number"1234" on 7-segment LED display

;************************************************************************

;BASIC DIRECTIVES

$MOD53

$TITLE(7SEG5.ASM)

$PAGEWIDTH(132)

$DEBUG

$OBJECT

$NOPAGING

;STACK


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DSEG AT 03FH


;RESET VECTORS

CSEG AT 0


ORG 100H


LOOP: MOV P1,#0 ; Turn off all display segments










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SJMP LOOP ; Return to the lable LOOP

Disp: ; Subroutine for writing digits

INC A

MOVC A,@A+PC

RET





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LCALL Disp ; Call mask for that digit

MOV P1,A ; Write units on display D4

LCALL Delay ; 25ms delay



MOV A,R3 ; Copy Register contaning tens

to A

LCALL Disp ; Call mask for that digit

MOV P1,A ; Write tens on display D3

LCALL Delay ; 25ms delay

SJMP LOOP

Delay:

MOV R1,#50 ; 5 ms delay

F01: MOV R0,#250

DJNZ R0,$

DJNZ R1,F01

RET

Disp: ; Subroutine for displaying

digits


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INC A

MOVC A,@A+PC

RET












Handling EEPROM

Thisprogramwritesdatatoon-chipEE

PROM memory.Inthiscase,thedataisahexadecimalnumber23

whichisto bewrittentothelocationwithaddress00.

Tomakesurethatthisnumberiscorrectly written,thesamelocationofEEPROM isread 10mSlaterin

ordertocomparethesetwonumbers.Ifthey match,Fwill bedisplayed. Otherwise,Ewill bedisplayedon

the LEDdisplay (Error).


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THE END EQU 071H ; Display "F"

ERROR EQU 033H ; Display "E"

;STACK

DSEG AT 03FH


;RESET VECTORS

CSEG AT 0


ORG 100H

XRESET: MOV IE,#00 ; All interrupts are disabled

MOV SP,#STACK_START

MOV DPTR,#0000H ; Choose location address in

EEPROM

ORL WMCON,#EEMEN ; Access to EEPROM is enabled

ORL WMCON,#EEMWE ; Write to EEPROM is enabled


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MOV TEMP,#23H ; Number written to EEPROM is

moved to

MOV A,TEMP ; register TEMP and Accumulator

MOVX @DPTR,A ; Write byte to EEPROM

CALL DELAY ; 10ms delay

MOVX A,@DPTR ; Read the same location and

compare to TEMP,

CJNE A,TEMP,ERROR ; If they don't match, jump to

label ERROR

MOV A,#KRAJ ; Display F (correct)

MOV P1,A

XRL WMCON,#EEMWE ; Write to EEPROM is disabled

XRL WMCON,#EEMEN ; Access to EEPROM is disabled


ERROR: MOV A,#ERROR ; Display E (error)

MOV P1,A

LOOP2: SJMP LOOP2

DELAY: MOV A,#0AH ; Delay

MOV R3,A

LOOP3: NOP


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LOOP4: DJNZ B,LOOP4

LOOP5: DJNZ B,LOOP5

DJNZ R3,LOOP3

RET


Data reception via UART

Inordertoenablesuccessful UART serialcommunication,itisnecessary tomeetspecificrulesofthe

RS232 standard.Itprimarily referstovoltagelevelsrequired by thisstandard. Accordingly,-10V stands

forlogicone (1)inthemessage,while +10V standsforlogiczero (0). Themicrocontrollerconverts

accurately dataintoserialformat, butitspowersupply voltageisonly 5V.Sinceitisnoteasy toconvert

0V into 10V and 5V into-10V,thisoperationison bothtransmitandreceivesidelefttoaspecializedIC

circuit. Here,the MAX232 by MAXIM isused becauseitiswidespread,cheapandreliable.

Thisexampleshowshowtoreceivemessagesent by a PC. TimerT1 generates boudrate.Sincethe

11.0592 MHzquartzcrystalisusedhere,itiseasy toobtainstandard baudratewhichamoutsto 9600

bauds.Eachreceiveddataisimmediately transferredtoport P1 pins.


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;************************************************************************

;* PROGRAM NAME : UartR.ASM

;* DESCRIPTION: Each data received from PC via UART appears on the port

;* P1.

;*

;************************************************************************

;BASIC DIRECTIVES


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$MOD53

$TITLE(UARTR.ASM)

$PAGEWIDTH(132)

$DEBUG

$OBJECT

$NOPAGING

;STACK

DSEG AT 03FH


;RESET VECTORS

CSEG AT 0


ORG 023H ; Starting address of UART interrupt

routine

JMP IR_SER

ORG 100H


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MOV SP,#STACK_START ; Initialization of Stack pointer

MOV TMOD,#20H ; Timer1 in mode2

MOV TH1,#0FDH ; 9600 baud rate at the frequency of

; 11.0592MHz

MOV SCON,#50H ; Receiving enabled, 8-bit UART

MOV IE,#10010000B ;UART interrupt enabled

CLR TI ; Clear transmit flag

CLR RI ; Clear receive flag

SETB TR1 ; Start Timer1


IR_SER: JNB RI,OUTPUT ; If any data is received,

; move it to the port

MOV A,SBUF ; P1

MOV P1,A


OUTPUT RETI


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Data transmission via UART

Thisprogramdescribeshowtouse UART totransmitdata. A sequenceofnumbers (0-255)istransmitted

toa PCat 9600 baudrate. The MAX 232 isusedasavoltageregulator.

;************************************************************************

;* PROGRAM NAME : UartS.ASM

;* DESCRIPTION: Sends values 0-255 to PC.

;************************************************************************

;BASIC DIRECTIVES

$MOD53

$TITLE(UARTS.ASM)

$PAGEWIDTH(132)

$DEBUG

$OBJECT

$NOPAGING

;STACK

DSEG AT 03FH


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;RESET VECTORS

CSEG AT 0


ORG 100H


MOV SP,#STACK_START ; Initialization of Stack pointer

MOV TMOD,#20H ; Timer1 in mode 2

MOV TH1,#0FDH ; 9600 baud rate at the frequency of

; 11.0592MHz

MOV SCON,#40H ; 8-bit UART

CLR TI ; Clear transmit bit


MOV R3,#00H ; Reset caunter

SETB TR1 ; Start Timer 1

START: MOV SBUF,R3 ; Move number from counter to a PC


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LOOP1: JNB TI,LOOP1 ; Wait here until byte transmission is

; complete

CLR TI ; Clear transmit bit

INC R3 ; Increment the counter value by 1

CJNE R3,#00H,START ; If 255 bytes are not sent return to

the

; label START



Writing message on LCD display

Thisexampleusesthemostfrequently usedtypeofLCDwhichdisplaystextintwolineswith 16

characterseach.InordertosaveI/O ports,only 4pinsareusedforcommunicationhere.Inthisway each

byteistransmittedintwosteps:firsthigherthenlowernible.

LCDneedsto beinitializedatthe beginningoftheprogram.Besides,partsoftheprogramwhichrepeatin

theprogramcreatespecialsubroutines. Allthismay seemextremely complicated, butthewholeprogram

basically performsseveralsimpleoperationsanddisplaysMikroelektronika Razvojni sistemi .


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$MOD53

$TITLE(LCD.ASM)

$PAGEWIDTH(132)

$DEBUG

$OBJECT

$NOPAGING

;Stack

DSEG AT 0E0h

Stack_Start: DS 020h

Start_address EQU 0000h

;Reset vectors

CSEG AT 0

ORG Start_address

JMP Inic

ORG Start_address+100h


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MOV IE,#00 ; All interrupts are disabled

MOV SP,#Stack_Start

Inic: CALL LCD_inic ; Initialize LCD

;*************************************************

;* MAIN PROGRAM

;*************************************************

START: MOV A,#80h ; Next character will appear on

the first

CALL LCD_status ; location in the first line of

LCD display.

MOV A,#'M' ; Display character M.

CALL LCD_putc ; Call subroutine for character

transmission.

MOV A,#'i' ; Display character i.

CALL LCD_putc

MOV A,#'k' ; Display character k.

CALL LCD_putc

MOV A,#'r' ; Display character r.

CALL LCD_putc


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MOV A,#'o' ; Display character o.

CALL LCD_putc

MOV A,#'e' ; Display character e.

CALL LCD_putc

MOV A,#'l' ; Display character l.

CALL LCD_putc


CALL LCD_putc


CALL LCD_putc

MOV A,#'t' ; Display character t.

CALL LCD_putc

MOV A,#'r' ; Display character r.

CALL LCD_putc


CALL LCD_putc

MOV A,#'n' ; Display character n.

CALL LCD_putc


CALL LCD_putc


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CALL LCD_putc

MOV A,#'a' ; Display character a.

CALL LCD_putc

MOV A,#0c0h ; Next character will appear on

the first

CALL LCD_status ; location in the second line

of LCD display.

MOV A,#'R' ; Display character R.

CALL LCD_putc ; Call subroutine for character

transmission.

MOV A,#'a' ; Display character a.

CALL LCD_putc

MOV A,#'z' ; Display character z.

CALL LCD_putc

MOV A,#'v' ; Display character v.

CALL LCD_putc


CALL LCD_putc

MOV A,#'j' ; Display character j.

CALL LCD_putc


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MOV A,#'n' ; Display character n.

CALL LCD_putc


CALL LCD_putc

MOV A,#' ' ; Display character .

CALL LCD_putc

MOV A,#'s' ; Display character s.

CALL LCD_putc


CALL LCD_putc

MOV A,#'s' ; Display character s.

CALL LCD_putc

MOV A,#'t' ; Display character t.

CALL LCD_putc


CALL LCD_putc

MOV A,#'m' ; Display character m.

CALL LCD_putc


CALL LCD_putc


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MOV R0,#20d ; Wait time (20x10ms)

CALL Delay_10ms ;

MOV DPTR,#LCD_DB ; Clear display

MOV A,#6d ;

CALL LCD_inic_status ;

MOV R0,#10d ; Wait time(10x10ms)

CALL Delay_10ms

JMP START

;*********************************************

;* Subroutine for wait time (T= r0 x 10ms)

;*********************************************

Delay_10ms: MOV R5,00h ; 1+(1+(1+2*r7+2)*r6+2)*r5

approximately

MOV R6,#100d ; (if r7>10)

MOV R7,#100d ; 2*r5*r6*r7

DJNZ R7,$ ; $ indicates current

instruction.

DJNZ R6,$-4


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LCD_read_write BIT P1.1 ;Bit for activating pin RW on

LCD.

LCD_reg_select BIT P1.2 ;Bit for activating pin RS on

LCD.

LCD_port SET P1 ; Port for connection to LCD.

Busy BIT P1.7 ; Port pin on which Busy flag

appears.

LCD_Start_I_red EQU 00h ; Address of the first message

character

; in the first line of LCD

display.

LCD_Start_II_red EQU 40h ; Address of the first message

character

; in the second line of LCD

display.

LCD_DB: DB 00111100b ; 0 -8b, 2/1 lines, 5x10/5x7

format

DB 00101100b ; 1 -4b, 2/1 lines, 5x10/5x7

format

DB 00011000b ; 2 -Display/cursor shift,right/left

DB 00001100b ; 3 -Display ON, cursor OFF,

cursor blink off


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DB 00000110b ; 4 -Increment mode, display

shift off

DB 00000010b ; 5 -Display/cursor home

DB 00000001b ; 6 -Clear display

DB 00001000b ; 7 -Display OFF, cursor OFF,

cursor blink off

LCD_inic:

;*****************************************

MOV DPTR,#LCD_DB

MOV A,#00d ; Triple initialization in 8-

bit

CALL LCD_inic_status_8 ; mode is performed at the

beginning

MOV A,#00d ; (in case of slow increment of

CALL LCD_inic_status_8 ; power supply when the power

supply is on

MOV A,#00d

lcall LCD_inic_status_8

MOV A,#1d ; Change from 8-bit into


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CALL LCD_inic_status_8 ; 4-bit mode

MOV A,#1d

CALL LCD_inic_status

MOV A,#3d ; As from this point the

program executes in

;4-bit mode


MOV A,#6d


MOV A,#4d


RET

LCD_inic_status_8:

;******************************************

PUSH B

MOVC A,@A+DPTR


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CLR LCD_reg_select ; RS=0 - Write command

CLR LCD_read_write ; R/W=0 - Write data on LCD

MOV B,LCD_port ; Lower 4 bits from LCD port

are memorized

ORL B,#11110000b

ORL A,#00001111b

ANL A,B

MOV LCD_port,A ; Data is moved from A to LCD

port

SETB LCD_enable ; high-to-low transition signal

; is generated on the LCD's EN

pin

CLR LCD_enable

MOV B,#255d ; Time delay in case of

improper reset

DJNZ B,$ ; during initialization

DJNZ B,$

DJNZ B,$


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CLR LCD_reg_select ; RS=O: Command is sent to LCD

CALL LCD_port_out

SWAP A ; Nibles are swapped in

accumulator

DJNZ B,$

DJNZ B,$

DJNZ B,$

CLR LCD_reg_select ; RS=0: Command is sent to LCD

CALL LCD_port_out

POP B

RET

;***************************************************************************

*

;* SUBROUTINE: LCD_putc

;* DESCRIPTION: Sending character to be displayed on LCD.

;***************************************************************************

*


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LCD_putc: PUSH B

MOV B,#255d

DJNZ B,$

SETB LCD_reg_select ; RS=1: Character is sent to

LCD

CALL LCD_port_out

SWAP A ; Nibles are swapped in

accumulator

DJNZ B,$

SETB LCD_reg_select ; RS=1: Character is sent to

LCD

CALL LCD_port_out

POP B

RET

;***************************************************************************

*

;* SUBROUTINE: LCD_port_out

;* DESCRIPTION: Sending commands or characters on LCD display


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;***************************************************************************

*

LCD_port_out: PUSH ACC

PUSH B

MOV B,LCD_port ; Lower 4 bits of LCD port are

memorized

ORL B,#11110000b

ORL A,#00001111b

ANL A,B

MOV LCD_port,A ; Data is copied from A to LCD

port

SETB LCD_enable ; high-to-low transition signal

; is generated on the LCD's EN

pin

CLR LCD_enable

POP B

POP ACC

RET


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Binary to decimal number conversion

Whenusing LEDand LCDdisplays,itisoftennecessary toconvertnumbersfrom binary todecimal.For

example,ifsomeregistercontainsanumberin binary formatthatshould bedisplayedonathreedigit

LEDdisplay itisfirstnecessary toconvertittodecimalformat.Inotherwords,itisnecessary todefine

whatshould bedisplayedonthemostrightdisplay (units),middledisplay (tens)andmostleftdisplay

(hundreds).

Thesubroutine belowperformsconversionofone byte.Binary numberisstoredintheaccumulator,while

digitsofthatnumberindecimalformatarestoredinregistersR3,R2 andaccumulator(units,tensand

hundreds,respectively).

;************************************************************************

;* SUBROUTINE NAME : BinDec.ASM

;* DESCRIPTION : Content of accumulator is converted into three decimal

digits

;************************************************************************

BINDEC: MOV B,#10d ; Store decimal number 10 in B

DIV AB ; A:B. Remainder remains in B

MOV R3,B ; Move units to register R3

MOV B,#10d ; Store decimal number 10 in B


MOV R2,B ; Move tens to register R2


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MOV B,#10d ; Store decimal number 10 in B


MOV A,B ; Move hundreds to accumulator

RET ; Return to the main program

key board interfacing

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