CLARE M-8880 Datasheet

M-8880DTMFTransceiver

AdvancedCMOStechnologyforlowpowerconsump

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tionandincreasednoiseimmunity
CompleteDTMFtransmitter/receiverinasinglechip

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Standard6500/6800seriesmicroprocessorport

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Centralofficequalityandperformance

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Adjustableguardtime

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Automatictoneburstmode

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Callprogressmode

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Single+5Voltpowersupply

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20-pinDIPandSOICpackages

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2MHzmicroprocessorportoperation

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Inexpensive3.58MHzcrystal

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Nocontinuousf2clockrequired,onlystrobe

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Applicationsinclude:pagingsystems,repeatersys

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tems/mobileradio,interconnectdialers,PBXsystems,
computersystems,faxmachines,paytelephones,
creditcardverification

TheM-8880isacompleteDTMFTransmitter/Receiverthatfea­turesadjustableguardtime,automatictoneburstmode,call
progressmode,andafullycompatible6500/6800microproces­sorinterface.Thereceiverportionisbasedontheindustrystan­dardM-8870DTMFReceiver,whilethetransmitterusesa
switched-capacitordigital-to-analogconverterfor
low-distortion,highlyaccurateDTMFsignaling.Toneburstscan
betransmittedwithprecisetimingbymakinguseoftheauto­matictoneburstmode.Toanalyzecallprogresstones,acall
progressfiltercanbeselectedbyanexternalmicroprocessor.

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Figure 1 Pin Diagram

Functional Description

M-8880functionsconsistofahigh-performanceDTMFreceiver
withaninternalgainsettingamplifierandaDTMFgeneratorthat
containsatoneburstcounterforgeneratingprecisetonebursts
andpauses.Thecallprogressmode,whenselected,allowsthe
detectionofcallprogresstones.Astandard6500/6800series
microprocessorinterfaceallowsaccesstoaninternalstatus
register,twocontrolregisters,andtwodataregisters.

Input Configuration

Theinputarrangementconsistsofadifferentialinputopera­tionalamplifierandbiassources(V
inputsatV
feedbackresistortotheop-ampoutput(GS)forgainadjust

/2.Provisionsaremadefortheconnectionofa

DD

)forbiasingtheamplifier

REF

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Figure 2 Block Diagram

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Figure 3 Single-Ended Input Configuration Figure 4 Differential Input Configuration

ment.Inasingle-endedconfiguration,theinputpinsshouldbe
connectedasshowninFigure3.Figure4showsthenecessary
connectionsforadifferentialinputconfiguration.

Receiver Section

Thelowandhighgrouptonesareseparatedbyapplyingthe
DTMFsignaltotheinputsoftwosixth-orderswitchedcapacitor
bandpassfilterswithbandwidthsthatcorrespondtothelowand
highgroupfrequencieslistedinTable2.Thelowgroupfilterin

­corporatesnotchesat350and440Hz,providingexcellentdial
tonerejection.Eachfilteroutputisfollowedbyasingle-order
switchedcapacitorfilterthatsmoothsthesignalspriortolimiting.
Limitingisperformedbyhigh-gaincomparatorswithhysteresis
topreventdetectionofunwantedlow-levelsignals.Thecom­paratoroutputsprovidefull-raillogicswingsattheincoming
DTMFsignalfrequencies.

Table 1 Pin Functions

Name Description

IN+

IN-

GS

V

REF

V

SS

OSC1
OSC2
TONE

R/W

CS

RS0

φ2

IRQ/CP

D0 - D3

ESt

St/GT

V

DD

Noninvertingop-amp input.
Inverting op-amp input.
Gain select. Gives access to output of front end differential amplifier for connection of feedback resistor.
Reference voltage output. NominallyVDD/2 is used to bias inputs at mid-rail.
Negative power supply input.
DTMF clock/oscillator input.
Clock output. A 3.5795 MHz crystal connected between OSC1 and OSC2 completes the internal oscillator circuit.
Dual tone multifrequency (DTMF) output.
Read/write input. Controls the direction of data transfer to and from the microprocessor and the receiver/transmitter.TTL

compatible.
Chip select.TTLinput (CS = 0 to select the chip).
Register select input. See Table 6.TTLcompatible.
System clock input. May be continuous orstrobedonly during read or write.TTLcompatible.
Interrupt request to microprocessor (open-drain output). Also, when call progress (CP) mode has been selected and inter

rupt enabled, theIRQ
op-amp. The input signal must be within the bandwidth limits of the call progress filter. See Figure 11

Microprocessor data bus.TTLcompatible.
Early steering output. Presents a logic high once the digital algorithm has detected a valid tone pair (signal condition). Any

momentary loss of signal condition will causeEStto return to a logic low.
Steering input/guard time output (bidirectional). A voltage greater thanV

detected tone pair and update the output latch. A voltage less thanV
GToutput acts to reset the external steering time-constant; its state is afuncitonofEStand the voltage on St.

Positive power supply input.

/CP pin will output a rectangular wave signal representative of the input signal applied at the input

Adecoderemploysdigitalcountingtechniquestodeterminethe
frequenciesoftheincomingtones,andtoverifythattheycorre
spondtostandardDTMFfrequencies.Acomplexaveragingal
gorithmprotectsagainsttonesimulationbyextraneoussignals
(suchasvoice),whiletoleratingsmalldeviationsinfrequency.
Thealgorithmprovidesanoptimumcombinationofimmunityto
talkoffwithtolerancetointerferingfrequencies(thirdtones)and
noise.Whenthedetectorrecognizesthepresenceoftwovalid
tones(referredtoas“signalcondition”),theearlysteering(ESt)
outputgoestoanactivestate.Anysubsequentlossofsignal
conditionwillcauseESttoassumeaninactivestate.

SteeringCircuit:Beforeadecodedtonepairisregistered,the
receiverchecksforavalidsignalduration(referredtoas“char­acterrecognitioncondition”).Thischeckisperformedbyanex­ternalRCtimeconstantdrivenbyESt.AlogichighonESt

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detected at St causes the device to register the

TSt

frees the device to accept a new tone pair. The

TSt

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M-8880

causesVC(seeFigure5)toriseasthecapacitordischarges.
Providedthatthesignalconditionismaintained(EStremains
high)forthevalidationperiod(t
(V

)ofthesteeringlogictoregisterthetonepair,latchingits

TSt

),VCreachesthethreshold

GTP

corresponding4-bitcode(seeTable2)intothereceivedatareg
ister.

Table 2 Tone Encoding/Decoding

F

697 1209 10001
697 1336 20010
697 1477 30011
770 1209 40100
770 1336 50101
770 1477 60110
852 1209 70111
852 1336 81000
852 1477 91001
941 1336 01010
941 1209
941 1477 #1100
697 1633 A 1101
770 1633 B 1110
852 1633 C 1111
941 1633 D 0000

F

LOW

0 = logic low, 1 = logic high

Digit D3 D2 D1 D0

HIGH

*

1011

Thesteeringcircuitworksinreversetovalidatetheinterdigit
pausebetweensignals.Thus,aswellasrejectingsignalstoo
shorttobeconsideredvalid,thereceiverwilltoleratesignalin
terruptions(dropout)tooshorttobeconsideredavalidpause.
Thiscapability,togetherwiththeabilitytoselectthesteering

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timeconstantsexternally,allowsthedesignertotailorperfor
mancetomeetawidevarietyofsystemrequirements.

GuardTimeAdjustment:Thesimplesteeringcircuitshownin
Figure5isadequateformostapplications.Componentvalues
arechosenaccordingtotheformula:

t

REC=tDP+tGTP

TID=tDA+t

ThevalueoftDPisadeviceparameterandt

GTA

REC

signaldurationtoberecognizedbythereceiver.AvalueforC1
of0.1µFisrecommendedformostapplications,leavingR1to
beselectedbythedesigner.Differentsteeringarrangements
maybeusedtoselectindependentlytheguardtimesfortone
present(t

)andtoneabsent(t

GTP

).Thismaybenecessaryto

GTA

meetsystemspecificationsthatplacebothacceptandreject
limitsonbothtonedurationandinterdigitpause.Guardtimead
justmentalsoallowsthedesignertotailorsystemparameters
suchastalkoffandnoiseimmunity.Increasingt
talkoffperformancesinceitreducestheprobabilitythattones
simulatedbyspeechwillmaintainsignalconditionlongenough
toberegistered.Alternatively,arelativelyshortt
t

wouldbeappropriateforextremelynoisyenvironments

DO

wherefastacquisitiontimeandimmunitytotonedropoutsare
required.Designinformationforguardtimeadjustmentisshown
inFigure6.

istheminimum

improves

REC

withalong

REC

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AtthispointtheStGToutputisactivatedanddrivesV

toVDD.

C

StGTcontinuestodrivehighaslongasEStremainshigh.
Finally,afterashortdelaytoallowtheoutputlatchtosettle,the
delayedsteeringoutputflaggoeshigh,signalingthatareceived
tonepairhasbeenregistered.Itispossibletomonitorthestatus
ofthedelayedsteeringflagbycheckingtheappropriatebitinthe
statusregister.Ifinterruptmodehasbeenselected,theIRQ

/CP

pinwillpulllowwhenthedelayedsteeringflagisactive.

Figure 5 Basic Steering Circuit

Thecontentsoftheoutputlatchareupdatedonanactivede
layedsteeringtransition.Thisdataispresentedtothe4-bit
bidirectionaldatabuswhenthereceivedataregisterisread.

Figure 6 Guard Time Adjustment

Call Progress Filter

Acallprogress(CP)modecanbeselected,allowingthedetec
tionofvarioustonesthatidentifytheprogressofatelephonecall

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onthenetwork.ThecallprogresstoneinputandDTMFinputare
common;however,callprogresstonescanonlybedetected
whentheCPmodehasbeenselected.DTMFsignalscannotbe

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M-8880

detectediftheCPmodehasbeenselected(seeTable3).Fig
ure7indicatestheusefuldetectbandwidthofthecallprogress
filter.Frequenciespresentedtotheinput(IN+andIN-)thatare
withinthe“accept”bandwidthlimitsofthefilterarehard-limited
byahigh-gaincomparatorwiththeIRQ

/CPpinservingasthe
output.Thesquarewaveoutputobtainedfromtheschmitttrig
gercanbeanalyzedbyamicroprocessororcounterarrange
menttodeterminethenatureofthecallprogresstonebeing
detected.Frequenciesinthe“reject”areawillnotbedetected,
andconsequentlytherewillbenoactivityonIRQ

/CPasaresult

ofthesefrequencies.

Figure 7 Call Progress Response

DTMF Generator

TheDTMFtransmitterusedintheM-8880iscapableofgenerat­ingall16standardDTMFtonepairswithlowdistortionandhigh
accuracy.Allfrequenciesarederivedfromanexternal3.58
MHzcrystal.Thesinusoidalwaveformsfortheindividualtones
aredigitallysynthesizedusingrowandcolumnprogrammable
dividersandswitchedcapacitordigital-to-analogconverters.
Therowandcolumntonesaremixedandfiltered,providinga
DTMFsignalwithlowtotalharmonicdistortionandhighaccu
racy.TospecifyaDTMFsignal,dataconformingtotheencod
ingformatshowninTable2mustbewrittentothetransmitdata
register.Notethatthisisthesameasthereceiveroutputcode.
Theindividualtonesthataregenerated(f

LOW

andf

HIGH

)arere
ferredtoaslow-groupandhigh-grouptones.Typically,the
high-grouptolow-groupamplituderatio(twist)is2dBtocom
pensateforhigh-groupattenuationonlongloops.

Operation:Duringwriteoperationstothetransmitdataregister,
4-bitdataonthebusislatchedandconvertedtoa2of8codefor
usebytheprogrammabledividercircuitrytospecifyatimeseg
mentlengththatwillultimatelydeterminethetonefrequency.
Thenumberoftimesegmentsisfixedat32,butthefrequencyis
variedbyvaryingthesegmentlength.Whenthedividerreaches
theappropriatecountasdeterminedbytheinputcode,areset
pulseisissuedandthecounterstartsagain.Thedivideroutput
clocksanothercounterthataddressesthesinewavelookup
ROM.Thelookuptablecontainscodesusedbytheswitched
capacitorD/Aconvertertoobtaindiscreteandhighlyaccurate
DCvoltagelevels.Twoidenticalcircuitsareusedtoproduce
rowandcolumntones,whicharethenmixedusingalow-noise
summingamplifier.Theoscillatordescribedneedsno“startup”
timeasinotherDTMFgenerators,sincethecrystaloscillatoris
runningcontinuously,thusprovidingahighdegreeoftoneburst

accuracy.Whenthereisnotoneoutputsignal,theTONEpin

­assumesaDClevelof2.5volts(typically).Abandwidthlimiting
filterisincorporatedtoattenuatedistortionproductsabove4
KHz.

BurstMode:Certaintelephonyapplicationsrequirethatgener

­atedDTMFsignalsbeofaspecificduration,determinedeither

­bytheapplicationorbyanyoftheexistingexchangetransmitter
specifications.StandardDTMFsignaltimingcanbeaccom
plishedbymakinguseoftheburstmode.Thetransmitterisca
pableofissuingsymmetricbursts/pausesofpredetermined

duration.Thisburst/pausedurationis51ms±1ms,astandard
intervalforautodialerandcentralofficeapplications.Afterthe
burst/pausehasbeenissued,theappropriatebitissetinthesta
tusregister,indicatingthatthetransmitterisreadyformoredata.

ThetimingdescribedisavailablewhentheDTMFmodehas
beenselected.However,whencallprogress(CP)modeisse
lected,asecondaryburst/pausetimeisavailablethatextends
thisintervalto102ms±2ms.Theextendedintervalisuseful
whenprecisetoneburstsoflongerthan51msdurationand51
mspausearedesired.NotethatwhenCPmodeandburstmode
havebeenselected,DTMFtonesmaybetransmittedonlyand

not

received.Inapplicationsrequiringanonstandard
burst/pausetime,useasoftwaretiminglooporexternaltimer.
Thisprovidesthetimingpulseswhentheburstmodeisdisabled
byenablinganddisablingthetransmitter.

TheM-8880isinitializedonpowerupsequencewithDTMF
modeandburstmodeselected.

Single-ToneGeneration:Asingle-tonemodeisavailable
wherebyindividualtonesfromthelowgrouporhighgroupcan
begenerated.ThismodecanbeusedforDTMFtestequipment
applications,acknowledgmenttonegeneration,anddistortion
measurements.RefertoTable4fordetails.

DistortionCalculations:TheM-8880iscapableofproducing
precisetoneburstswithminimalerrorinfrequency(seeTable

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3).Theinternalsummingamplifierisfollowedbyafirst-order

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low-passswitchedcapacitorfiltertominimizeharmoniccompo
nentsandintermodulationproducts.Thetotalharmonicdistor

single

tionfora

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Figure9)whichistheratioofthetotalpowerofalltheextrane
ousfrequenciestothepowerofthefundamentalfrequencyex

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pressedasapercentage.TheFouriercomponentsofthetone

tone can be calculated using Equation 1, (see

outputcorrespondtoV2f...Vnfasmeasuredontheoutput
waveform.Thetotalharmonicdistortionfora

dual

calculatedusingEquation2(seeFigure9).

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Table 3 Actual Frequencies vs. Standard

Requirements

Active

Cell

L1 697 699.1 + 0.30
L2 770 766.2 - 0.49
L3 852 847.4 - 0.54
L4 941 948.0 + 0.74
H1 1209 1215.9 + 0.57
H2 1336 1331.7 - 0.32
H3 1447 1471.9 - 0.35
H4 1633 1645.0 + 0.73

Output Frequency (Hz)

Specified Actual

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tone can be

% Error

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