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MT88L85AN1

3 V Integrated DTMF Transceiver with Power-Down and Adaptive Micro Interface

Microsemi Corporation 

MT88L85

Data Sheet

EXPLANATION OF EVENTS

A)

TONE BURSTS DETECTED, TONE DURATION INVALID, RX DATA REGISTER NOT UPDATED.

B)

TONE #n DETECTED, TONE DURATION VALID, TONE DECODED AND LATCHED IN RX DATA REGISTER.

C)

END OF TONE #n DETECTED, TONE ABSENT DURATION VALID, INFORMATION IN RX DATA REGISTER RETAINED

UNTIL NEXT VALID TONE PAIR.

D)

TONE #n+1 DETECTED, TONE DURATION VALID, TONE DECODED AND LATCHED IN RX DATA REGISTER.

E)

ACCEPTABLE DROPOUT OF TONE #n+1, TONE ABSENT DURATION INVALID, DATA REMAINS UNCHANGED.

F)

END OF TONE #n+1 DETECTED, TONE ABSENT DURATION VALID, INFORMATION IN RX DATA REGISTER

RETAINED UNTIL NEXT VALID TONE PAIR.

EXPLANATION OF SYMBOLS

Vin

DTMF COMPOSITE INPUT SIGNAL.

ESt

EARLY STEERING OUTPUT. INDICATES DETECTION OF VALID TONE FREQUENCIES.

St/GT

STEERING INPUT/GUARD TIME OUTPUT. DRIVES EXTERNAL RC TIMING CIRCUIT.

RX0-RX3 4-BIT DECODED DATA IN RECEIVE DATA REGISTER

b3

DELAYED STEERING IN STATUS REGISTER (BIT 3) INDICATES THAT VALID FREQUENCIES HAVE BEEN

PRESENT/ABSENT FOR THE REQUIRED GUARD TIME THUS CONSTITUTING A VALID SIGNAL. ACTIVE LOW FOR

THE DURATION OF A VALID DTMF SIGNAL.

b2

RECEIVE DATA REGISTER FULL (BIT 2) IN STATUS REGISTER INDICATES THAT VALID DATA IS IN THE RECEIVE

DATA REGISTER. THE BIT IS CLEARED AFTER THE STATUS REGISTER IS READ.

IRQ/CP INTERRUPT IS ACTIVE INDICATING THAT NEW DATA IS IN THE RX DATA REGISTER. THE INTERRUPT IS

CLEARED AFTER THE STATUS REGISTER IS READ.

tREC

MAXIMUM DTMF SIGNAL DURATION NOT DETECTED AS VALID. TYPICALLY 20MS.

tREC

MINIMUM DTMF SIGNAL DURATION REQUIRED FOR VALID RECOGNITION. TYPICALLY 40MS.

tID

MINIMUM TIME BETWEEN VALID SEQUENTIAL DTMF SIGNALS. TYPICALLY 40MS.

tDO

MAXIMUM ALLOWABLE DROPOUT DURING VALID DTMF SIGNAL. TYPICALLY 20MS.

tDP

TIME TO DETECT VALID FREQUENCIES PRESENT.

tDA

TIME TO DETECT VALID FREQUENCIES ABSENT.

tGTP

GUARD TIME, TONE PRESENT.

tGTA

GUARD TIME, TONE ABSENT.

Figure 8 - Description of Timing Events

Call Progress Filter

A call progress mode, using the MT88L85, can be selected to allow the detection of various tones, which identify

the progress of a telephone call on the network. The call progress tone input and DTMF input are common,

however, call progress tones can only be detected when CP mode has been selected. DTMF signals cannot be

detected if CP mode has been selected (see Table 7). Figure 10 indicates the useful detect bandwidth of the call

progress filter. Frequencies presented to the input, which are within the ‘accept’ bandwidth limits of the filter, are

hard-limited by a high gain comparator with the IRQ/CP pin serving as the output. The square-wave output obtained

from the schmitt trigger can be analyzed by a microprocessor or counter arrangement to determine the nature of

the call progress tone being detected. Frequencies which are in the ‘reject’ area will not be detected and

consequently the IRQ/CP pin will remain low.

DTMF Generator

The DTMF transmitter employed in the MT88L85 is capable of generating all sixteen standard DTMF tone pairs

with low distortion and high accuracy. All frequencies are derived from an external 3.579545 MHz crystal. The

sinusoidal waveforms for the individual tones are digitally synthesized by using row and column programmable

dividers and switched capacitor D/A converters. The row and column tones are mixed and filtered to provide a

DTMF signal with low total harmonic distortion and high accuracy. To specify a DTMF signal, data conforming to the

encoding format shown in Table 1 must be written to the transmit Data Register. Note that Table 1 is the same as

the receiver output code. The individual tones which are generated (fLOW and fHIGH) are referred to as Low Group

and High Group tones. As seen from the table, the low group frequencies are 697, 770, 852 and 941 Hz. The high

group frequencies are 1209, 1336, 1477 and 1633 Hz. Typically, the high group to low group amplitude ratio (twist)

is 2 dB to compensate for high group attenuation on long loops.

9

Zarlink Semiconductor Inc.

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