MT88L85

Advance Information

4-76

Figure 9 - Description of Timing Events

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

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.

4-BIT DECODED DATA IN RECEIVE DATA REGISTER

b3

DELAYED STEERING. 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

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.

MAXIMUM DTMF SIGNAL DURATION NOT DETECTED AS VALID.

MINIMUM DTMF SIGNAL DURATION REQUIRED FOR VALID RECOGNITION.

MINIMUM TIME BETWEEN VALID SEQUENTIAL DTMF SIGNALS.

MAXIMUM ALLOWABLE DROPOUT DURING VALID DTMF SIGNAL.

TIME TO DETECT VALID FREQUENCIES PRESENT.

TIME TO DETECT VALID FREQUENCIES ABSENT.

GUARD TIME, TONE PRESENT.

GUARD TIME, TONE ABSENT.

DTMF signals cannot be detected if CP mode has

been selected (see Table 7). Figure 8 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 squarewave 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 using row

and column programmable dividers and switched

capacitor D/A converters. The row and column tones

are mixed and filtered providing 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 this is the

same as the receiver output code. The individual

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 com-pensate for high group

attenuation on long loops.

Figure 8 - Call Progress Response

LEVEL

(dBm)

FREQUENCY (Hz)

-25

0250

500

750

= Reject

= May Accept

= Accept