MT8880C/MT8880C-1

4-39

Burst Mode

In certain telephony applications it is required that

DTMF signals being generated are of a specific

duration

determined

either

by

the

particular

application or by any one of the exchange transmitter

specifications currently existing. Standard DTMF

signal timing can be accomplished by making use of

the Burst Mode. The transmitter is capable of issuing

symmetric bursts/pauses of predetermined duration.

This burst/pause duration is 51 ms±1 ms which is a

standard interval for autodialer and central office

applications. After the burst/pause has been issued,

the appropriate bit is set in the Status Register

indicating that the transmitter is ready for more data.

The timing described above is available when DTMF

mode has been selected. However, when CP mode

(Call Progress mode) is selected, a second burst/

pause

time of 102 ms ±2 ms is available. This

extended interval is useful when precise tone bursts

of longer than 51 ms duration and 51 ms pause are

desired. Note that when CP mode and Burst mode

have been selected, DTMF tones may be transmitted

only and

In applications where a non-standard burst/pause

duration is required, burst mode must be disabled

and the transmitter gated on and off by an external

hardware or software timer.

Single Tone Generation

A single tone mode is available whereby individual

tones from the low group or high group can be

generated. This mode can be used for DTMF test

equipment

applications,

acknowledgment

tone

generation and distortion measurements. Refer to

Control Register B description for details.

Distortion Calculations

The MT8880C/C-1 is capable of producing precise

tone bursts with minimal error in frequency (see

Table 1). The internal summing amplifier is followed

by a first-order lowpass switched capacitor filter to

minimize harmonic components and intermodulation

products. The total harmonic distortion for a

single

ratio of the total power of all the extraneous

frequencies

to

the

power

of

the

fundamental

frequency expressed as a percentage. The Fourier

harmonic distortion for a

Figure 11 - 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.

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.