MT8889C/MT8889C-1

4-112

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 Generator

The DTMF transmitter employed in the MT8889C/

MT8889C-1 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

tones

which

are

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.

The period of each tone consists of 32 equal time

segments. The period of a tone is controlled by

varying the length of these time segments. During

write operations to the Transmit Data Register the 4

bit data on the bus is latched and converted to 2 of 8

coding for use by the programmable divider circuitry.

This code is used to specify a time segment length,

which will ultimately determine the frequency of the

tone. When the divider reaches the appropriate

count, as determined by the input code, a reset pulse

is issued and the counter starts again. The number

of time segments is fixed at 32, however, by varying

the

segment

length

as

described

above

the

frequency can also be varied. The divider output

clocks

another

counter,

which

addresses

the

sinewave lookup ROM.

The lookup table contains codes which are used by

the switched capacitor D/A converter to obtain

discrete and highly accurate DC voltage levels. Two

identical circuits are employed to produce row

and

column tones, which are then mixed using a low

noise summing amplifier. The oscillator described

needs

no

“start-up”

time

as

in

other

DTMF

generators since the crystal oscillator is running

continuously thus providing a high degree of tone

burst

accuracy.

A

bandwidth

limiting

filter

is

incorporated and serves to attenuate distortion

products above 8 kHz. It can be seen from Figure 6

that the distortion products are very low in amplitude.