FUNCTIONAL PIN DESCRIPTION (DIP18 only)

V+

Pin 1. V+ is the positive supply for the circuit and

must meet the maximum and minimum voltage re-

quirements. (see electrical specifications).

MODE

Input. Pin 2. MODE determines the dialer’s default

operating mode. When the device is powered up or

the hookswitch input is switched from on-hook, (V+),

to off-hook, (V–), the default determines the signal-

ing mode. A V+ connection defaults to tone mode

operation and a V– connection defaults to pulse

mode operation.

When dialing in the pulse mode, a softswitch feature

will allow a change to the tone mode whenever the

* key, or softswitch, is depressed. Subsequent * key

inputs will causethe DTMF code for an * to be dialed.

The softswitch will only switch from pulse to tone. Af-

ter returning to on-hook and back to off-hook, the

part will be in pulse mode. Redial by the LND key will

repeat the softswitch.

C1, C2, C3, C4, R4, R3, R2, R1

Keyboard inputs. The MK53761 interfaces with

either the standard 2-of-8 with negative common or

the singlecontact (Form A) keyboard.

A valid keypad entry is either a single Row con-

nected to a single Column or V– simultaneously pre-

sented to both a single Row and Column. In its qui-

escent or standby state, during normal off-hook op-

eration, either the Rows or the Columns are at a

logic level 1 (V+). Pulling one input low enables the

on-chip oscillator. Keyboard scanning then begins.

Scanning consists of Rows and Columns alternately

switching high through on-chip pullups. After both a

Row and Column key have been detected, the de-

bounce counter is enabled and any noise (bouncing

contacts, etc.) is ignored for a debounce period

and if both Row and Column information are valid,

the information is buffered into the LND location. If

switched on-hook (pin 17 to pin 1), the keyboard in-

puts all pull high through on-chip pullup resistors.

Figure 2 : Keypad Configuration.

In the tone mode, if 2 or more keys in the same row

or if 2 or more keys in the same column are de-

pressed a single tone will be output. The tone will

corres-pond to the row or column for which the 2

keys were pushed. This feature is for test purposes,

and single tones will not be redialed.

Also in the tone mode, the outputtone is continuous

is manual dialing as long as the key is pushed. The

output tone duration follows the table 1.

Table 1 : Output Tone Duration.

Key-Push Time, T*

Tone Outpu t*

T

≤ 32ms

No Output Ignored by

MK53761

32ms

75ms Duration Output

T

* TKD is the key pad debounce time which is typically 32 ms.

When redialing in the tone mode, eachDTMF output

is 75ms duration, and the tone separation (intersig-

nal delay) is 50ms.

V–

Input. Pin 6 is the negative supply input to the de-

vice. This is the voltage reference for all specifica-

tions.

OSC1, OSC2

Pin 7 (input), pin 8 (output). OSC1 and OSC2 are

connections to an on-chip inverter used as the tim-

ing reference for the circuit. It has sufficient loopgain

to oscillate when used with a low-cost television

color-burst crystal. The nominal crystal frequency is

3.579545MHz and any deviation from this standard

is directly reflected in the Tone output frequencies.

The crystal oscillator provides the time reference for

all circuit functions. A ceramic resonator with toler-

ance of

± 0.25 % may also be used.

DTMF OUTPUT

Output. Pin 10. An NPN transistor emitter with a col-

lector tied to V+ drives the DTMF OUTPUT pin. The

transistor base is connected to an on-chip opera-

tional amplifier that mixes the Row and Column

tones. Figure 7 shows the timing at this pin.

The DTMF OUTPUT is the summation of a single

Row frequency and a single Column frequency. A

typical single tone sine wave is shown in Figure 4.

This waveform is synthesized using a resistor tree

with sinusoidally weighted taps.

The MK53761 is designed to operate from an un-

regulated supply ; the TONE LEVEL is supply inde-

pendent, and the single row tone output level will be

typically :

± 1dB

The DC component of the DTMF output while active

is described by the following equation :

MK53761

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