Control Register A Description

Bit

Name

Function

Description

b0

TOUT

Tone output

A logic 1 enables the tone output. This function can be implemented in either the burst

mode or nonburst mode.

b1

CP/DTMF

Mode control

In DTMF mode (logic 0), the device is capable of generating and receiving DTMF signals.

When the call progress (CP) mode is selected (logic 1), a 6th-order bandpass filter is enabled to allow

call progress tones to be detected. Call progress tones within the specified bandwidth will be presented

at the IRQ/CP pin in rectangular wave format if the IRQ bit has been enabled (b2 = 1). Also, when the

CP mode and burst mode have both been selected, the transmitter will issue DTMF signals with a burst

and pause of 102 ms (typ) duration. This signal duration is twice that obtained from the DTMF transmit-

ter, if DTMF mode had been selected. Note that DTMF signals cannot be decoded when the CP mode

has been selected.

b2

IRQ

Interrupt enable

A logic 1 enables the interrupt mode. When this mode is active and the DTMF mode has

been selected (b1 = 0), the IRQ/CP pin will pull to a logic 0 condition when either (1) a valid DTMF sig-

nal has been received and has been present for the guard time or (2) the transmitter is ready for more

data (burst mode only).

b3

RSEL

Register select

A logic 1 selects control register B on the next write cycle to the control register address. Subsequent

write cycles to the control register are directed back to control register A.

Active Cell

Output Frequency(Hz)

% Error

Specified

Actual

L1

697

699.1

+ 0.30

L2

770

766.2

- 0.49

L3

852

847.4

- 0.54

L4

941

948.0

+ 0.74

H1

1209

1215.9

+ 0.57

H2

1336

1331.7

- 0.32

H3

1447

1471.9

- 0.35

H4

1633

1645.0

+ 0.73

M-8888

www.clare.com

5

Rev. 1

The divider output clocks another counter that

addresses the sinewave lookup ROM. The lookup

table contains codes used by the switched capacitor

D/A converter to obtain discrete and highly accurate

DC voltage levels. Two identical circuits are used to

produce row and column tones, which are then mixed

using a low-noise summing amplifier. The oscillator

described needs no startup time as in other DTMF

generators, since the crystal oscillator is running con-

tinuously, thus providing a high degree of tone burst

accuracy. When there is no tone output signal, the

TONE pin assumes a DC level of 2.5 volts (typically).

A bandwidth limiting filter is incorporated to attenuate

distortion products above 4 KHz.

Burst Mode:

Certain telephony applications require that generated

DTMF signals be of a specific duration, determined

either by the application or by any of the existing

exchange transmitter specifications. 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, 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 trans-

mitter is ready for more data.

The timing described in the previous paragraph is

available when the DTMF mode has been selected.

However, when call progress (CP) mode is selected, a

secondary burst/pause time is available that extends

this interval to 102 ms ± 2 ms. The 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 not received. In

applications where a nonstandard burst/pause time is

desirable, a software timing loop or external timer can

be used to provide the timing pulses when the burst

mode is disabled by enabling and disabling the trans-

mitter.

The M-8888 is initialized on powerup sequence with

DTMF mode and burst mode selected.

Single-Tone Generation:

A single-tone mode is available whereby individual

tones from the low group or high group can be gener-

ated. This mode can be used for DTMF test equipment

applications, acknowledgment tone generation, and

distortion measurements. Refer to the Control Register

B Description below for details.

Actual Frequencies vs Standard Requirements