detected if the CP mode has been selected (see Table 3). Fig-

ure 7 indicates the useful detect bandwidth of the call progress

filter. Frequencies presented to the input (IN+ and IN-) that 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 square wave output obtained from the schmitt trig-

ger can be analyzed by a microprocessor or counter arrange-

ment to determine the nature of the call progress tone being

detected. Frequencies in the “reject” area will not be detected,

and consequently there will be no activity on IRQ /CP as a result

of these frequencies.

DTMF Generator

The DTMF transmitter used in the M-8880 is capable of generat-

ing all 16 standard DTMF tone pairs with low distortion and high

accuracy. All frequencies are derived from an external 3.58

MHz crystal. The sinusoidal waveforms for the individual tones

are digitally synthesized using row and column programmable

dividers and switched capacitor digital-to-analog converters.

The row and column tones are mixed and filtered, providing a

DTMF signal with low total harmonic distortion and high accu-

racy. To specify a DTMF signal, data conforming to the encod-

ing format shown in Table 2 must be written to the transmit data

register. Note that this is the same as the receiver output code.

ferred to as low-group and high-group tones. Typically, the

high-group to low-group amplitude ratio (twist) is 2 dB to com-

pensate for high-group attenuation on long loops.

Operation: During write operations to the transmit data register,

4-bit data on the bus is latched and converted toa2 of8 code for

use by the programmable divider circuitry to specify a time seg-

ment length that will ultimately determine the tone frequency.

The number of time segments is fixed at 32, but the frequency is

varied by varying the segment length. When the divider reaches

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

pulse is issued and the counter starts again. 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 continuously, 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 gener-

ated 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 accom-

plished by making use of the burst mode. The transmitter is ca-

pable 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 sta-

tus register, indicating that the transmitter is ready for more data.

The timing described is available when the DTMF mode has

been selected. However, when call progress (CP) mode is se-

lected, 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 requiring a nonstandard

burst/pause time, use a software timing loop or external timer.

This provides the timing pulses when the burst mode is disabled

by enabling and disabling the transmitter.

The M-8880 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 generated. This mode can be used for DTMF test equipment

applications, acknowledgment tone generation, and distortion

measurements. Refer to Table 4 for details.

Distortion Calculations: The M-8880 is capable of producing

precise tone bursts with minimal error in frequency (see Table

3). The internal summing amplifier is followed by a first-order

low-pass switched capacitor filter to minimize harmonic compo-

nents and intermodulation products. The total harmonic distor-

tion for a

single tone can be calculated using Equation 1, (see

Figure 9) which is the ratio of the total power of all the extrane-

ous frequencies to the power of the fundamental frequency ex-

pressed as a percentage. The Fourier components of the tone

output correspond to V2f... Vnf as measured on the output

waveform. The total harmonic distortion for a

dual tone can be

calculated using Equation 2 (see Figure 9).

Page 4

M-8880

40-406-00012, Rev. G

www.clare.com

Figure 7 Call Progress Response

Active

Cell

Output Frequency (Hz)

Specified

Actual

% Error

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

Table 3 Actual Frequencies vs. Standard

Requirements