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.
Figure 7 Call Progress Response
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.
The individual tones that are generated (fLOW and fHIGH) are re-
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 to a 2 of 8 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
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).
Table 3 Actual Frequencies vs. Standard
Output Frequency (Hz)
40-406-00012, Rev. G