CM8888 Datasheet, PDF (9/13 Pages) California Micro Devices Corp – CMOS INTEGRATED DTMF TRANCEIVER

CALIFORNIA MICRO DEVICES
CM8888/8888-2
DTMF Generator
The DTMF transmitter employed in the CM888 is capable
of generating all sixteen 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 D/A converters. The row and column
tones are mixed and filtered providing a DTMF signal,
data conforming to the encoding format shown in Figure
5 must be written to the Transmit Data Register. Note
that this is the same as the receiver output code. The
individual tones which are generated (fLOW and fHIGH)
are referred to as low-group and high-group tones. As
seen from Table 1, 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 2dB to
compensate for High-Group attenuation on long loops.
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. Under conditions when
there is no tone output signal, the TONE pin assumes a
DC level of 2.5 volts (typ.) A bandwidth limiting filter is
incorporated and serves to attenuate distortion products
above 4 KHz. It can be seen from Figure 7 that the
distortion products are very low in amplitude.
Figure 7 Spectrum Plot
Figure 6 Call Progress Response
DTMF Generator Operation
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, 4-bit data on the
bus is latched and converted to 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 for 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
ACTIVE
CELL
OUTPUT FREQUENCY
(Hz)
SPECIFIED ACTUAL
L1
697
699.1
L2
770
766.2
L3
852
847.4
L4
941
948.0
H1
1209
1215.9
H2
1336
1331.7
H3
1447
1471.9
H4
1633
1645.0
%
ERROR
+0.30
-0.49
-0.54
+0.74
+0.57
-0.32
-0.35
+0.73
Table 1 Actual Frequencies Versus
Standard Requirements
Burst Mode
In certain telephony applications it is required that DTMF
signals being generated be of a specific duration
determined either by the particular application or by any
one of the exchange transmitter specifications currently
existing. 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 which is 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 transmitter is ready for more
data. The timing described above is available when the
DTMF Mode has been selected. However, when CP
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