CM8888 Datasheet, PDF (10/13 Pages) California Micro Devices Corp – CMOS INTEGRATED DTMF TRANCEIVER

CALIFORNIA MICRO DEVICES
CM8888/8888-2
Mode (Call Progress Mode is selected, a secondary burst/
pause time is available such that this interval is extended
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 certain 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 transmitter.
The CM8888 is initialized on power-up sequences such
that DTMF mode and burst mode are selected. CP mode
and burst mode have been selected, DTMF tones may
be transmitted only and not received. In certain
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
transmitter.
The CM8888 is initialized on power-up sequence such
that DTMF mode and burst mode are 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 Control Register B
(Table 6) description for details.
Distortion Calculations
The CM8888 is capable of producing precise tone bursts
with minimal error in frequency (See Table1). The
internal summing amplifier is followed by a first-order
low-pass switched-capacitor filter to minimize
harmonic components and intermodulation products.
calculated using equation 2. VL and VH correspond to
the low-group amplitude, respectively, and V2IMD is
the sum of all the intermodulation components. The
interval switched-capacitor filter following the D/A
converter keeps distortion products down to a very low
level as shown in Figure 7.
DTMF Clock Circuit
The internal clock circuit is complete with the addition of
a standard television color burst crystal having a resonant
frequency of 3.579545 MHz. A number of CM8888
devices can be connected as shown in Figure 8 such that
only one crystal is required.
Figure 8 Common Crystal Connection
Table 2. Internal Register Functions
Table 3. CRA Bit Positions
Equation 1. THD (%) For a Single Tone
Table 4. CRA Bit Positions
Equation 2. THD (%) For a Dual Tone
The total harmonic distortion for a single tone can be
calculated using Equation 1 which is the ratio of the
total power of all the extraneous frequencies to the power
of the fundamental frequency expressed 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
Microprocessor Interface
The CM8888 employs a microprocessor interface which
allows precise control of transmitter and receiver functions.
There are five internal registers associated with the
microprocessor interface which can be subdivided into
three categories, ie; data transfer, transceiver control and
transceiver status. There are two registers associated with
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