2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
1
CALIFORNIA MICRO DEVICES
CM8888/8888-2
CMOS INTEGRATED DTMF TRANCEIVER
Block Diagram
C0400798D
Fea tur es
Advanced CMOS technology for low power
consumption and increased noise immunity
Complete DTMF
Transmitter/Receiver
Standard 8051, 8086/8 microprocessor port
Central office quality and performance
Adjustable guard time
Automatic tone burst mode
Call progress mode
Single +5 volt power supply
20-pin DIP, 20-pin DIP EIAJ, 20-pin SOIC,
28-pin PLCC packages
2 MHz microprocessor port operation
Product Description
The CAMD CM8888/8888-2 is a fully integrated DTMF Transceiver, featuring adjustable guard time, automatic tone burst
mode, call progress mode, and a fully compatible 8051, 8086/8 microprocessor interface. The CM8888/8888-2 is
manufactured using state-of-the-art advanced CMOS technology for low power consumption and precise data handling.
The CM8888/8888-2 is based on the industry standard CM8870 DTMF Receiver, while the transmitter utilizes a switched-
capacitor D/A converter for low distortion, highly accurate DTMF signaling. Internal counters provide an automatic tone
burst mode which allows tone bursts to be transmitted with precise timing. A call progress filter can be selected by an
external microprocessor for analyzing call progress tones. The CM8888-2 is electrically equivalent to the CM8888 but does
not include the call progress function.
Applications
Paging systems
Repeater systems/mobile radio
Interconnect dialers
PABX systems
Computer systems
Fax machines
Pay telephones
Credit card verification
All trademarks are the property of their respective holders.
2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
2
CALIFORNIA MICRO DEVICES
CM8888/8888-2
This device contains input protection
against damage due to high static voltages
or electric fields; however, precautions
should be taken to avoid application of
voltages higher than the maximum rating.
Note:
Exceeding these ratings may cause
permanent damage, functional
operation under these conditions is
not implied.
Operating Characteristics - Gain Setting Amplifier: All voltages referenced to V
SS
, V
DD
= 5.0V + 5%,
fc = 3.579545 MHz, T
A
= -40C to +85C unless otherwise noted.
DC Characteristics: All voltages referenced to V
SS
, V
DD
= 5.0V + 5%,
fc = 3.579545 MHz, T
A
= -40C to +85C unless otherwise noted.
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Absolute Maximum Ratings: (See Note)
2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
3
CALIFORNIA MICRO DEVICES
CM8888/8888-2
AC Characteristics: All voltages referenced to V
SS
, V
DD
= 5.0V + 5%, fc = 3.579545 MHz,
T
A
= -40C to +85C unless otherwise noted.
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2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
4
CALIFORNIA MICRO DEVICES
CM8888/8888-2
Notes:
1. dBm = decibels above or below a reference power
of 1 mW into a 600 ohm load.
2. Digit sequence consists of all 16 DTMF tones.
3. Tone duration = 40mS. Tone pause = 40 mS.
4. Nominal DTMF frequencies are used.
5. Both tones in the composite signal have
an equal amplitude.
6. The tone pair is deviate by + 1.5% + 2Hz.
7. Bandwidth limited (3 KHz) Gaussian Noise.
8. The precise dial tone frequencies are 350 and 440 Hz (+2%)
9. For an error date of less than 1 in 10,000.
10. Referenced to the lowest amplitude tone in the DTMF signal.
11. Referenced to the minimum valid accept level.
AC Characteristics: V
DD
= 5.0V + 5%, V
SS
= OV, T
A
= -40C to +85C
* Typical values are for use as design aids only, and are not guaranteed or subject to production testing.
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2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
5
CALIFORNIA MICRO DEVICES
CM8888/8888-2
2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
6
CALIFORNIA MICRO DEVICES
CM8888/8888-2
Explanation Of Events
A.
Tone bursts detected, tone duration invalid RX
data register not updated.
B.
Tone #n detected, tone duration valid, tone
decoded and latched in RX data register.
C.
End of tone #n detected, tone absent duration
valid, information in RX data register retained
until next valid tone pair.
D.
Tone #n + 1 detected, tone duration valid, tone
decoded and latched in RX data register.
E.
Acceptance dropout of tone #n + 1, tone
absent duration invalid, data remains
unchanged.
F.
End of tone #n + 1 detected, tone absent
duration valid, inforamtion in RX data register
retained until next valid tone repair.
Explanation Symbols
V
IN
DTMF composite input signal.
ESt
Early steering output, Indicates detection
of valid tone frequencies.
St/GT
Steering input/guard time output. Drives
external RC timing circuit.
RX
0
-RX
3
4-bit decoded data in receive data register.
b3
Delayed steering. Indicates that valid
freqeuncies have been present/absent for
the required guard time thus consituting a
valid siganal.
b2
Indicates that valid data is in the receive
data register. The bit is cleared after the
status register is read.
IRQ/CP Interupt is active indicating that new data
is in the RX data register. The interupt is
cleared after the status register is read.
tREC
Maximum DTMF signal duration not
detected as valid.
tREC
Minimum DTMF signal duration required
for valid recognition.
t
ID
Minimum time between valid sequencial
DTMF signals.
t
DO
Maximum allowable dropout during valid
DTMF signal.
t
DP
Time to detect valid frequencies present.
t
DA
Time to detect valid frequencies absent.
t
GTP
Guard time, tone present.
t
GTA
Guard time, tone absent.
Functional Description
The CM8888 Integrated DTMF Transceiver provides the design
engineer with not only low power consumption, but central
office quality performance. The CM8888s internal architecture
consists of high performance DTMF receiver with an internal
Gain Setting Amplifier and DTMF gernerator. The DTMF
Generator contains a Tone Burst Counter for generating precise
tone bursts and pauses. The Call Progress mode, when
selected, allows the detection of call progress tones. A
standard 8051, 8086/8 series microprocessor interface allows
access to an internal status register, two control registers and
two data registers within the CM8888.
Input Configuration
The CM8888 input arrangement consists of a differential input
operational amplifier and bias sources (V
REF
) for biasing the
amplifier inputs at V
DD
/2. Provisions are made for the
connection of a feedback resistor to the op amp output (GS) for
gain adjustment. In the single-ended configuration, the input
pins should be connected as shown in Figure 1, while Figure 2
shows the necessary connections for a differential input
configuration.
2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
7
CALIFORNIA MICRO DEVICES
CM8888/8888-2
Figure 1. Single Ended Input Configuration
Figure 2. Differential Input Configuration
VOLTAGE GAIN
Receiver
Section
Separation of the low and high group tones is
achieved by applying the DTMF signal to the inputs to
two sixth order switched capacitor bandpass filters, the
bandwidths of which correspond to the low and high-
group frequencies as shown in Figure 5. The low-
group filter incorporates notches at 350 Hz and 440
Hz for excellent dial-tone rejection. Each filter output
is followed by a single-order switched capacitor filter
section which smoothes the signals prior to limiting.
Limiting is performed by high-gain comparators with
hysteresis to prevent detection of unwanted low-level
signals. The outputs of the comparators provide full-
rail logic swings at the incoming DTMF signals
frequencies.
Following the filter section is a decoder which employs
digital counting techniques to determine the
frequencies of the incoming tones, and to verify that
the incoming tones correspond to standard DTMF
frequencies. A complex averaging algorithm protects
against tone simulation by extraneous signals (e.g.
voice), while still providing tolerance to small
deviations in frequency. The averaging algorithm was
developed to ensure and optimum combination of
immunity to talk-off , as well as a tolerance to the
presence of two valid tones (sometimes referred to as
signal condition in industry publications), the Early
Steering (EST) output will go to and active state. Any
subsequent loss of signal condition will cause ESt to
assume and inactive state.
SteeringCircuit
Before registration of a decoded tone pair, the receiver
checks for a valid signal duration (referred to as
Character Recognition Condition). This check is
performed by an external RC time constant driven by
ESt. A logic high on ESt causes VC (See Figure 3) to
rise as the capacitor discharge. Provided that the
signal condition is maintained (ESt remains high) for
the validation period (tGTP), VC reaches the threshold
(VTSt) of the steering logic to register the tone pair,
latching its corresponding 4-bit code (See Figure 5)
into the Receive Data Register. At this point the GT
continues to drive high as long as ESt remains high.
Finally, after a short delay to allow the output latch to
settle, the Delayed Steering output flag goes high,
signalling that a received tone pair has been registered.
It is possible to monitor the status of the Delayed
Steering flag by checking the appropriate bit in the
Status Register. If Interrupt Mode has been selected,
the IRQ/CP pin will pull low when the Delayed Steering
flag is active.
The contents of the output latch are updated on an active
Delayed Steering transition. This data is presented to the
4-bit bi directional data bus when the Receive Data
Register is read. The steering circuit works in reverse to
validate the interdigit pause between signals. Thus, as
well as rejecting signals too short to be considered valid,
the receiver will tolerate signal interruptions (drop out)
too short to be considered a valid pause. This facility,
together with the capability of selecting the steering time
constants externally, allows the designer to tailor
performance to meet a wide variety of system
requirements.
2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
8
CALIFORNIA MICRO DEVICES
CM8888/8888-2
Guard Time Adjustment
The simple steering circuit shown in Figure 3 is adequate
for most applications. Component values are chosen
according to the formula:
tREC = tDP + tGTP
tID = tDA + tGTA
The value of tDP is a device parameter and tREC is the
minimum signal duration to be recognized by the
receiver. A value for C of 0.1 uF is recommended for
most applications, leaving R to be selected by the
designer. Different steering arrangements may be used
to select independently the guard times for tone present
(tGPT) and tone absent (tGTA). This may be necessary
to meet system specifications which place both accept
and reject limits on both tone duration and interdigital
pause. Guard Time adjustment also allows the designer
to tailor system parameters such as talk-off and noise
immunity. Increasing tREC improves talk-off performance
Figure 3. Basic Steering Circuit
Figure 4. Guard Time Adjustment
since it reduces the probability that tones simulated by
speech will maintain signal condition long enough to
be registered. Alternatively, a relatively short tREC with
a ling tDO would be appropriate for extremely noisy
environments where fast acquisition time and immunity
to tone drop-outs are required. Design information for
Guard Time adjustments is shown in
Figure 4.
Call Progress Filter (CM8888)
A Call Progress (CP) Mode can be selected allowing the
detection of various tones which identify the progress of a
telephone call on the network. The Call Progress tone input
and DTMF input are common, however, call progress tones
can only be detected when the CP Mode has been selected.
DTMF signals cannot be detected if the CP Mode has been
selected (see Table 5). Figure 6. indicates the useful detect
bandwidth of the Call Progress filter. Frequencies presented
to the input (IN + and IN-) which 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 trigger
can be analyzed by a microprocessor or counter arrangement
to determine the nature of the Call Progress tone being
detected. Frequencies which are in the reject area will
not be detected, and consequently there will be no activity
on IRQ/CP as a result of these frequencies.
F
W
O
L
F
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G
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C
1
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1
D
0
0
0
0
Figure 5. Function Encode/Decode
0=LOGIC LOW, 1=LOGIC HIGH
2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
9
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.
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
Figure 6 Call Progress Response
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.
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
Figure 7 Spectrum Plot
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Table 1 Actual Frequencies Versus
Standard Requirements
2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
10
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.
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
Equation 1. THD (%) For a Single Tone
Equation 2. THD (%) For a Dual Tone
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.
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
Figure 8 Common Crystal Connection
Table 2. Internal Register Functions
Table 3. CRA Bit Positions
Table 4. CRA Bit Positions
2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
11
CALIFORNIA MICRO DEVICES
CM8888/8888-2
data transfer operations. The Receive Data Register contains
the output code of the last valid DTMF tone pair to be
decoded and is a read-only register. The data entered in
the Transmit Data Register will determine which tone
pair is to be generated (See Figure 5 fro coding details).
Data can only be written to the Transmit Data Register.
Transceiver control is accomplished with two Control
Registers (CRA and CRB) which occupy the same address
space. A write operation to CRB can be executed by
setting the appropriate bit in CRA. The following write
operation to the same address will then be directed to
CRB and subsequent write cycles will then be directed
Table 5. Control Regsiter A Description
N
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back to CRA. Internal reset circuitry will clear the control
registers on power-up; however, as a precautionary
measure the initialization software should include a
routine to clear the registers. Refer to Table 5 and 6 for
details concerning the Control Registers. The IRQ/CP
pin can be programmed such that it will provide and
interrupt request signal upon validation of DTMF signals,
or when the transmitter is ready for more data (Burst
mode only). The IRQ/CP pin is configured as an open-
drain output device and as such requires a pull-up resistor
(See Figure 9).
2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
12
CALIFORNIA MICRO DEVICES
CM8888/8888-2
2000 California Micro Devices Corp. All rights reserved.
5/00
215 Topaz Street, Milpitas, California 95035 Tel: (408) 263-3214 Fax: (408) 263-7846 www.calmicro.com
13
CALIFORNIA MICRO DEVICES
CM8888/8888-2
Bit
Name
Status Flag Set
Status Flag Cleared
bo
IRQ
Interrupt has occured. Bit one
Interrupt is inactive. Cleared after
(b1) and/or bit two (b2) is set.
Status Register is read.
b1
Transmit Data Register
Pause duration has terminated
Cleared after Status Register is read
and transmitter is ready for
or when in Non-Burst Mode.
new data.
b2
Receive Data Register Full
Valid data is in the Receive
Cleared after Status Register is
Data Register.
read.
b3
Delayed Steering
Set upon the valid detection of
Cleared upon the detection of a
the absence of a DTMF signal.
valid DTMF signal.
Table 7. Status Register Description
Pin Function Table
Name
Description
Name
Description
N+
Non-inverting op-amp input.
IN-
Inverting op-amp input
GS
Gain Select. Gives access to output of
front end differential amplifier for
connection of feedback resistor.
V
REF
References voltage output. Nominally
V
DD
/2 is used to bais inputs at inputs at
mid-rail (see application circuit).
V
SS
Negative power supply input.
OSC1
DTMF clock/oscillator input.
OSC2
Clock output. A 3.5795 MHz crystal
connected between OSC1 AND OSC2
completes the internal oscillator circuit.
TONE
Dual tone Multi-Frequency (DTMF)
output.
WR
Write input. A low on this pin when
CS is low enables data transfer from
the microprocessor. TTL compatible.
CS
Chip Select. TTl input. (CS =0 to
select the chip).
RSO
Register select input. See register
decode table. TTL compatible.
RD
Read input. A low on this pin when
CS is low enables data transfer to the
microprocessor. TTL compatible.
IRQ/CP Interupt request to microprocessor
(open-drain output). Also, when
Call Progress (CP) Mode has been
selected and Interrupt enabled the
IRQ/CP pin will output a rectangular
wave signal representative of the
input siganl applied at the input op-
amp. The input signal must be
within the bandwidth limits of the
Call Progress filter. See Filter 6.
DO-D3 Microprocessor data bus. TTL
compatible.
ESt
Early Spring output. Presents a
logic high once the digital algorithm
has detected a valid tone pair (signal
condition). Any momentary loss of signal
condition will cause EST to
return to a logic low.
StGT
Steering input/Guard Time output
(bidierectional). A voltage greater
than VTS, detected at St causes the
device to register the detected tone
pair and update the output latch. A
voltage less than VTS, frees the device
to accept a new tone pair. The GT output
acts to reset the external steering time-
constant; its state is a function of ESt and
the voltage on St.
V
DD
Positive power supply input.
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