Part #
Description
M-8870-01
18-pin plastic DIP
M-8870-01SM
18-pin plastic SOIC
M-8870-01SMTR
18-pin plastic SOIC, tape and reel
M-8870-02
18-pin plastic DIP, power-down,
option
M-8870-02SM
18-pin plastic SOIC, power-down,
option
M-8870-02T
18-pin plastic SOIC, power-down
option, tape and reel
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DS-M8870-R3
M-8870
DTMF Receiver
1
Block Diagram
Pin Configuration
Ordering Information
Features
Low Power Consumption
Adjustable Acquisition and Release Times
Central Office Quality and Performance
Power-down and Inhibit Modes (-02 only)
Inexpensive 3.58 MHz Time Base
Single 5 Volt Power Supply
Dial Tone Suppression
Applications
Telephone switch equipment
Remote data entry
Paging systems
Personal computers
Credit card systems
Description
The M-8870 is a full DTMF Receiver that integrates
both bandsplit filter and decoder functions into a single
18-pin DIP or SOIC package. Manufactured using
CMOS process technology, the M-8870 offers low
power consumption (35 mW max) and precise data
handling. Its filter section uses switched capacitor
technology for both the high and low group filters and
for dial tone rejection. Its decoder uses digital counting
techniques to detect and decode all 16 DTMF tone
pairs into a 4-bit code. External component count is
minimized by provision of an on-chip differential input
amplifier, clock generator, and latched tri-state inter-
face bus. Minimal external components required
include a low-cost 3.579545 MHz color burst crystal, a
timing resistor, and a timing capacitor.
The M-8870-02 provides a "power-down" option
which, when enabled, drops consumption to less
than 0.5 mW. The M-8870-02 can also inhibit the
decoding of fourth column digits (see Tone Decoding
table on page 5).
Operating Characteristics - Gain Setting Amplifier
Parameter
Symbol
Min
Typ
Max
Units
Test Conditions
Input leakage current
I
N
-
100
-
nA
V
SS
< V
IN
< V
DD
Input resistance
R
IN
4
-
-
M
-
Input offset voltage
V
OS
-
25
-
mV
-
Power supply rejection
PSRR
50
-
-
dB
1 KHz
Common mode rejection
CMRR
55
-
-
dB
-3.0V < V
IN
< 3.0V
DC open loop voltage gain
A
VOL
60
-
-
dB
-
Open loop unity gain bandwidth
f
C
1.2
1.5
-
MHz
-
Output voltage swing
V
O
3.5
-
-
V
P-P
RL
100 K to V
SS
Tolerable capacitive load (GS)
C
L
-
-
100
pF
-
Tolerable resistive load (GS)
R
L
-
-
50
k
-
Common mode range
V
CM
2.5
-
-
V
P-P
No load
*Typical figures are at 25C and are for design aid only; not guaranteed and not subject to production testing.
Notes:
1. All voltages referenced to V
SS
unless otherwise noted. For typical values, V
DD
= 5.0V, V
SS
= 0V, TA = 25C.
DC Characteristics
Parameter
Symbol
Min
Typ
Max
Units
Test Conditions
Operating supply voltage
V
DD
4.75
-
5.25
V
-
Operating supply current
I
DD
-
3.0
7.0
mA
-
Standby supply current (see Note 3)
I
DD
Q
-
-
100
A
PD=V
DD
Power consumption
P
O
-
15
35
mW
f = 3.579 MHz, V
DD
= 5.0 V
Low level input voltage
V
IL
-
-
1.5
V
-
High level input voltage
V
IH
3.5
-
-
V
-
Input leakage current
I
IH
/I
IL
-
0.1
-
A
V
IN
= V
SS
or V
DD
(see Note 2)
Pullup (source) current on OE
I
SO
-
6.5
15.0
A
OE = 0 V
Input impedance, signal inputs 1, 2
R
IN
8
10
-
m
@ 1 kHz
Steering threshold voltage
V
TSt
2.2
-
2.5
V
-
Low level output voltage
V
OL
-
-
0.03
V
No load
High level output voltage
V
OH
V
DD
- 0.03
-
-
V
No load
Output low (sink) current
I
OL
1.0
2.5
-
mA
V
OUT
= 0.4 V
Output high (source) current
I
OH
0.4
0.8
-
mA
V
OUT
= V
DD
- 0.4 V
Output voltage V
REF
V
REF
2.4
-
2.7
V
No load
Output resistance V
REF
R
OR
-
10
-
k
-
*Typical figures are at 25C and are for design aid only; not guaranteed and not subject to production testing.
Absolute Maximum Ratings
Parameter
Symbol
Value
Power supply voltage (V
DD
- V
SS
)
V
DD
6.0 V max
Voltage on any pin
V
DC
V
SS
-0.3, VDD +0.3
Current on any pin
I
DD
10 mA max
Operating temperature
T
A
-40C to + 85C
Storage temperature
T
S
-65C to + 150C
Note:
Exceeding these ratings may cause permanent damage. Functional operation under
these conditions is not implied.
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2
M-8870
Rev. 3
Absolute Maximum Ratings are stress ratings. Stresses in
excess of these ratings can cause permanent damage to
the device. Functional operation of the device at these or
any other conditions beyond those indicated in the opera-
tional sections of this data sheet is not implied. Exposure of
the device to the absolute maximum ratings for an extend-
ed period may degrade the device and effect its reliability.
M-8870
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3
Rev. 3
Basic Steering Circuit
Single-Ended Input Configuration
Functional Description
M-8870 operating functions (see block diagram on
page 1) include a bandsplit filter that separates the
high and low tones of the received pair, and a digital
decoder that verifies both the frequency and duration
of the received tones before passing the resulting 4-bit
code to the output bus.
Filter
The low and high group tones are separated by apply-
ing the dual-tone signal to the inputs of two 6th order
switched capacitor bandpass filters with bandwidths
that correspond to the bands enclosing the low and
high group tones. The filter also incorporates notches
at 350 and 440 Hz, providing excellent dial tone rejec-
tion. Each filter output is followed by a single-order
switched capacitor section that smooths the signals
prior to limiting. Signal limiting is performed by high-
gain comparators provided with hysteresis to prevent
detection of unwanted low-level signals and noise.
The comparator outputs provide full-rail logic swings
at the frequencies of the incoming tones.
Decoder
The M-8870 decoder uses a digital counting tech-
nique to determine the frequencies of the limited tones
and to verify that they correspond to standard DTMF
frequencies. A complex averaging algorithm is used to
protect against tone simulation by extraneous signals
(such as voice) while tolerating small frequency varia-
tions. The algorithm ensures an optimum combination
of immunity to talkoff and tolerance to interfering sig-
nals (third tones) and noise. When the detector rec-
ognizes the simultaneous presence of two valid tones
(known as signal condition), it raises the Early
Steering flag (ESt). Any subsequent loss of signal
condition will cause ESt to fall.
Steering Circuit
Before a decoded tone pair is registered, the receiver
checks for a valid signal duration (referred to as char-
acter-recognition-condition). This check is performed
by an external RC time constant driven by ESt. A logic
high on ESt causes VC (see block diagram on page 1)
to rise as the capacitor discharges. Provided that sig-
nal condition is maintained (ESt remains high) for the
validation period (t
GTF
), V
C
reaches the threshold (V
TSt
)
of the steering logic to register the tone pair, thus latch-
ing its corresponding 4-bit code (see DC
Characteristics on page 2) into the output latch. At this
point, the GT output is activated and drives V
C
to V
DD
.
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 (StD)
goes high, signaling that a received tone pair has been
registered. The contents of the output latch are made
available on the 4-bit output bus by raising the three-
state control input (OE) to a logic high. 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 (dropouts) too short to be consid-
ered a valid pause. This capability, together with the
ability to select the steering time constants externally,
allows the designer to tailor performance to meet a
wide variety of system requirements.
Pin Functions
Pin
Name
Description
1
IN+
Non-inverting input
Connections to the front-end differential amplifier.
2
IN-
Inverting input
3
GS
Gain select. Gives access to output of front-end amplifier for connection of feedback resistor.
4
V
REF
Reference voltage output (nominally VDD/2). May be used to bias the inputs at mid-rail.
5
INH*
Inhibits detection of tones representing keys A, B, C, and D.
6
PD*
Power down. Logic high powers down the device and inhibits the oscillator. Internal pulldown.
7
OSC1
Clock input
3.579545 MHz crystal connected between these pins completes the internal oscillator.
8
OSC2
Clock output
9
VSS
Negative power supply (normally connected to 0 V).
10
OE
Tri-statable output enable (input). Logic high enables the outputs Q1 - Q4. Internal pullup.
11-14 Q1, Q2, Tri-statable data outputs. When enabled by OE, provides the code corresponding to the last valid tone pair
Q3, Q4
received (see Tone Decoding table on page 5).
15
StD
Delayed steering output. Presents a logic high when a received tone pair has been registered and the output latch is
updated. Returns to logic low when the voltage on St/GT falls below VTSt.
16
ESt
Early steering output. Presents a logic high immediately when the digital algorithm detects a recognizable tone pair (signal
condition). Any momentary loss of signal condition will cause ESt to return to a logic low.
17
St/GT
Steering input/guard time output (bidirectional). A voltage greater than VTSt detected at St causes the device to register the
detected tone pair and update the output latch. A voltage less than VTSt frees the device to accept a new tone pair. The GT
output acts to reset the external steering time constant, and its state is a function of ESt and the voltage on St. (See
Common Crystal Connection on page 5).
18
V
DD
Positive power supply. (Normally connected to +5V.)
* -02 only. Connect to V
SS
for -01 version
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4
M-8870
Rev. 3
Guard Time Adjustment
Where independent selection of signal duration and
interdigit pause are not required, the simple steering
circuit of Basic Steering Circuit is applicable.
Component values are chosen according to the formu-
la:
t
REC
= t
DP
+ t
GTP
t
GTP
@ 0.67 RC
The value of tDP is a parameter of the device and
tREC is the minimum signal duration to be recognized
by the receiver. A value for C of 0.1 F is recommend-
ed for most applications, leaving R to be selected by
the designer. For example, a suitable value of R for a
t
REC
of 40 ms would be 300 k
. A typical circuit using
this steering configuration is shown in the Single -
Ended Input Configuration on page 4. The timing
requirements for most telecommunication applications
are satisfied with this circuit. Different steering arrange-
ments may be used to select independently the guard
times for tone-present (t
GTP
) and tone-absent (t
GTA
).
This may be necessary to meet system specifications
that place both accept and reject limits on both tone
duration and interdigit pause.
Guard time adjustment also allows the designer to tai-
lor system parameters such as talkoff and noise immu-
nity. Increasing t
REC
improves talkoff performance,
since it reduces the probability that tones simulated by
speech will maintain signal condition long enough to be
registered. On the other hand, a relatively short t
REC
with a long t
DO
would be appropriate for extremely
noisy environments where fast acquisition time and
immunity to dropouts would be required. Design infor-
mation for guard time adjustment is shown in the
Guard Time Adjustment below.
Power-down and Inhibit Mode (-02 only)
A logic high applied to pin 6 (PD) will place the device
into standby mode to minimize power consumption. It
Figure 5 Guard Time Adjustment
Tone Decoding
FLOW
FHIGH
Key (ref.)
OE
Q4
Q3
Q2
Q1
697
1209
1
H
0
0
0
1
697
1336
2
H
0
0
1
0
697
1477
3
H
0
0
1
1
770
1209
4
H
0
1
0
0
770
1336
5
H
0
1
0
1
770
1477
6
H
0
1
1
0
852
1209
7
H
0
1
1
1
852
1336
8
H
1
0
0
0
852
1477
9
H
1
0
0
1
941
1336
0
H
1
0
1
0
941
1209
S
H
1
0
1
1
941
1477
#
H
1
1
0
0
697
1633
A
H
1
1
0
1
770
1633
B
H
1
1
1
0
852
1633
C
H
1
1
1
1
941
1633
D
H
0
0
0
0
ANY
ANY
ANY
L
Z
Z
Z
Z
L = logic low, H = logic high, Z = high impedance
M-8870
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5
Rev. 3
Differential Input Configuration
Common Crystal Connection
stops the oscillator and the functioning of the filters.
On the M-8870-01 models, this pin is tied to ground
(logic low).
Inhibit mode is enabled by a logic high input to pin 5
(INH). It inhibits the detection of 1633 Hz. The output
code will remain the same as the previous detected
code (see Pin functions table on page 4). On the M-
8870-01 models, this pin is tied to ground (logic low).
Input Configuration
The input arrangement of the M-8870 provides a dif-
ferential input operational amplifier as well as a bias
source (V
REF
) to bias the inputs at mid-rail. Provision
is made for connection of a feedback resistor to the
op-amp output (GS) for gain adjustment.
In a single-ended configuration, the input pins are
connected as shown in the Single - Ended Input
Configuration on page 3 with the op-amp connected
for unity gain and V
REF
biasing the input at 1/2V
DD
.
The Differential Input Configuration bellow permits
gain adjustment with the feedback resistor R
5
.
DTMF Clock Circuit
The internal clock circuit is completed with the addition
of a standard 3.579545 MHz television color burst crys-
tal. The crystal can be connected to a single M-8870 as
shown in the Single - Ended Input Configuration on
page 3, or to a series of M-8870s. As illustrated in the
Common Crystal Connection below, a single crystal
can be used to connect a series of M-8870s by cou-
pling the oscillator output of each M-8870 through a 30
pF capacitor to the oscillator input of the next M-8870.
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