MT8870 Mitel Networks Corporation, MT8870 Datasheet - Page 3

• Manufacturer Part Number: MT8870
• Description: ISO2-CMOS Integrated DTMF Receiver
• Manufacturer: Mitel Networks Corporation

Functional Description
The
receiver offers small size, low power consumption
and high performance. Its architecture consists of a
bandsplit filter section, which separates the high and
low group tones, followed by a digital counting
section which verifies the frequency and duration of
the received tones before passing the corresponding
code to the output bus.
Filter Section
Separation of the low-group and high group tones is
achieved by applying the DTMF signal to the inputs
of two sixth-order switched capacitor bandpass
filters, the bandwidths of which correspond to the low
and high group frequencies. The filter section also
incorporates notches at 350 and 440 Hz for
exceptional dial tone rejection (see Figure 3). Each
filter output is followed by a single order switched
capacitor filter section which smooths the signals
prior to limiting. Limiting is performed by high-gain
comparators which are provided with hysteresis to
prevent detection of unwanted low-level signals. The
outputs of the comparators provide full rail logic
swings at the frequencies of the incoming DTMF
signals.
Decoder Section
Following the filter section is a decoder employing
digital
frequencies of the incoming tones and to verify that
they correspond to standard DTMF frequencies. A
complex averaging algorithm protects against tone
simulation by extraneous signals such as voice while
ATTENUATION
MT8870D/MT8870D-1
counting
(dB)
0
10
20
30
40
50
techniques
to
monolithic
X
Y
determine
AA A A AA
AAA AA AA AA
AAA
AA
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AA
AA A A
AAA
AAA AA
AA AA AA AA AA
AA
AA
AAA
AAA
AAA
AA
AA
AAA
AA
AA
AA
AA AA
AA
AA AAA
AA
AA
AAA
AA
AA
AA
AA
AA AA AA AA AA AA AA AA AA AA AA AA A A
AAA
AA AA A A
AA
AAA
AA
AA
AAA
AA
AA
AA
AA
AA
AA
AA A A
AA
AA A A
AAA
AA
AA
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AA
Figure 3 - Filter Response
AAA
AA A A AA
AAA
AA
AA
AA
AA
AA
AA
AA AAA
AA
AAA
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AA A A
AA
AA A A
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AA AA
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A B
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AA
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AA
AA
DTMF
AA A A AA
AA AA A A A A A A AA AA AA
AA
AA
A
AA
AA
AA
AA
AA
AA
AA
A
AA
AA
AA
AA
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AA
FREQUENCY (Hz)
AA
AA
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A
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A
A
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A
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the
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AAA
AAA
ISO
AA
AA AA
AA
AA AA
AA
AA
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AA
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AAA
AA
AA
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AAA
C
AA
AA
AA
AA AA AA AA AA
AA AA AA AA
AA
AA
AA
AA AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AA
AAA
AA
AA
AAA
AA
AA
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AA
AA
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AA
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AAA
AA
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AAA
AAA
AA
AA
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AA
AA
AA
AA AA AA
AA
AAA
AA AA AA AA AA
AAA
AAA
AAA
AAA
AAA
AA AA AA
AA
AA
AA
AA AA AA AA
AA
AA
AA
AA
AA
AA
AA
AAA
AAA
AA
AA
AA
AA
AAA
AAA
AAA
AAA
AAA
AAA
AAA
AAA
AA
AA
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AA
AAA
AA
AA
AAA
AA
AA
AA AA AA
AA
AA AA AA AA AA AA
AA AA AA
AA AA
AA
AA
AA
AA
D
AAA AA AA
AAA
AAA AA
AA
AAA
AAA
AAA
AA AA AA
AA AA AA AA AA
AA AA
AAA
AA
AA
AA
AA
AA
AA AA
AA
AA
AA
AA AA AA AA AA AA AA AA
AA AA
AA
AA
AA
AA
AAA AA AA AA AA AA AA AA AA
AA A A AA
AAA
AAA
AA
AAA
AAA
AA AA AA AA AA AA
AA
AA
AA
AA AA AA AA
AA
AA
AA
AA
AA
AA AA AA
AA
2
AAA AA AA AA AA AA AA AA AA AA
AAA AA
AA
AAA
AAA
AAA
AA AA AA AA AA AA
AA AA AA
AAA
AAA
AA
AA AA AA
1kHz
-CMOS
AAA AA AA AA AA AA AA AA
AAA
AA AA AA AA AA AA AA AA AAA
AA AA AA AA
AAA
AAA
AA
AA AA
providing tolerance to small frequency deviations
and variations. This averaging algorithm has been
developed to ensure an optimum combination of
immunity to talk-off and tolerance to the presence of
interfering frequencies (third tones) and noise. When
the detector recognizes the presence of two valid
tones (this is referred to as the “signal condition” in
some industry specifications) the “Early Steering”
(ESt) output will go to an active state. Any
subsequent loss of signal condition will cause ESt to
assume an inactive state (see “Steering Circuit”).
Steering Circuit
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 v
rise as the capacitor discharges. Provided signal
AAA AA AA
AAA AA AA AA AA AA AA AA
AAA
AAA
AA AA AA AA AA AA AA AA AA AA AA AAA
AAA
AAA
AAA
AA
AA AA AA
AA
AAA AA AA AA AA AA AA AA AA AA AA AA AA AA
AAA
AA AA AA AA AA AA AA AA AA AA AAA
AA AA AA AA A A
AAA
AAA
AAA AA AA AA AA AA AA AA
AA
AAA
AA AA AA AA A A
AAA
AA AA AA AA AAA
AAA
AA
AAA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AAA
AA AA AA AA AA AA AA AA AA AA AA AAA
AAA
AAA
AAA
AA
AA AA AAA
AAA
AAA
AA
AA AA AA AA AA
V
AAA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AAA
AAA
AAA
AAA
AAA
AA AA AA AA AA AA
AAA AA AA AA AA
AA AA AA AAA
AA AA AAA
AAA
AAA
AAA
MT8870D-1
AAA AA AA AA AA AA AA AA AA AA AA AA AA AA AA A A AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AAA
AA AA AA AA AA AA
AA AA AA AA AA AA A A
AA AA AA AA AA AA
AAA
DD
MT8870D/
AAAA AAAA
AAAA
E
AAAA AAAA
St/GT
F
V
ESt
StD
Figure 4 - Basic Steering Circuit
DD
AAAA AAAA
MT8870D/MT8870D-1
G
AAAA AAA
AAA AA AA
AA AA
A A A
AA
AA AA A
AA AA AA AA AA
AA AA AA AA AA AA A A AA AA AA AA AA AA A A
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
H
AA AA AA AA AA AA A A
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AAA
AA AA AA AA AA AA A A
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AAA
AA AA AA AA AA AA AA AA AA AA AA AA
R
AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA
AA AA AA AA AA AA AA AA AA AA AA AA
t
t
AA
AAA
AA AA AA AA AA AA AA AA AA AA
AAA
GTA
GTP
AAA
AAA
AAA AA AA AA AA AA AA AA AA
AAA
AAA
AAA
AAA
AAA
AAA
AA AA AA
AA AA AA
AAA
AAA AA AA AA AA AA AA
AAA
AAA
=(RC)In(V
=(RC)In[V
AA AA
AA AA AA AA AA AA AA AA AA AA AA AA
AA AA
AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA AA
AAA
AAA AA AA AA AA AA AA
AAA
AAA
v
C
AA AA AA
AA AA AA
AA AA AA AA AA AA AA
AA AA
c
c
(see Figure 4) to
PRECISE
DIAL TONES
X=350 Hz
Y=440 Hz
DTMF TONES
A=697 Hz
B=770 Hz
C=852 Hz
D=941 Hz
E=1209 Hz
F=1336 Hz
G=1477 Hz
H=1633 Hz
DD
DD
/V
/(V
TSt
DD
)
-V
TSt
)]
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