Advance Information
Pin Description
Pin #
20
20
24
24
8,9
16,
17
MT88L89
28
28
3,5,
10-11
16
23-
25
Name
V
DD
NC
Positive power supply (3V typ.).
No Connection.
Description
Functional Description
The MT88L89 Integrated DTMF Transceiver consists
of a high performance DTMF receiver with an
internal gain setting amplifier and a DTMF generator,
which employs a burst counter to synthesize precise
tone bursts and pauses. A call progress mode can
be selected so that frequencies within the specified
passband can be detected. The adaptive micro
interface allows microcontrollers, such as the
68HC11, 80C51 and TMS370C50, to access the
MT88L89 internal registers.
Receiver Section
Separation of the low 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 (see Table 1). The filters
also incorporate notches at 350 Hz and 440 Hz for
exceptional dial tone rejection. 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.
MT88L89
IN+
IN-
Power Down
The MT88L89 provides enhanced power down
functionality to facilitate minimization of supply
current consumption. DTMF transmitter and receiver
circuit blocks may be independently powered down
via register control. When asserted, RxEN control
bit powers down all analog and digital circuitry
associated solely with the DTMF and Call Progress
receiver. The TOUT control bit is used to disable the
transmitter and put all circuitry associated only with
the DTMF transmitter in power down mode. With the
TOUT control bit asserted, the TONE output pin is
held in a high impedance (floating) state. When both
power down control bits are asserted, circuits utilized
by both the DTMF transmitter and receiver are also
powered down. This includes the crystal oscillators,
and the V
Ref
generator. In addition, the IRQ , TONE
output and DATA pins are held in a high impedance
state.
C
R
IN
R
F
VOLTAGE GAIN
(A
V
) = R
F
/ R
IN
GS
V
Ref
Figure 3 - Single-Ended Input Configuration
MT88L89
C1
R1
IN+
IN-
C2
R4
R5
GS
R3
R2
V
Ref
DIFFERENTIAL INPUT AMPLIFIER
C1 = C2 = 10 nF
R1 = R4 = R5 = 100 kΩ
R2 = 60kΩ, R3 = 37.5 kΩ
R3 = (R2R5)/(R2 + R5)
VOLTAGE GAIN
INPUT IMPEDANCE
(A
V
diff) - R5/R1
(Z diff) = 2 R1
2
+ (1/ωC)
2
IN
Input Configuration
The input arrangement of the MT88L89 provides a
differential-input operational amplifier as well as a
bias source (V
Ref
), which is used to bias the inputs at
V
DD
/2. 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 Figure 3.
Figure 4 shows the necessary connections for a
differential input configuration.
Figure 4 - Differential Input Configuration
4-127
MITEL [ MITEL NETWORKS CORPORATION ]
