MT8885AP datasheet(3/20 Pages) MITEL

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Advance Information

MT8885

4-53

Functional Description

The MT8885 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

the

68HC11, 80C51 and TMS370C50, to access the

MT8885 internal registers.

Power Down

The

MT8885

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, the 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 VRef

generator. In addition, the IRQ , TONE output and

DATA pins are held in a high impedance state.

Finally, the whole device is put in a power down state

when the PWDN pin is asserted.

Input Configuration

The input arrangement of the MT8885 provides a

differential-input operational amplifier as well as a

bias source (VRef), which is used to bias the inputs at

VDD/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.

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.

Figure 3 - Single-Ended Input Configuration

Figure 4 - Differential Input Configuration

RIN

IN+

IN-

VRef

VOLTAGE GAIN

(AV) = RF / RIN

MT8885

IN+

IN-

VRef

DIFFERENTIAL INPUT AMPLIFIER

C1 = C2 = 10 nF

R1 = R4 = R5 = 100 k

Ω

R2 = 60k

Ω, R3 = 37.5 kΩ

R3 = (R2R5)/(R2 + R5)

VOLTAGE GAIN

(AV diff) - R5/R1

INPUT IMPEDANCE

(ZINdiff) = 2 R1

2 + (1/

ωC)2

MT8885

8,9

3,5,

10,11

16,

20,

No Connection.

Pin Description

Pin #

Name

Description

MT8885AP Datasheet(PDF) 3 Page - Mitel Networks Corporation

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