MITEL MT8888CS, MT8888CN Datasheet

4-91



Features

• Central office quality DTMF transmitter/receiver

• Low power c onsu mpt ion

• High speed Intel micro interface

• Adjustable guard time

• Automat ic tone bu rst mo de

• Call prog ress ton e det ection t o -30dBm

Applications

• Credit card systems

• Paging systems

• Repeater systems/mobile radio

• Interconn ect di alers

• Persona l comp uters

Description

The MT8888C is a monolithic DTMF transceiver with call progress filter. It is fabricated in CMOS technology offering low power consumption and high reliability.

The receiver section is based upon the industry standard MT8870 DTMF receiver while the transmitter utilizes a switched capacitor D/A converter for low distortion, high accuracy DTMF signalling. Internal counters provide a burst mode such that tone bursts can be transmitted with precise timing. A call progress filter can be selected allowing a microprocessor to analyze ca ll progress tones.

The MT8888C utiliz es an Intel micro int erface, which allows the device to be connected to a number of popular microcontrollers with minimal external logic. The MT8888C-1 is functionally identical to the MT8888C except the receiver is enhanced to accept lower level signals, and also has a specified low signal rejection level.

Ordering Information

MT8888CE/CE-1 20 Pin Plastic DIP MT8888CC/CC-1 20 Pin Ceramic DIP MT8888CS/CS-1 20 Pin SOIC MT8888CN/C N-1 24 Pin SSOP

-40°C to +85°C

Figure 1 - Functional Block Diagram

TONE

IN+

IN-

OSC1 OSC2

DDVRefVSS

ESt St/GT

D0 D1 D2 D3

IRQ

/CP

RD CS R/W RS0

∑

D/A

Converters

Row and

Column

Counters

Transmit Data

Data

Bus

Buffer

Tone Burst

Gating Cct.

Oscillator

Circuit

Bias

Circuit

Control

Logic

Digital Algorithm and Code Converter

Control

Logic

Steering

Logic

Status

Control

Receive Data

Interrupt

Logic

I/O

Control

Low Group

Filter

High Group

Filter

Dial Tone Filter

ISSUE 2 May 1995

MT8888C/MT8888C-1

Integrated DTMFTransceiver

with Intel Micro Interface

MT8888C/MT8888C-1

4-92

Figure 2 - Pin Connections

Pin Description

Pin #

Name Description

20 24

11 IN+Non-inverting op-amp input. 22 IN-Inverting op-amp input. 33 GSGain Se le c t. Giv es access to output of front end differential amplif ier fo r connection of

feedback resistor.

44V

Ref

Reference Voltage output (VDD/2).

55V

Ground (0V). 66OSC1Oscillator input. This pin can also be driven directly by an external clock. 77OSC2Oscillator output. A 3.579545 MHz crystal conn ected between OSC1 and OSC2 completes

the internal oscillat or circui t. Leave open cir cuit when OSC1 is driven externally. 8 10 TONE Output from inte rnal DTMF transmitte r. 911 WR

Write microprocessor input. TTL compatible.

10 12 CS

Chip Se le c t input . Active Low. This signal must be qualified externally by address latch

enable (ALE) signal, se e Figure 12.

11 13 RS0 Register Select input. Refer to Table 3 for bit inte rp retat ion . TTL compat ibl e. 12 14 RD

Read microprocessor input. TTL compatible.

13 15 IRQ

/CPInterrupt Request/ Cal l Progress (open drain) output . In interrupt mode, this output goes

low when a valid DTMF tone burst has been transmitted or received. In call progress mode,

this pin will output a rectangular signal representative of the input signal appli ed at the input

op-amp. The input signal must be wit hin the bandwidt h limit s of the call progress filter, see

Figure 8.

14-1718-21D0-D3 Microprocessor Data Bus. High impedance when CS

= 1 or RD = 1.

TTL compatib le.

18 22 ES t Early Steering out put. Presen ts a logic high once the digita l algorit hm has det ected a valid

tone pair (signal condition ). Any mome ntary loss of signal condition will cause ES t to return

to a logic low.

19 23 St/GT Steering Input/Guard Time output (bidirectional). A voltage greater than V

TSt

detected at St causes the device to register the detected tone pair and update the outp ut latch. A voltage less than V

TSt

frees the device to accept a new tone pair. The GT output acts to reset the

external steering time-const ant; its stat e is a function of ESt and the volt age on St.

20 24 V

Positive power supply (5V typ.).

8,9

16,17

NC No Connection.

1 2

3 4 5

6 7 8 9

20 19 18 17 16 15 14 13

IN+

IN-

VRef

VSS OSC1 OSC2 TONE

R/W

VDD St/GT ESt D3 D2 D1 D0 IRQ

/CP

RD RS0

1 2

3 4 5

6 7 8

9 10 11 12

24 23 22 21 20 19 18 17

IN+

IN-

VRef

VSS OSC1 OSC2

TONE

R/W

VDD St/GT ESt D3 D2 D1 D0 NC NC IRQ

/CP RD RS0

24 PIN SSOP

20 PIN CERDIP/PLASTIC DIP/SOIC

MT8888C/MT8888C-1

4-93

Functional Description

The MT8888C/MT8888C-1 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 Intel micro interface allows microcontrollers, such as the 8080, 80C31/51 and 8085, to access the MT8888C/MT8888C-1 internal registers.

Input Configuration

The input arrangement of the MT8888C/MT8888C-1 provides a differential-input operational amplifier as well as a bias source (V

Ref

), which is used to bias the

inputs at V

/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 Se ction

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). These filters 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 s mooths 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

IN+

IN-

Ref

VOLTAGE GAIN

) = RF / R

MT8888C/ MT8888C-1

Figure 4 - Differential Input Configura tion

0= LOGIC LOW, 1 = LOGIC HIGH

Table 1. Functional Encode/Decode Table

LOW

HIGH

DIGIT D

697 1209 1 0 0 0 1 697 1336 2 0 0 1 0 697 1477 3 0 0 1 1 770 1209 4 0 1 0 0 770 1336 5 0 1 0 1 770 1477 6 0 1 1 0 852 1209 7 0 1 1 1 852 1336 8 1 0 0 0 852 1477 9 1 0 0 1 941 1336 0 1 0 1 0 941 1209 * 1 0 1 1 941 1477 # 1 1 0 0 697 1633 A 1 1 0 1 770 1633 B 1 1 1 0 852 1633 C 1 1 1 1 941 1633 D 0 0 0 0

IN+

IN-

Ref

MT8888C/

DIFFERENTIAL INPUT AMPLIFIER

C1 = C2 = 10 nF R1 = R4 = R5 = 100 kΩ R2 = 60kΩ, R3 = 37.5 kΩ R3 = (R2R5)/(R2 + R5)

VOLTAGE GAIN

diff) - R5/R1

INPUT IMPEDANCE

diff) = 2 R12 + (1/ωC)

MT8888C-1

MT8888C/MT8888C-1

4-94

Following the filter section is a decoder employing digital counting techniques to determine the 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 vo ice while 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 inactiv e state.

Steering Circuit

Before registration of a decoded tone pair, the receiver checks f or a v alid 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

(see Figure 5) to rise as the capacitor discharges. Provided that the signal condition is maintained (ESt remains high) for the validation period (t

GTP

), vc reaches the threshold

TSt

) of the steering logic to register the tone pair, latching its corresponding 4-bit code (see Table 1) into the Receive D ata Regist er. At this point the GT output is activated and drives v

to VDD. GT continues to drive high as long as ESt remains high. Finally, after a short de lay to a ll ow th e o ut put latch to settle, the delayed steering output flag goes high, signalling that a received tone pair has been registered. The status of the delayed steering flag can be monitored 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 s t e er ing flag is a cti v e.

The contents of the output latch are updated on an active delayed steering transition. This data is presented to the four bit bidirectional 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.

Figure 5 - Basic Steering Circuit

Guard Time Adjustment

The simple steering circuit shown in Figure 5 is adequate for most applications. Component values are chosen according to the following inequalities (see Figure 7):

REC

≥ t

DPmax+tGTPmax

- t

DAmin

REC

≤ t

DPmin+tGTPmin

- t

DAmax

tID ≥ t

DAmax+tGTAmax

- t

DPmin

tDO ≤ t

DAmin+tGTAmin

- t

DPmax

The value of tDP is a device parameter (see AC Electrical Characteristics) and t

REC

is the minimum signal duration to be recognized by the receiver. A value for C1 of 0.1 µF is recommended for most

Figure 6 - Guard Time Adjustment

St/GT

ESt

MT8888C/

GTA

= (R1C1) In (VDD / V

TSt

)

GTP

= (R1C1) In [VDD / (VDD-V

TSt

)]

MT8888C-1

St/GT

ESt

St/GT

ESt

GTA

= (R1C1) In (VDD/V

TSt

)

GTP

= (RPC1) In [VDD / (VDD-V

TSt

)]

= (R1R2) / (R1 + R2)

GTA

= (RpC1) In (VDD/V

TSt

)

GTP

= (R1C1) In [VDD / (VDD-V

TSt

)]

= (R1R2) / (R1 + R2)

a) decreasing tGTP; (tGTP < tGTA)

b) decreasing tGTA; (tGTP > tGTA)

MT8888C/MT8888C-1

4-95

applications, leaving R1 to be selected by the designer. Different steering arrangements may be used to select independent tone present (t

GTP

) and

tone absent (t

GTA

) guard times. This may be necessary to meet system specifications which place both accept and reject limits on tone duration and interdigital pause. Guard time adjustment also allows the designer to tailor system parameters such as talk off and noise imm unity.

Increasing t

REC

improves talk-off performance since it reduces the probability that tones simulated by speech will maintain a valid signal condition long enough to be registered. Alternatively, a relatively short t

REC

with a long 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 adjustment is shown in Figure 6. The receiver timing is shown in Figure 7 with a description of the events in Figure 9.

Call Progress Filter

A call progress mode, using the MT8888C/ MT8888C-1, 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 CP

mode has been selected. DTMF signals cannot be detected if CP mode has been selected (see Table

7). Figure 8 indicates the useful detect bandwidt h of the call progress filter. Frequencies presented to the input, 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 squarewave 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 the IRQ

/CP pin will remain low.

Figure 8 - Call Progress Response

AAA

AAAA

AAA

AAAA

LEVEL

(dBm)

FREQUENCY (Hz)

-25

0 250 500 750

= Reject = May Accept = Accept

Figure 7 - R ece iver Tim ing Diag ram

ESt

St/GT

-RX

Read Status Register

IRQ

/CP

EVENTS

ABCDEF

REC

TONE #n

TONE #n + 1

GTP

GTA

PStRX

PStb3

DECODED TONE # (n-1)

# n

# (n + 1)

TSt

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