cellphone landrover Chapter 3

7/31/2019 cellphone landrover Chapter 3

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CHAPTER

DTMF DECODER CIRCUIT

Introduction

Cost of development

Working of MT8870

Basic circuit for DTMF decoding

Connecting a Typical TRS headset with mt8870


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DTMF DECODER CIRCUIT

INTRODUCTION

So far DTMF is introduced and fully explained. After knowing much about

DTMF now we are ready to introduce a popular IC mt8870 developed by Mitel Corporation.

This IC is used in decoding DTMF signal transmitted over the network. There are several

applications of this IC among which following are important:

Receiver system for British Telecom (BT) or CEPT Spec

Paging systems

Repeater systems/mobile radio

Credit card systems

Remote control

Personal computers

Telephone answering machine

This project is based on the application of remote controlling using mobile

phones. In this chapter functioning, construction and practical implementation of the IC is

discussed. Problems encountered during construction of this circuit have also been dealt in

what follows.

Cost of development:

Cost of the above IC is approximately 50 per IC. Total costing of the circuit

(if implemented on breadboard) is expected to be 80*, however due to our relatively newness

towards this it costs us more. If implemented on PCB it would cost about 120* including

PCB designing cost. Thus it is seen that this circuit is very cost effective as compared to costly

RF devices and also quite suitable for home appliances control.

*Cost excluding Mobile phone and corresponding accessories.
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Working of MT8870

The MT8870is a complete DTMF receiver integrating both the band split filter

and digital decoder functions. The filter section uses switched capacitor techniques for high

and low group filters; the decoder uses digital counting techniques to detect and decode all 16

DTMF tone-pairs into a 4-bit code. There is another DTMF decoder IC available in market that

is ht9170, which was quite popular several years ago but now is being replaced by MT8870

due to following features:

Complete DTMF Receiver

Low power consumption

Internal gain setting amplifier

Adjustable guard time

Central office quality

Power-down mode

Inhibit mode

Pin configuration:

Pin diagram of MT8870 is given below and also pin description is given in

what follows:

Fig. Pin configuration of MT8870

As seen above there are two variants of MT8870 are available in market.

For this project we used 18 Pin plastic DIP IC because, this is the one we can find easily.


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Pin description of this IC is given below:

*Pin description as given in Zarlink semiconductor Inc. mt8870 datasheet


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Functional diagram and internal architecture of MT8870:

Fig. Functional diagram of MT8870

Functional description

The MT8870 monolithic DTMF receiver offers small size, low power

consumption and high performance. Its architecture consists of a band split 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.

The MT-8870 is a full DTMF Receiver that integrates both band split filter and

decoder functions into a single 18-pin DIP. 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 interface bus. Minimal external components required include a

low-cost 3.579545 MHz crystal, a timing resistor, and a timing capacitor. The MT-8870-02 can

also inhibit the decoding of fourth column digits. MT-8870 operating functions include a band


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split 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.The low and high group tones are separated by applying the dual-tonesignal to the inputs of two 6th order switched capacitor band pass 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 rejection. Each filter output is followed by a single-order switched capacitor section that

smoothes 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

MT-8870 decoder uses a digital counting technique to determine the frequencies of the limited

tones and to verify that they correspond to standard DTMF frequencies. When the detector

recognizes 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.

Before a decoded tone pair is registered, the receiver checks for valid signal duration (referred

to as character- recognition-condition). This check is performed by an external RC time

constant driven by ESt.

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 logic high. 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. On

the M- 8870 models, this pin is tied to ground (logic low).The input arrangement of the MT-

8870 provides a differential input operational amplifier as well as a bias source (VREF) 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. The internal clock circuit is completed with the addition of a

standard 3.579545 MHz crystal. The input arrangement of the MT-8870 provides a differential

input operational amplifier as well as a bias source (VREF) 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. The internal clock circuit is completed with the addition of a standard 3.579545

MHz crystal. The function of all the pins have already been discussed and now only state of

pins corresponding to input is shown.


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Functional decode Table

Note: L=LOGIC LOW,

H=LOGIC HIGH,

Z=HIGH IMPEDANCE

X = DONT CARE

Basic circuit for DTMF decoding


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Basic circuit for DTMF decoding can be understood using the pin description of

MT8870. The only thing is to be remembered is frequency of crystal oscillator, which should

be 3.579545 MHz. The internal oscillator circuit is completed only with the given value of

crystal oscillator. Basic circuit for DTMF decoding is shown in next page:

Fig.: Single Ended Input configuration for DTMF Decoding

The circuit shown above is not concerned with unbalanced loading. If

unbalanced loading is concerned two capacitors each of 30pf is two be connected.

Actual size PCB of the circuit used for our project is also shown below:


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Fig.: Timing diagram of MT8870


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Connecting a Typical TRS headset with mt8870

What is TRS Headset

In this project we used 3.5mm pin headset with three wires, which are TRS and

are explained below:

T: Tip: Tip is the wire that goes to left earphone. Usually blue or green colour is

given to Tip.

R: Ring: Ring is the wire that connects to right earphone. Usually Red colour is

adopted for Ring wire.

Colour convention described above is still controversial because some

manufacturers use blue for Ring and Red for Tip. So it is not a quite good idea to make any

conclusion by looking at the colours of wire rather one should always refer the practice of

looking right or left wire so that actual status can be found.

S: Sleeve: Sleeve is the wire that works as reference level or ground. The colour

of Sleeve is generally brown and is opted by most of the manufacturers. Sleeve wire goes to

both of the earphones viz. right and left along with Tip and Ring.

The reason behind using TRS 3.5mm headset is the availability of this headset

and most of the group members having smart phones that is equipped with 3.5mm audio jack.Another reason is the cost consideration. Since we had to test with several headsets many times

it was a bad idea to test with a headset that is too much costly and is rarely found. Cost of the

headset used in this project is approximately 80 and is compatible with most of the smart

phones available today in the market which widens the scope of this project.

Connecting a typical TRS Headset to input seems to be quite simple thing but,

when we tried it in practice this was quite cumbersome task. This was because we had no idea

about the structure of such headset and soldering was also difficult because it was mentioned

by some online communities that the wires of headset is one of the thinnest wire in the world.

But, thanks to HOD, Comp. Science and Engg. dept. that he told us how to use and solder such

type of wires. We cut the headset from its mid point and then what we get was three coloured

wires and one stranding. Following are the steps for soldering such wires:
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Cut the wire of headset either from midpoint or nearby the earphones

as per requirement. If Tip and Ring have to be used separately then second

approach is better because this will give two wires i.e. Tip and Ring

separately. In this project we didnt need separate Tip and Ring so we used

first approach.

After that the wire is stripped for approx. 1in so we get 4 cables

including nylon stranding.

Once Tip, Ring and Sleeve is identified we burn the insulation of all

three cables failing to which there would be no identification of tone and

project will not run. For burning the insulation we simply pass it over a

burning matchstick quickly. The above process should be done quickly

otherwise the conductor cable will also melt after which above steps have to

be repeated.

Once three cables are obtained we solder the Tip and Ring together to the

DTMF input as shown in circuit diagram and connect sleeve to the ground

of the circuit.

After above process is done we test the continuity between DTMF Input and

the lead of the headset and also between the leads bottom part and ground

of the circuit.

Once all the components are mounted on PCB and Headset is successfully

connected to the designed circuit we connect 9V battery to input of 7805 IC which after

regulating gives 4.88V input as VDD to the IC MT8870. For testing purpose only we

connected four LED at the output pins using breakaway headers.

Now it is time to test the DTMF decoder circuit. We tested this circuit using

several mobile phones that are LG P500 (Optimus One), Nokia 6300 and Nokia 3610C. Each

of the phone was tested separately. First LG P500 was tested. We connected headset to the

audio jack of slave mobile phone (LG P500) and made a call to slave from master or controller

mobile phone (We used Nokia 3230 as master). Then key from 0 to 9, * and # were pressed

one by one and we get the output from LED as given in Functional Decode Table.


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On this project we can mount slave mobiles on auto answer mode so that we do

not have any tension about receiving calls manually. One thing that is to be remembered is the

keypad tones of master mobile phone should be on and at full volume setting or otherwise

DTMF tones shall not be transmitted over network and project will not work properly.

Conclusion

Since we are done with DTMF decoder circuit its now time to approach to

motor driving circuit and motor operation. The motor driver circuit is thoroughly explained in

next section.

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cellphone landrover Chapter 3