cellphone landrover chapter 3
TRANSCRIPT
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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.