DTMF decoding and controller design

This circuit detects the dial tone from a telephone line and decodes the keypad pressed on the remote telephone. The dial tone we heard when we pick up the phone set is call Dual Tone Multi-Frequency, DTMF in short. The name was given because the tone that we heard over the phone is actually make up of two distinct frequency tone, hence the name dual tone. The DTMF tone is a form of one way communication between the dialer and the telephone exchange. A complete communication consist of the tone generator and the tone decoder. In this article, we are using the IC MT8870DE as the brain of this device to decode the input dial tone to 5 digital outputs. These digital bits can be interfaced to a computer or microcontroller for further application like remote control, phone line transfer operation, LED sequencers, etc.

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In the early days, our phone system used to be operated by human operator in a telephone exchange room. The caller picks up the phone, give instructions to the operator to connect their line to the destination over the other end of the telephone. As more and more people find phone technology a useful communication tool, line connection operated by human operators has become a tedious task.

As technology matures, pulse/dial tone method was inverted for telephony communication. It uses electronics and computer to assist in the phone line connection. Basically on the caller side, it is a dial tone generator. When a key is being pressed on the matrix keypad, it generate a unique tone consisting of two audible tone frequency. For example, if the key ‘1’ is being press on the phone, the tone you hear is actually consist of a 697 Hz & 1209 Hz sine signal. Pressing key ‘9’ will generate the tone form by 852 Hz & 1477 Hz. The frequency use in the dial tone system is of audible range suitable for transmission over the telephone cable.

On the telephone exchange side, it has a decoder circuit to decode the tone to digital code. For example, the tone of 941 Hz + 1336 Hz will be decoded as binary ‘1010’ as the output. This digital output will be read in by a computer, which will then act as a operator to connect the caller’s telephone line to the designated phone line. The telephone exchange center will generate a high voltage signal to the receiving telephone, so as to ring the telephone bell, to notified the receiving user that there is an incoming call.

This project article focus on a simple DTMF (dual tone multi-frequency) decoder circuit. This circuit can be interface to a computer, allowing caller to computer interaction. Many communication application can be build for example, a computerize call receiving/diverting phone network system. Remote control to Home/Office electrical appliances using a mobile phone network.

Detection of dial tones is reflected on the bit TOE, while the output Q4, Q3, Q2, Q1 indicate the dial tone that is being detected on the telephony system.

Decoder table  :

Digit      TOE        INH     EST  Q4     Q3  Q2   Q1

1                H          X          H      0       0      0      1

2               H          X           H      0      0        1      0

3               H          X          H       0      0         1        1

4               H         X           H     0     1         0        0

Output Logic behavior from the DTMF decoder IC. :

When no button pressed
TOE: Logic 0
Q4: Logic 0
Q3: Logic 0
Q2: Logic 0
Q1: Logic 0

When ‘1’ pressed and hold
TOE: Logic 1
Q4: Logic 0
Q3: Logic 0
Q2: Logic 0
Q1: Logic 1

release from button ‘1’
TOE: Logic 0
Q4: Logic 0
Q3: Logic 0
Q2: Logic 0
Q1: Logic 1

When ‘2’ pressed and hold
TOE: Logic 1
Q4: Logic 0
Q3: Logic 0
Q2: Logic 1
Q1: Logic 0

release from button ‘2’
TOE: Logic 0
Q4: Logic 0
Q3: Logic 0
Q2: Logic 1
Q1: Logic 0

‘0’ press and hold
TOE: Logic 1
Q4: Logic 1
Q3: Logic 0
Q2: Logic 1
Q1: Logic 0

       Pin Description:

dtmf

 

  1.  IN+ Non-Inverting Op-Amp (Input).
  2.  IN- Inverting Op-Amp (Input).

 

  1. GS Gain Select. Gives access to output of front end differential amplifier for connection of feedback resistor.

 

  1. V-Ref Reference Voltage (Output). Nominally VDD/2 is used to bias inputs at          mid-rail .

 

  1. INH Inhibit (Input). Logic high inhibits the detection of tones representing characters A, B, C and D. This pin input is internally pulled down.

 

  1. PWDN Power Down (Input). Active high. Powers down the device and inhibits the oscillator. This pin input is internally pulled down.

 

  1. OSC1 Clock (Input).

 

  1. OSC2 Clock (Output). A 3.579545 MHz crystal connected between pins OSC1 and OSC2 completes the internal oscillator circuit.

 

  1. VSS Ground (Input). 0 V typical.

 

  1. TOE Three State Output Enable (Input). Logic high enables the outputs Q1-Q4. This pin is pulled up internally.

   11-14. Q1-Q4 Three State Data (Output). When enabled by TOE, provide the code corresponding to the last valid tone-pair received (see Table 1). When TOE is logic low, the data outputs are high impedance.

  1. StD Delayed Steering (Output).Presents a logic high when a received tone-pair has been registered and the output latch updated; returns to logic low when the voltage on St/GT falls below VTSt.
  2.  ESt Early Steering (Output). Presents a logic high once the digital algorithm has detected a valid tone pair (signal condition). Any momentary loss of signal condition will cause ESt to return to a logic low.

    17.    St/GT Steering Input/Guard time (Output) Bidirectional. A voltage greater than VTSt detected at St causes the device to register the detected tone pair and update the output latch. A voltage less than VTSt frees the device to accept a new tone pair. The GT output acts to reset the external steering time-constant; its state is a function of ESt and the voltage on St.

  1.  VDD Positive power supply (Input). +5 V typical
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