How to Use DTMF Decoder: Examples, Pinouts, and Specs

The MT8870, manufactured by Mitel Networks Corporation, is a DTMF (Dual-Tone Multi-Frequency) Decoder IC designed to detect and decode the audio tones generated by telephone keypads. It converts these tones into a 4-bit binary output, making it ideal for telecommunication systems, remote control applications, and interactive voice response (IVR) systems. The MT8870 is widely used in projects requiring tone-based signaling and control.

• Telephone-based remote control systems
• Interactive voice response (IVR) systems
• Caller ID systems
• Security systems with tone-based access
• Robotics and automation projects

• Operating Voltage: 4.75V to 5.25V DC
• Current Consumption: 3mA (typical)
• Input Signal Range: -35 dBm to +3 dBm
• Frequency Detection: Standard DTMF frequencies (low group: 697 Hz to 941 Hz, high group: 1209 Hz to 1633 Hz)
• Output: 4-bit binary code corresponding to the detected DTMF tone
• Operating Temperature: -40°C to +85°C
• Package Type: 18-pin DIP or SOIC

The MT8870 has 18 pins, each serving a specific function. The table below provides a detailed description of each pin:

1. Power Supply: Connect the VDD pin to a regulated +5V DC supply and the VSS pin to ground.
2. Input Signal: Feed the DTMF signal to the IN+ pin. If using differential input, connect the complementary signal to IN-. For single-ended input, connect IN- to ground.
3. Oscillator: Connect a 3.579545 MHz crystal between OSC1 and OSC2. Add two 22pF capacitors to ground for stability.
4. Output: The decoded 4-bit binary output is available on Q1 to Q4. Use the STD pin to check if a valid tone pair is detected.
5. Steering Circuit: Connect a resistor and capacitor to the St/GT pin to set the tone detection guard time. Typical values are 300kΩ and 0.1µF.

• Ensure the input signal is within the specified range (-35 dBm to +3 dBm) for accurate decoding.
• Use a low-noise power supply to avoid interference with the DTMF detection.
• The TOE pin can be used to enable or disable the binary output. When TOE is high, the output pins are in a high-impedance state.
• For applications requiring low power consumption, use the PWDN pin to put the IC into power-down mode.

Below is an example of how to interface the MT8870 with an Arduino UNO to read the decoded DTMF tones:

// Pin definitions for MT8870 connections
#define Q1 2  // Connect Q1 to Arduino digital pin 2
#define Q2 3  // Connect Q2 to Arduino digital pin 3
#define Q3 4  // Connect Q3 to Arduino digital pin 4
#define Q4 5  // Connect Q4 to Arduino digital pin 5
#define STD 6 // Connect STD to Arduino digital pin 6

void setup() {
  // Set MT8870 output pins as inputs
  pinMode(Q1, INPUT);
  pinMode(Q2, INPUT);
  pinMode(Q3, INPUT);
  pinMode(Q4, INPUT);
  pinMode(STD, INPUT);

  // Initialize serial communication for debugging
  Serial.begin(9600);
}

void loop() {
  // Check if a valid DTMF tone is detected
  if (digitalRead(STD) == HIGH) {
    // Read the 4-bit binary output
    int dtmfCode = (digitalRead(Q4) << 3) | (digitalRead(Q3) << 2) |
                   (digitalRead(Q2) << 1) | digitalRead(Q1);

    // Print the decoded DTMF code
    Serial.print("DTMF Code: ");
    Serial.println(dtmfCode, DEC);

    // Wait for the tone to end
    while (digitalRead(STD) == HIGH);
  }
}

1. No Output Detected:

   • Ensure the input signal is within the specified range (-35 dBm to +3 dBm).
   • Verify the crystal oscillator is functioning correctly (check connections to OSC1 and OSC2).
   • Check the power supply voltage (should be 5V DC).

2. Incorrect Decoding:

   • Verify the resistor and capacitor values connected to the St/GT pin.
   • Ensure the input signal is not distorted or noisy.

3. Output Pins Always High or Low:

   • Check the TOE pin. If it is high, the output pins will be in a high-impedance state.
   • Verify the connections to the Q1-Q4 pins.

1. Can the MT8870 decode custom frequencies?

   • No, the MT8870 is designed to decode standard DTMF frequencies only.

2. What is the purpose of the STD pin?

   • The STD pin indicates when a valid DTMF tone pair is detected. It can be used to trigger external circuits or microcontrollers.

3. Can the MT8870 operate at voltages other than 5V?

   • No, the MT8870 requires a regulated 5V DC supply for proper operation.

4. How do I adjust the tone detection timing?

   • The tone detection timing can be adjusted by changing the resistor and capacitor values connected to the St/GT pin.