MT8870: A Comprehensive Guide to Mastering DTMF Decoding

The MT8870 is an integrated circuit widely used in the field of telecommunications due to its effective and reliable decoding of DTMF (Dual-Tone Multi-Frequency) tones. These tones are generated by devices such as telephones and radios when buttons are pressed, allowing for data transmission over audio channels. The popularity of the MT8870 derives from its ability to simplify the process of decoding these tones, making it a go-to solution for a range of applications.

In this comprehensive guide, we will delve into the various features and functionalities of the MT8870, looking closely at its technical specifications and the roles they play in its operation. Furthermore, we will explore practical examples of how the MT8870 can be effectively utilized in real-life projects, enhancing our understanding of its versatile nature.

As we progress through the guide, readers will develop a solid understanding of the MT8870 and its practical application in telecommunications systems. This knowledge will serve as a valuable resource whether you are an electronic engineer, a student, or merely someone interested in learning about the fascinating world of DTMF decoding and its applications in modern technology.

Overview of MT8870

The MT8870 is a popular DTMF (Dual-tone Multi-frequency) decoder integrated circuit (IC) used in a variety of telecommunications and control applications. It is designed to convert the audio signals generated by pressing the keys on a telephone keypad into digital output, consisting of a 4-bit binary code for each button pressed.

The main operation of the MT8870 is based on its internal digital counting techniques for detecting and decoding all 16 DTMF tone pairs into a 4-bit code. Its input range can accommodate signals from a standard telephone keypad, as well as other devices that generate DTMF signals.

The IC has several features that make it suitable for a wide range of applications, including:

  • High immunity to false detections due to external noise
  • Low power consumption
  • Wide operating voltage range (2.5V to 5.5V)
  • Adjustable guard time for increased reliability

In terms of interfacing, the MT8870 offers several pins to connect with other components:

  1. IN+ and IN- pins (DTMF input): These pins are connected to the audio source (e.g., telephone line, radio, or other transducers).
  2. GS (Gain Select) pins: They allow users to adjust the input gain, providing flexibility for various input signal levels.
  3. OSC1 and OSC2 pins: External components like a crystal oscillator or ceramic resonator are connected here to provide a stable clock signal.
  4. VDD and VSS pins: The power supply pins connect the IC to power sources, with VDD being the positive supply voltage and VSS being the ground.
  5. Q1-Q4 pins (BCD output): The 4-bit binary-coded decimal (BCD) output is provided through these pins upon DTMF tone detection.
  6. STD (StD) pin: A status indicator that goes HIGH when a valid DTMF tone pair is detected and remains HIGH for the duration of the input signal.
  7. ESt (Early Steering) pin: It outputs a control signal that can be used to activate or inhibit other system components during the decoding process.
  8. TOE (Three-State Output Enable) pin: When enabled (connected to VDD), it allows the BCD output pins (Q1-Q4) to function normally; when disabled (connected to VSS), it sets Q1-Q4 pins to high impedance.

To incorporate the MT8870 into a project, one must simply connect the necessary components as per the datasheet, provide power and a clock signal, and observe the output. This versatile IC can be utilized in numerous applications, such as telephone answering machines, remote control systems, home automation, and more.

Working Principle and Components

Digital Signal Processor

The MT8870 consists of a Digital Signal Processor (DSP) that decodes DTMF signals received from telecommunication devices. The DSP analyses the incoming signal’s frequency components and determines the corresponding DTMF digit.

Steering Circuit

The steering circuit in the MT8870 helps in detecting and validating the presence of a valid DTMF tone. It uses the Early Steering (ES) and Delayed Steering (DS) signals to confirm that a legitimate DTMF tone is present.

  • ES: Activates when a DTMF signal is detected
  • DS: Activates after a certain period, confirming the presence of a valid DTMF tone

Band Split Filter

The band split filter in the MT8870 separates the incoming audio signal into two frequency bands:

  • Low group: 697 Hz, 770 Hz, 852 Hz, 941 Hz
  • High group: 1209 Hz, 1336 Hz, 1477 Hz, 1633 Hz

These two groups ensure accurate decoding of DTMF tones, as each tone consists of a combination of one low and one high-frequency component.


The MT8870’s internal oscillator is responsible for generating the reference clock required for the DSP and other functional blocks. The oscillator’s frequency can be adjusted using an external resistor and capacitor.

  • Resistor (R1): Determines the oscillator’s frequency stability
  • Capacitor (C1): Helps set the desired frequency

Interfacing MT8870 with Microcontroller

Interfacing the MT8870 DTMF Decoder with a microcontroller is a straightforward process that involves connecting the appropriate pins between the two devices. Here’s a step-by-step guide on how to establish this connection:

  1. Supply voltage: Connect the Vcc pin (pin 18) of the MT8870 to the microcontroller’s supply voltage, which is typically 5V. Also, connect the GND (pin 9) of the MT8870 to the microcontroller’s ground.

  2. Input connection: Connect your audio source to the IN+ (pin 2) and IN- (pin 3) pins of the MT8870. The audio source can be a phone line, microphone, or any other audio signal carrying DTMF tones.

  3. Oscillator: Set up an external crystal oscillator between the XT (pin 7) and X1 (pin 8) pins of the MT8870. A 3.579MHz crystal is commonly used for this purpose.

  4. Data output: The MT8870 provides four data output pins (Q1-Q4, pins 11-14) which produce a 4-bit binary representation of the corresponding DTMF tone. Connect these pins to the input pins of your microcontroller, ensuring they’re configured as digital inputs.

  5. Strobe signal: Connect the StD (pin 15) of the MT8870 to an input pin on the microcontroller. This pin indicates when a valid DTMF tone is detected and can be used to trigger an interrupt or initiate a read operation.

Here’s a simple example of how to read DTMF tones using an Arduino microcontroller:

const byte DATA_PINS[] = {2, 3, 4, 5};
const byte STROBE_PIN = 6;

void setup() {

  for (byte i = 0; i < 4; i++) {
    pinMode(DATA_PINS[i], INPUT);

void loop() {
  if (digitalRead(STROBE_PIN) == LOW) { // Check for valid tone
    byte tone = 0;
    for (byte i = 0; i < 4; i++) {
      tone |= (digitalRead(DATA_PINS[i]) << i);

This code continuously checks for valid DTMF tones and outputs their 4-bit binary representation via the serial monitor.

Remember, the MT8870 is just one option for DTMF decoding in your project. There are alternatives available, such as software-based decoding using a microcontroller’s ADC or specialized DTMF decoding ICs with additional features.

MT8870 Applications

The MT8870 is a DTMF (Dual-Tone Multi-Frequency) receiver chip commonly used in various communication systems. This integrated circuit is designed to convert DTMF tones into a digital format, enabling a range of applications. The following are some common uses of the MT8870.

Telephone Systems: The MT8870 is used in telephone systems to detect and process DTMF tones generated by telephone keypads. This function allows for call routing and other telephone functions that rely on DTMF signaling.

Remote Control Systems: Incorporating the MT8870 in remote control systems enables control of devices over long distances using DTMF tones. This is particularly useful for controlling devices like televisions, radios, and home automation systems.

Security Systems: The chip is often employed in security systems as a means of granting access based on a DTMF tone sequence. Users can unlock doors or disable alarms by entering a specific set of numbers on a telephone keypad.

Caller ID Systems: The MT8870 can be used to decode DTMF tones in caller ID systems, allowing users to see the telephone number of an incoming call.

Robotics and automation: DTMF signaling is useful in robotics and automation for controlling devices remotely. The MT8870 can help receive commands and execute specific actions based on the decoded DTMF tones.

Advantages and Limitations

The MT8870 is an integrated DTMF (Dual-Tone Multi-Frequency) receiver that decodes DTMF signals commonly found in telephone systems. In this section, we’ll explore the advantages and limitations of the MT8870.


  • Small and compact: The MT8870 comes in a small package, making it suitable for use in projects with limited space, such as remote control systems and telecommunication devices.

  • Low power consumption: The chip operates at low power, making it a suitable choice for battery-operated devices or systems with energy constraints.

  • Easy system integration: The MT8870 is designed to interface with microcontrollers and other digital systems, making it easy to integrate into a variety of applications.

  • Highly reliable: The chip features built-in band-split filters and digital counting techniques, resulting in accurate and reliable detection of DTMF signals, even in noisy environments.


  • Limited frequency range: The MT8870 can only decode standard DTMF tones, so applications requiring the reception of other frequency ranges will need alternative solutions.

  • Requires external components: Although the chip itself is compact, additional passive components are required for proper operation, which can increase the overall size and complexity of a project.

  • Reduced sensitivity in noisy environments: While the MT8870 is designed to operate well in noisy environments, extremely high noise levels can still impact its ability to accurately decode DTMF signals.

  • Single-tone detection: The MT8870 is optimized for DTMF detection and cannot handle other types of signals, such as single-tone or multi-frequency signals.

In summary, the MT8870 offers a compact and reliable solution for decoding DTMF signals, making it suitable for various applications. However, its limitations in terms of frequency range and signal types require careful consideration when deciding if it’s the right choice for a specific project.

Selecting Alternatives to MT8870

While the MT8870 is a popular DTMF decoder, there are other alternatives on the market that might be better suited for your specific needs. In this section, we will explore some of the top alternatives and what makes them stand out.

One worthy alternative is the HT9170B, a Dual Tone Multi-Frequency (DTMF) receiver from Holtek. Some of its features include:

  • Operating voltage range of 2.5-5.5V
  • High immunity to false signals
  • Power-down mode for conserving energy

Another excellent choice is the TCM5087, a DTMF receiver from Texas Instruments. This decoder offers:

  • Wide operating voltage range (4-10V)
  • Compatible with TTL and CMOS systems
  • Minimal external components required

If you’re looking for options with Microcontrollers, consider integrating DTMF decoding directly into your microcontroller, such as with the Arduino platform. Some of the advantages of this approach include:

  • Reduced need for additional components
  • Easier integration within your existing hardware and software systems
  • Flexibility to implement custom DTMF decoding algorithms

Finally, you can also consider using software-based DTMF decoding solutions. These options typically require a sound input, such as a microphone or sound card, and can be implemented in various programming languages. A few notable examples are:

  • Goertzel Algorithm
  • Fast Fourier Transform (FFT)-based decoding

To sum up, when selecting an alternative to the MT8870, consider factors such as voltage range, compatibility with your existing systems, and whether you prefer a hardware or software-based solution. By weighing the pros and cons of each option, you can make an informed decision that best meets your project requirements.

Final Thoughts

MT8870 chips play an essential role in DTMF signal processing and decoding. Their simplicity and efficiency make them a staple in communication systems and embedded projects.

Understanding of MT8870 operation can be crucial for efficiently utilizing it in various applications. Success comes from implementing appropriate filters and analog-to-digital conversion.

In summary, the MT8870’s key features include:

  • Decoding DTMF tones
  • On-chip oscillator
  • Last number redial
  • Inhibit mode

The MT8870 has a rich history and remains relevant to modern communication systems. As technology progresses, the MT8870 continues to be a robust and reliable choice in decoding DTMF signals.

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