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How to crack mt8870 dtmf

How to crack mt8870 dtmf


The mt8870 is a popular DTMF decoder integrated circuit. DTMF stands for Dual Tone Multi-Frequency and is used in telephone systems for dialing and signaling. The mt8870 can detect and decode all 16 DTMF tone pairs into a 4-bit digital output. This allows microcontrollers and other devices to interface with telephone systems and interpret keypad presses.

However, in some cases, you may need to “crack” or gain access to the decoding functionality of an mt8870 without authorization. This article provides technical information on understanding and exploiting the mt8870 DTMF decoder. We cover the mt8870 internal architecture, communication protocols, decoding techniques, and potential methods to override the tone detection and output functions.

Keyword: mt8870 dtmf

MT8870 Internal Architecture

The mt8870 consists of a bandsplit filter section, row decoder, column decoder, timing logic, and digital output latch.

Bandsplit Filters

  • Separate the high and low tone frequencies
  • High pass and low pass filter the tones
  • Removes harmonics and noise

Row Decoder

  • Detects the amplitude of low tones
  • Identifies 1 of 4 frequencies: 697 Hz, 770 Hz, 852 Hz, 941 Hz

Column Decoder

  • Detects the amplitude of high tones
  • Identifies 1 of 4 frequencies: 1209 Hz, 1336 Hz, 1477 Hz, 1633 Hz

Timing Logic

  • Validates tone durations
  • Sets decoding time windows
  • Prevents falsing

Digital Output Latch

  • Encodes the row and column data into a 4-bit output
  • Represents the decoded DTMF digit

This architecture allows the mt8870 to detect DTMF signals from audio inputs and output the digit value. To crack the mt8870, we need to understand how to manipulate these internal blocks.

Communication Protocols

The mt8870 uses 3 main communication ports – digital input/output, analog input, analog output.

Digital Input/Output

  • 4-bit decoded digit value output
  • Digital control pins like chip enable and latch enable

Analog Input

  • Connect audio signal with DTMF tones
  • Levels from 100 mVpp to 4 Vpp

Analog Output

  • Gain adjusted tone input signal
  • Used for feedback monitoring

To override the behavior, we can manipulate both the analog input tones and digital output values.

Decoding Techniques

The mt8870 uses multiple techniques to decode DTMF signals.

Low Group Filter

  • 6th order bandpass filter
  • Centered on 697 Hz, 770 Hz, 852 Hz, 941 Hz
  • Low tone level envelope detected

High Group Filter

  • 6th order bandpass filter
  • Centered on 1209 Hz, 1336 Hz, 1477 Hz, 1633 Hz
  • High tone level envelope detected

Steering Logic

  • Determines valid tone pair combinations
  • Rejects invalid tones like 941 Hz + 1477 Hz

Timing Logic

  • Tone valid if present between 40 ms and 120 ms
  • Prevents falsing from transients

Output Encoder

  • Row tone value maps to bit 0 and bit 1
  • Column tone value maps to bit 2 and bit 3
  • 4-bit digit output

To override the DTMF decoder, we need to feed tones within the filter bands that pass the steering logic and timing checks.

Methods to Crack mt8870

There are a few potential methods that could be used to crack or exploit the mt8870 DTMF decoder.

1. Tone Generator

  • Use an arbitrary waveform generator to create DTMF tone pairs
  • Bypass any telephone audio input and directly input synthesized tones
  • Could brute force through all tone combinations
  • Requires knowledge of filter bands and timing windows

2. Microcontroller Fuzzing

  • Use a microcontroller to generate randomized DTMF tones
  • Fuzzing technique to throw random inputs at the mt8870
  • Monitor digital output for unexpected values
  • May find flaws in steering logic or timing windows

3. Output Latch Hijacking

  • Use a microcontroller to force digital values to output latch
  • Could override internal decoding circuits
  • Requires debugging to tap into chip latch I/O
  • Can directly set the 4-bit digital output value

4. Packet Sniffing

  • Monitor communication lines between mt8870 and upstream MCU
  • Packet sniff the digital output values
  • Analyze or spoof the digit data packets
  • Requires gaining access to board wiring and buses

Concerns and Issues

While exploiting the mt8870 in unauthorized ways may enable access to functionality, there are also legal and ethical concerns.

  • Gaining access to secured systems could constitute hacking or intrusion
  • Overriding copy protection mechanisms may violate intellectual property laws
  • Social engineering attacks have problematic consent issues
  • Regulatory requirements like telecom standards apply

Any person considering testing or modifying mt8870 behavior should be aware of applicable laws and act in an ethical manner. There are often better alternatives to circumvention like contacting the vendor or using open source options.


This article provided technical background on how the mt8870 DTMF decoder IC functions and some potential methods to override its standard behavior. While interesting academically, actually employing such techniques requires thoughtful awareness of legal and ethical boundaries. There are often better options available to achieve the desired access or functionality through official channels.

Frequently Asked Questions

1. What are the key components inside the mt8870?

The key internal components of the mt8870 are the bandsplit filters, row and column decoders, timing logic, and digital output latch. These blocks allow it to detect DTMF tone frequencies, validate the signals, and output a decoded digit value.

2. What communication protocols does the mt8870 use?

The main communication protocols are the 4-bit digital parallel output, analog audio input, and analog feedback output. Control pins enable digital functions like latching the decoded data.

3. How does the timing logic prevent falsing of codes?

The timing logic ensures a tone pair is present between about 40ms to 120ms. Shorter or longer tones are rejected to prevent random noises or transients from falsely triggering a decoded output.

4. What methods might be used to override the mt8870 functionality?

Potential methods include using a tone generator to brute force tones, microcontroller fuzzing of random inputs, hijacking the output latch pins, or packet sniffing the digital data output.

5. What legal or ethical concerns apply when circumventing the mt8870?

Circumventing secured systems, overriding copy protection, and social engineering may constitute hacking, intellectual property violations, or problematic consent issues. Laws and ethics should be considered.

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