Introduction to the MT8870
The MT8870 is a DTMF receiver IC manufactured by various semiconductor companies, including Mitel, Zarlink, and others. It is designed to decode DTMF tones, which are the audible tones generated when pressing keys on a telephone keypad or other tone-generating devices.
DTMF signaling is widely used in telecommunications, remote control systems, and automated response systems. Each key on a standard telephone keypad generates a unique combination of two tones, one from a low-frequency group and one from a high-frequency group, allowing the MT8870 to decode the pressed key.
Key Features of the MT8870
The MT8870 offers several key features that make it a popular choice for DTMF decoding applications:
- DTMF decoding: The MT8870 can decode all 16 DTMF tone pairs, corresponding to the digits 0-9, letters A-D, and symbols * and #.
- Binary output: The decoded DTMF tone is presented as a 4-bit binary code on the output pins (Q1, Q2, Q3, Q4) of the IC.
- Input signal conditioning: The MT8870 includes an input amplifier, bandsplit filter, and dial tone rejection circuit to condition the input signal for reliable decoding.
- Adjustable gain: The input gain can be adjusted using an external resistor (GS) to optimize performance for different input signal levels.
- Steering logic output: The StD pin provides a steering logic output signal to indicate the presence of a valid DTMF tone.
- Low power consumption: The MT8870 operates at a low supply voltage (3.5V to 5V) and consumes minimal current, making it suitable for battery-powered applications.
MT8870 Pinout and Pin Description
The MT8870 is available in various package types, with the most common being the 18-pin DIP and 18-pin SOIC. The pinout and pin descriptions are as follows:
| Pin Number | Pin Name | Description |
|---|---|---|
| 1 | IN+ | Positive input (non-inverting) |
| 2 | IN- | Negative input (inverting) |
| 3 | GS | Gain select input |
| 4 | VRef | Voltage reference output |
| 5 | ESt | Early steering output |
| 6 | StD | Delayed steering output |
| 7 | TOE | Three-state output enable input |
| 8 | Q4 | 4-bit binary output (MSB) |
| 9 | Q3 | 4-bit binary output |
| 10 | Q2 | 4-bit binary output |
| 11 | Q1 | 4-bit binary output (LSB) |
| 12 | OSC1 | Oscillator input/output |
| 13 | OSC2 | Oscillator input/output |
| 14 | VSS | Negative power supply (ground) |
| 15 | VDD | Positive power supply |
| 16 | PWDN | Power down input (active high) |
| 17 | StGT | Steering guard time output |
| 18 | St/GT | Steering input/guard time output |
MT8870 Application Circuits
The MT8870 can be used in various application circuits depending on the specific requirements of the system. Here are two common application circuits:
Basic MT8870 Application Circuit
This basic application circuit shows the essential components required for the MT8870 to function properly:
- The input signal is AC-coupled to the IN+ and IN- pins through capacitors C1 and C2.
- The gain is set by resistor R1 connected to the GS pin.
- A crystal or ceramic resonator (X1) is connected between the OSC1 and OSC2 pins to provide the internal clock signal.
- The power supply (VDD) is decoupled with capacitor C3.
- The binary output is available on pins Q1-Q4, and the steering output is available on the StD pin.
MT8870 Application Circuit with Microcontroller Interface
This application circuit demonstrates how to interface the MT8870 with a microcontroller:
- The input signal conditioning and oscillator connections remain the same as in the basic application circuit.
- The binary output pins (Q1-Q4) are connected to the microcontroller’s input pins.
- The steering output (StD) is connected to an interrupt pin on the microcontroller to detect the presence of a valid DTMF tone.
- The TOE pin is connected to a microcontroller output pin to enable or disable the three-state output as needed.
MT8870 DTMF Decoding Table
The MT8870 decodes the received DTMF tones into a 4-bit binary code. The following table shows the correspondence between the DTMF tones, the keypad symbols, and the binary output:
| DTMF Tone | Keypad Symbol | Q4 | Q3 | Q2 | Q1 |
|---|---|---|---|---|---|
| 697 + 1209 Hz | 1 | 0 | 0 | 0 | 1 |
| 697 + 1336 Hz | 2 | 0 | 0 | 1 | 0 |
| 697 + 1477 Hz | 3 | 0 | 0 | 1 | 1 |
| 770 + 1209 Hz | 4 | 0 | 1 | 0 | 0 |
| 770 + 1336 Hz | 5 | 0 | 1 | 0 | 1 |
| 770 + 1477 Hz | 6 | 0 | 1 | 1 | 0 |
| 852 + 1209 Hz | 7 | 0 | 1 | 1 | 1 |
| 852 + 1336 Hz | 8 | 1 | 0 | 0 | 0 |
| 852 + 1477 Hz | 9 | 1 | 0 | 0 | 1 |
| 941 + 1336 Hz | 0 | 1 | 0 | 1 | 0 |
| 941 + 1209 Hz | * | 1 | 0 | 1 | 1 |
| 941 + 1477 Hz | # | 1 | 1 | 0 | 0 |
| 697 + 1633 Hz | A | 1 | 1 | 0 | 1 |
| 770 + 1633 Hz | B | 1 | 1 | 1 | 0 |
| 852 + 1633 Hz | C | 1 | 1 | 1 | 1 |
| 941 + 1633 Hz | D | 0 | 0 | 0 | 0 |
Frequently Asked Questions (FAQ)
1. What is the difference between the MT8870 and the MT8870D?
The MT8870D is a variant of the MT8870 with improved dial tone rejection. It is designed to provide better immunity against dial tones that may interfere with DTMF decoding. In most other aspects, the two ICs are functionally identical.
2. Can the MT8870 decode DTMF tones from a telephone line?
Yes, the MT8870 can decode DTMF tones from a telephone line. However, it is essential to use proper isolation and signal conditioning circuits to protect the MT8870 from the high voltages present on the telephone line and to ensure compliance with relevant telephone regulations.
3. What is the purpose of the steering logic outputs (StD and ESt)?
The steering logic outputs, StD (delayed steering) and ESt (early steering), indicate the presence of a valid DTMF tone. The ESt output goes high as soon as a valid tone is detected, while the StD output goes high after a short delay (usually a few milliseconds) to ensure the tone is stable. These outputs can be used to trigger interrupts or control logic in the host system.
4. How can I adjust the input gain of the MT8870?
The input gain of the MT8870 can be adjusted by selecting an appropriate value for the external resistor connected to the GS pin. The gain is inversely proportional to the resistance value. Refer to the MT8870 datasheet for recommended resistor values based on the expected input signal level.
5. What is the purpose of the PWDN pin on the MT8870?
The PWDN (power down) pin is an active-high input that can be used to put the MT8870 into a low-power standby mode when DTMF decoding is not required. When PWDN is high, the IC’s power consumption is significantly reduced, making it suitable for battery-powered applications where power conservation is critical. To resume normal operation, the PWDN pin should be brought low.
Conclusion
The MT8870 is a versatile and widely used DTMF decoder IC that finds applications in various systems involving tone-based signaling. Its low power consumption, adjustable gain, and easy-to-use binary output make it an attractive choice for designers.
By understanding the key features, pinout, application circuits, and decoding table of the MT8870, engineers and hobbyists can effectively integrate this IC into their projects. As with any electronic component, it is essential to refer to the manufacturer’s datasheet for the most up-to-date and accurate information.
Leave a Reply