MT8870DSR1

The MT8870DSR1 is a complete DTMF (Dual-Tone Multi-Frequency) receiver integrated circuit originally developed by Zarlink Semiconductor (now part of Microchip Technology). This device provides full DTMF signal detection and decoding functionality in a single chip, eliminating the need for complex analog filtering and digital signal processing. The MT8870 integrates a band-split filter section and a digital decoder section. The filter section uses switched capacitor technology to separate the high and low tone groups, providing excellent performance in rejecting speech and other non-DTMF signals. The decoder section uses digital counting techniques to measure the frequencies of the received tones and determine which DTMF digit was dialed. The device accepts the 16 standard DTMF digit pairs (0-9, A, B, C, D, *, #) and outputs the decoded digit as a 4-bit binary code on tristate output pins. The early steering (ESt) and delayed steering (StD) outputs provide signaling for tone presence and valid digit detection, with guard time adjustable via an external capacitor. The MT8870 operates from a single 5V power supply and uses a standard 3.579545 MHz color burst crystal, making it both cost-effective and easy to implement. The DSR1 suffix indicates the SOIC-18 surface-mount package with industrial temperature range. Widely used in telecommunications equipment, IVR systems, remote control devices, and security systems, the MT8870 remains a reliable standard solution for DTMF reception.

The MT8870DSR1 operates as a DTMF receiver through a two-stage process: filtering and digital detection. 1) Input Stage: The DTMF signal from the telephone line is applied to the IN- input after appropriate AC coupling. The input amplifier accepts signals in the range of 35 mV to 1.2 Vrms. 2) Dial-tone Filter: A high-pass filter at the input removes low-frequency dial tones (350 Hz and 440 Hz) that could interfere with detection. 3) Band-split Filter: A 6th-order switched capacitor band-split filter separates the incoming signal into low group (697-941 Hz) and high group (1209-1633 Hz) frequency bands. The filter uses switched capacitor technology with a clock derived from the 3.579545 MHz crystal, providing precise frequency response without external precision components. 4) Digit Detector: Each band (low and high) passes through a detector circuit that uses digital counting techniques to accurately measure the frequency of the tone present. The device compares the measured frequencies against the eight standard DTMF frequencies (four low, four high) to determine which row and column of the DTMF keypad is being pressed. 5) Steering Logic: The steering logic validates that both a low group and a high group tone are present simultaneously and have been present for the minimum guard time. The ESt (Early Steering) output goes high when valid dual tones are first detected, while StD (Steered, Delayed) goes high after the guard time has elapsed, indicating a valid digit has been latched. 6) Output Latch: When a valid digit is confirmed, the 4-bit binary code corresponding to the detected DTMF digit is latched onto the output pins (D0-D3). The outputs are tristate, controlled by the TOE (Three-state Output Enable) pin. The MT8870 incorporates sophisticated tone validation algorithms to reject speech, music, and other signals that might falsely trigger DTMF detection.

• Interactive Voice Response (IVR) and voicemail systems
• Remote telephone control systems for home automation
• Security alarm systems with DTMF remote arming/disarming
• Utility meter reading via telephone line
• PBX and key telephone system DTMF receivers
• Paging system decoders
• Automated phone banking and payment systems
• Radio repeater control and remote base station control