G8870

The G8870 has the following features including CMOS technology for low power consumption-35 mW max;Full DTMF receiver;Provides DTMF high and low group filtering;Adjustable acquisition and release times;Dial tone suppression;Integrated bandsplit filter and digital decoder functions;On-chip differential amplifier, clock oscillator, and latched three-state bus;Uses inexpensive 3.58 MHz crystal;Central office quality and performance;Single +5 volt power supply;18-pin DIP or 20-pin PLCC package.

The GTE 68870 provides full DTMF receiver capability by integrating both the bandsplit filter and digital decoder functions into a single 18-pin DIP or 20-pin PLCC package. The 68870 is manufactured using state-of-the-art CMOS process technology for low power consumption (35 mW max.) and precise data handling. The filter section uses a switched capacitortechnique for both high and low group filters and dial tone rejection. The 68870 decoder uses digital counting techniques for the detection and decoding of all 16 DTMF tone pairs into a 4-bit code.The 68870 minimizes external component count by providing an on-chip differential input amplifier, clock generator, and a latched three-state intertace bus. The on-chip clock generator requires only a low cost TV crystal as an external component.This device contains input protection against damage due to high static voltages or electric,fields; however, precautions should be taken to avoid application of voltages higher than the maximum rating.

The G8870 decoder uses a digital counting technique to determine the frequencies of the limited tones and to verify that these tones correspond to standard l7TMF frequencies.A complex averaging algorithm is used to protect against tone simulation  by  extraneous signals  (such as voice)  while providing  tolerance  to  small  frequency  variations.  The averaging algorithm has been developed to ensure an optimum combination of immunity to "talk-off" and tolerance to the presence of interfering signals (third tones) and noise.When the detector recognizes the simultaneous presence of two valid tones (known as "signal condition'), it raises the "Early Steering" flag (ESt). Any subsequent loss of signal condition will cause ESt to fall.The value of t}P is a parameter of the device and fast is the minimum signal duration to be recognized by the receiver. A value for C of 0.1 uF is recommended for most applications,leaving R to be selected by the designer. For example, a suitable value of R for a fast of 40 milliseconds would be 300K. A typical circuit using this steering configuration is shown in Figure 1. The timing requirements for most telecommunication applications are satisfied with this circuit. Different steering arrangements may be used to select independently the guard-times for tone-present (tGTP)  and tone-absent (tGTA).