DTMF stands for dual tone multiple frequency.  When you press a button on a telephone keypad, the telephone will generate two sine waves with different frequency at the same time.  These frequencies are called DTMF and there is a table below listing the frequencies for each number.

Tone generator chip

The MT8880C telephone dialer chip from MITEL can generate the standard telephone tones.  The spec sheet for the chip can be found at the MITEL home page - www.mitelsemi.com

The MT8880C chip can generates dial tone to make a phone call over 2-wire telephone line, tip and ring, connection.  All frequencies are derived from an external 3.579545MHz crystal that corresponds to digit in the following table:
 
 

The chip uses a 4 bit data bus inputs; the FPGA can send the corresponding number to the chip to generate the required DTMF signal.  The CP output is used to determine the call progress (busy or not).  The CP output converts the busy tone signal to 0.5s high/low square wave (see appendix of MT8880C CP progress p4-52) by using a band pass filter.

The IN^- input is connected to the VX output of the MT88435 phone interface chip for detecting busy tone signal.  The tone output is connected to VR^- of  the MT88435 input to send out DTMF tone dialing signal onto the phone line when the MT88435 is in the off-hook state.

Since the system has to poll the DTMF generator status register for busy signal or not busy signal, therefore a buffer has to be used.

Here's the complete schematic diagram for the tone generator chip.

The MH88435AD-P chip from MITEL is used to connect between the DTMF dialer chip and the phone line.  This device provides a complete interface between audio or data transmission equipment and a telephone line.  All functions are integrated into a single thickflim-hybrid module, which provides high voltage isolation, very high reliability and optimum circuit design.  The spec sheet for this chip can be found at the MITEL home page - www.mitelsemi.com

The main function of this chip is to manage the phone line.  When the parallel phone is off-hook, it will set the LCD (loop current detect) pin to high to notify the FPGA that the phone line is busy.

The LC (loop control) pin is driven by the FPGA.  When LC is asserted, the MH88435 chip will be in the off-hook state, tone dialing and voice transmission can progress.

There is a 15kohms resistor across NB1 and NB2 (network balance) and a 4.7kohms resistor at ZA (line impedance).  These resistors are used to program the phone interface matching the phone line impedance (600 ohms in North America, see appendix of MH88435 p2-53).  There is a 360kohms resistor across VLOOP1 & 2 and this resistor is used to program the loop voltage (6V across TIP and RING, see appendix of MH88435 p2-42) on the phone line.  There is a 100kohms resistor at the input of VR^- and the input of VR⁺.  These resistors are used to program the overall gain at the MH88435 phone interface input.  Since the input impedance is 47kohms, the overall gain is -9.9 dB.

Overall receive gain = 0dB + 20 log(47k/(47k+100k)) = -9.9 dB using 100kohms resistor

The receive gain is the gain from the VR⁺ and VR^- to the differential signal across TIP and RING.

Here's the complete schematic diagram for the tone generator chip.

Reference:

            - MITEL Corporation, copyright 1999