DESCRIPTION
The device consists of the speech and the dialer functions. It provides the DC line interface circuit that terminates the telephone line, analog amplifier for speech transmission and necessary signals for either DTMF and pulse dialing. When mated with a tone ringer, a complete telephone can be produced with just two ICs.

SPEECH CIRCUIT
■ 2 TO 4 WIRES CONVERSION
■ PRESENT THE PROPERDC PATH FOR THE LINE CURRENT AND THE FLEXIBILITY TO ADJUST IT ANDALLOWPARALLELPHONE OPERATION
■ SYMMETRICAL HIGH IMPEDANCE MICROPHONE INPUTS SUITABLE FOR DYNAMIC, ELECTRET OR PIEZOELECTRIC TRANSDUCER
■ ASYMMETRICAL EARPHONE OUTPUT SUITABLE FOR DYNAMIC TRANSDUCER
■ LINE LOSS COMPENSATION FIXED INTERNALLY BY A.G.C.
■ INTERNAL MUTING TO DISABLE SPEECH DURING DIALING AND EXTERNAL MUTING TO DISABLE TRANSMIT AMPLIFIER DURING CONVERSATION MODE
■ LED INDICATOR EITHER FOR KEYBOARDILLUMINATION (OR ON-LINE INDICATION) DURING DIALING AND CONVERSATION OR FOR TONE MODE INDICATION, THROUGH MU/MFI PIN

DIALER CIRCUIT
■ STORE UP TO 32 DIGITS FOR LAST NUMBER REDIAL BUFFER, LNR IS INHIBITED IF THERE ARE MORE THAN 32 DIGITS STORED
■ ALLOWMIXED MODE DIALINGIN PULSE MODE
■ PACIFIER TONE PROVIDES AUDIBLE INDICATION OF VALID KEY PRESSED IN A BUZZER OR/AND IN THE EARPHONE
■ TIMED PABX PAUSE / 10PPS PULSE RATE
■ MAKE/BREAK RATIO : 40/60 (ST3917A) AND 33/67 (ST3917B)
■ 4 SELECTABLE OPTIONS ON FLASH DURATION AND SOFTSWITCH INHIBITION IN ONE OF THE OPTION WITH 100ms FLASH TIMING
■ 2 SELECTABLE OPTIONS : TRANSMIT MUTE TOGETHER WITH LED FOR KEYBOARD ILLUMINATIONOR LED FOR TONE MODE INDICATION
■ CONTINUOUS TONE FOR EACH DIGIT UNTIL KEY RELEASE
■ USES INEXPENSIVE 3.579545MHz CERAMIC RESONATOR
■ POWERED FROM TELEPHONE LINE, LOW OPERATING VOLTAGE FOR LONG LOOP APPLICATION