I The keyboard input may be from either the standard 4 × 4 keyboard
or an inexpensive single contact (form A) keyboard. Electronic
input from a µC can also be used. A valid key entry is defined as a
single row being connected to a single column.
I, O A built-in inverter provides oscillation with an inexpensive
3.579545 MHz crystal or ceramic resonator.
O The T/P MUTE is a conventional CMOS N-channel open drain
output. The output transistor is switched on during pulse and tone
mode dialing sequence and flash break. Otherwise, it is switched
I Pulling mode pin to VSS places the dialer in tone mode. Pulling
mode pin to VDD places the dialer in pulse mode (10 ppS). Leaving
mode pin floating places the dialer in pulse mode (20 ppS).
I Hook switch input. HKS = 1: On-hook state. Chip in sleeping mode,
no operation. HKS = 0: Off-hook state. Chip enabled for normal
operation. The HKS pin is pulled to VDD by an internal resistor.