The MT9092 HPhone-II is a fully featured integrated digital telephone circuit which includes an HDLC data formatter. Voice band signals are converted to digital PCM and vice versa by a switched capacitor Filter/Codec. The Filter/Codec uses an ingenious differential architecture to achieve low noise operation over a wide dynamic range with a single 5V supply. A Digital Signal Processor provides handsfree speaker-phone operation. The DSP is also used to generate tones (DTMF, Ringer and Call Progress) and control audio gains. Internal registers are accessed through a serial microport conforming to INTEL MCS-51TM specifications. The device is fabricated in Mitel's low power ISO2-CMOS technology.