The MC145745 is a selectable modem chip compatible with ITU V.21 (300 baud full duplex asynchronous) and V.23 mode 2 (1200 baud half duplex asynchronous). The builtin differential line driver has the capability of driving 0 dBm into a 600 load with a 5 V single power supply. This device also includes a DTMF generator, DTMF receiver, callprogress tone detector, answer tone generator, and a receive timing control circuit. Besides having a clock generator with a crystal oscillator connected to it, the device has a divider circuit to which input of a double frequency clock is possible from external sources, such as from a microcontroller unit (MCU). The serial control port (SCP) permits the MCU to access internal registers for exercising the builtin features.