> 3.18.3 | Behavior of the RING0 Line (ASC0 Interface only)

> The RING0 line is available on the first serial interface ASC0 (see also Section 3.10). The signal serves to indicate incoming calls and other types of URCs (Unsolicited Result Code).
> Although not mandatory for use in a host application, it is strongly suggested that you connect the RING0 line to an interrupt line of your application. In this case, the application can be designed to receive an interrupt when a falling edge on RING0 occurs. This solution is most effective, particularly, for waking up an application from power saving. Note that if the RING0 line is not wired, the application would be required to permanently poll the data and status lines of the serial interface at the expense of a higher current consumption. Therefore, utilizing the RING0 line provides an option to significantly reduce the overall current consumption of your application.
> The behavior of the RING0 line varies with the type of event:

> • | When a voice/fax/data call comes in the RING0 line goes low for 1s and high for another 4s. Every 5 seconds the ring string is generated and sent over the /RXD0 line.

> If there is a call in progress and call waiting is activated for a connected handset or handsfree device, the RING0 line switches to ground in order to generate acoustic signals that indicate the waiting call.
> Figure 38: Incoming voice/fax/data call

> • | All other types of Unsolicited Result Codes (URCs) also cause the RING0 line to go low, however for 1 second only.

> Figure 39: URC transmission