Auto Wake-on-Break

During Sleep mode, all clocks to the UART are suspended. Because of this, the Baud Rate Generator is inactive and a proper character reception cannot be performed. The Auto Wake-on-Break feature allows the controller to wake up due to activity on the RX line.

The Auto-Wake-up feature is enabled by setting both the WUE bit and the UxIE bit in the PIEx register. Once set, the normal receive sequence on RX is disabled, and the UART remains in an Idle state, monitoring for a wake-up event independent of the CPU mode. A wake-up event consists of a transition out of the Idle state on the RX line (this coincides with the start of a Break or a wake-up signal character for the LIN protocol).

The UART module generates a WUIF interrupt coincident with the wake-up event. The interrupt is generated synchronously to the Q clocks in normal CPU operating modes (Figure 1), and asynchronously, if the device is in Sleep mode (Figure 2). The interrupt condition is cleared by clearing the WUIF bit.

Figure 1. Auto-Wake-Up Timing During Normal Operation
Figure 2. Auto-Wake-Up Timing During Sleep
To generate an interrupt on a wake-up event, all the following bits must be set:

The WUE bit is automatically cleared by the transition to the Idle state on the RX line at the end of the Break. This signals to the user that the Break event is over. At this point, the UART module is in Idle mode, waiting to receive the next character.