24.3.2.10 Multiprocessor Communication Mode

The Multiprocessor Communication mode (MCPM) effectively reduces the number of incoming frames that have to be handled by the receiver in a system with multiple microcontrollers communicating via the same serial bus. This mode is enabled by writing a '1' to the MCPM bit in the Control B register (USARTn.CTRLB). In this mode, a dedicated bit in the frames is used to indicate whether the frame is an address or data frame type.

If the receiver is set up to receive frames that contain five to eight data bits, the first Stop bit is used to indicate the frame type. If the receiver is set up for frames with nine data bits, the ninth bit is used to indicate frame type. When the frame type bit is one, the frame contains an address. When the frame type bit is zero, the frame is a data frame. If 5- to 8-bit character frames are used, the transmitter must be set to use two Stop bits, since the first Stop bit is used for indicating the frame type.

If a particular slave MCU has been addressed, it will receive the following data frames as usual, while the other slave MCUs will ignore the frames until another address frame is received.