The Universal Asynchronous Receiver Transmitter (UART) has the same features as that of the standard 8051 microcontrollers. It means that it can operate in 1 out of 4 different modes, which is controlled by bits SM0 and SM1 of the SCON register.

Multiprocessor Communication

Multiprocessor communication (the SM2 bit of the SCON register is set) enables automatic address recognition by allowing the serial port to examine the adress of each incoming command. The process of writing a program is much easier therefore as the microcontrollers sharing the same interface don't have to check each address received via the serial port. Let's make it clear. Two special function registers, SADDR and SADEN, enable multiprocessor communication. Each device has an individual address that is specified in the SADDR register, while the so called mask address is written to the SADEN register. The mask address contains don't care bits which provide the flexibility to address one or more slaves at a time. In other words, it defines which bits of the SADDR register are to be used and which are to be ignored. When the master wants to transmit data to one of several slaves, it first sends out an address byte which identifies the target device. An address byte differs from a data byte in that the 9th bit is 1 in an address byte and 0 in a data byte. After receiving the address byte, all slaves check whether it matches their address. The adressed slave clears its SM2 bit and prepares to receive the data bytes to come. The slaves that weren't addressed leave their SM2 bits set and ignores the coming data bytes. The most simple example is a “mini-network” comprising only 3 microcontrollers: Microcontroller A is the master and communicates with devices “B” and “C”.

Microcontroller B: SADDR = 1100 0000
                   SADEN = 1111 1101
                 Address = 1100 00X0
                        
Microcontroller C: SADDR = 1100 0000
                   SADEN = 1111 1110
                 Address = 1100 000X

Although both microcontrollers B and C are assigned the same address (1100 0000), the mask in register SADEN is used to differentiate between them. It enables the master to communicate with both of them separately or at the same time: If transmit address is 1100 0010, the data will be sent to slave device B. If transmit address is 1100 0001 the data will be sent to slave device C. If transmit address is 1100 0000 the data will be sent to both slave devices.