XC886/888CLM
Serial Interfaces
User’s Manual
12-7
V1.3, 2010-02
Serial Interfaces, V 1.0
12.1.2
Multiprocessor Communication
Modes 2 and 3 have a special provision for multiprocessor communication using a
system of address bytes with bit 9 = 1 and data bytes with bit 9 = 0. In these modes,
9 data bits are received. The 9th data bit goes into RB8. The communication always
ends with one stop bit. The port can be programmed such that when the stop bit is
received, the serial port interrupt will be activated only if RB8 = 1.
This feature is enabled by setting bit SM2 in SCON. One of the ways to use this feature
in multiprocessor systems is described in the following paragraph.
When the master processor wants to transmit a block of data to one of several slaves, it
first sends out an address byte that identifies the target slave. 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. With
SM2 = 1, no slave will be interrupted by a data byte. An address byte, however, will
interrupt all slaves, so that each slave can examine the received byte and see if it is being
addressed. The addressed slave will clear its SM2 bit and prepare to receive the data
bytes that will be coming. The slaves that were not being addressed retain their SM2s
as set and ignore the incoming data bytes.
Bit SM2 has no effect in mode 0. SM2 can be used in mode 1 to check the validity of the
stop bit. In a mode 1 reception, if SM2 = 1, the receive interrupt will not be activated
unless a valid stop bit is received.
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