C511_1 Datasheet, PDF (61/128 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Units
6.3.1 Multiprocessor Communications
Modes 2 and 3 have a special provision for multiprocessor communications. In these modes, 9 data
bits are received. The 9th data bit goes into RB8. Then comes a 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. A way to use this feature in
multiprocessor systems is as follows.
When the master processor wants to transmit a block of data to one of several slaves, it first sends
out an address byte which 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 beeing addressed. The addressed slave will clear its SM2 bit and
prepare to receive the data bytes that will be coming. The slaves that weren’t being addressed leave
their SM2s set and go on about their business, ignoring the incoming data bytes.
SM2 has no effect in mode 0, and in mode 1 can be used 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.
Semiconductor Group
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