SAB80515 Datasheet, PDF (50/270 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller Family

On-Chip Peripheral Components
7.2.2 Multiprocessor Communication Feature
Modes 2 and 3 of the serial interface 0 have a special provision for multi-processor communication.
In these modes, 9 data bits are received. The 9th bit goes into RB8. Then a stop bit follows. The
port can be programmed such that when the stop bit is received, the serial port 0 interrupt will be
activated (i.e. the request flag RI is set) only if RB8 = 1. This feature is enabled by setting bit SM2
in SCON. A way to use this feature in multiprocessor communications is as follows.
lf the master processor wants to transmit a block of data to one of the 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 being addressed. The addressed slave will clear its SM2 bit and
prepare to receive the data bytes that will be coming. After having received a complete message,
the slave sets SM2 again. The slaves that were not addressed leave their SM2 set and go on about
their business, ignoring the incoming data bytes.
SM2 has no effect in mode 0. In mode 1 SM2 can be used to check the validity of the stop bit. lf
SM2 = 1 in mode 1, the receive interrupt will not be activated unless a valid stop bit is received.
7.2.3 Baud Rates
As already mentioned there are several possibilities to generate the baud rate clock for the serial
interface depending on the mode in which it is operated.
To clarify the terminology, something should be said about the difference between "baud rate clock"
and "baud rate". The serial interface requires a clock rate which is 16 times the baud rate for internal
synchronization, as mentioned in the detailed description of the various operating modes in section
7.2.4.
Therefore, the baud rate generator have to provide a "baud rate clock" to the serial interface which -
there divided by 16 - results in the actual "baud rate". However, all formulas given in the following
section already include the factor and calculate the final baud rate.
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