SAB80C517-M16 SIEMENS [Siemens Semiconductor Group], SAB80C517-M16 Datasheet - Page 53

• Manufacturer Part Number: SAB80C517-M16
• Description: 8-Bit CMOS Single-Chip Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

On-Chip Peripheral Components
7.2.1.2 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 RB80. 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 RI0 is set) only if RB80 = 1. This feature is enabled by setting bit
SM20 in S0CON. 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 SM20 = 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
SM20 bit and prepare to receive the data bytes that will be coming. After having received a complete
message, the slave sets SM20 again. The slaves that were not addressed leave their SM20 set and
go on about their business, ignoring the incoming data bytes.
SM20 has no effect in mode 0. In mode 1 SM20 can be used to check the validity of the stop bit. lf
SM20 = 1 in mode 1, the receive interrupt will not be activated unless a valid stop bit is received.
7.2.1.3 Baud Rates of Serial Channel 0
As already mentioned there are several possibilities to generate the baud rate clock for the serial
interface 0 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
7.2.3.
Therefore, the baud rate generators 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.
Semiconductor Group
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