SAB80C517-M16 SIEMENS [Siemens Semiconductor Group], SAB80C517-M16 Datasheet - Page 62

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

On-Chip Peripheral Components
7.2.2.2 Multiprocessor Communication Feature
Mode A of the serial interface 1 has a special provision for multiprocessor communication. In this
mode, 9 data bits are received. The 9th bit goes into RB81. Then follows a stop bit. The port can be
programmed such that when the stop bit is received, the serial port interrupt (i.e. the request flag
Rl1 is set) will be activated only if RB81 = 1. This feature is enabled by setting bit SM21 in S1CON.
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 SM21 = 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
SM21 bit and prepare to receive the data bytes that will be coming. After having received a complete
message, the slave is setting SM21 again. The slaves that were not addressed leave their SM21
set and go on about their business, ignoring the incoming data bytes.
In mode B SM21 can be used to check the validity of the stop bit. lf SM21 = 1 in mode B, the receive
interrupt will not be activated unless a valid stop bit is received.
7.2.2.3 Baud Rates of Serial Channel 1
As already mentioned serial interface 1 uses its own dedicated baud rate generator for baud rate
generation in both operating modes (see figure 7-14).
f
This baud rate generator consists of a free running 8-bit timer with
/2 input frequency. The timer
OSC
is automatically reloaded at overflow by the contents of register S1REL (see figure 7-15). The timer
must be started by writing the desired reload value to register S1REL. The baud rate in operating
modes A and B can be determined by following formula:
oscillator frequency
Mode A, B baud rate =
32 x (256 – S1REL)
At 12-MHz oscillator frequency a baud rate range from about 1.5 kbaud up to 375 kbaud is covered.
Using the fast baud rates offers the same functionality as the operating mode 2 in serial interface 0
with its fixed baud rates.
Semiconductor Group
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