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C509-L_97 Datasheet, PDF (170/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller
On-Chip Peripheral Components
C509-L
6.5.2.2 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 a stop bit follows. The port can be
programmed such that when the stop bit is received, the serial port 1 interrupt will be activated (i.e.
the request flag Rl1 is set) 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.
6.5.2.3 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 6-40). This baud rate generator consists of a free
running 10-bit timer with fOSC/2 (S1P=1) or fOSC (S1P=0) as input frequency. On overflow of this
timer (next count step after counter value 3FFH) there is an automatic 10-bit reload from the
registers S1RELL and S1RELH. The lower 8 bits of the timer are reloaded from S1RELL, while the
upper two bits are reloaded from bit 0 and 1 of register S1RELH. The baud rate timer is reloaded
by writing to S1RELL.
Figure 6-40
Baud Rate Generator for Serial Interface 1
Semiconductor Group
6-92
1997-10-01