C517A_99 Datasheet, PDF (147/218 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C517A
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 6-35). This baud rate generator consists of a free
running 10-bit timer with f_OSC/2 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.
Baud Rate Generator
S1RELH .1 .0
S1RELL
f_OSC /2 _{Input Clock}
10-Bit Timer
Figure 6-35
Baud Rate Generator for Serial Interface 1
Owerflow
MCS03331
Baud
Rate
Clock
Semiconductor Group
6-81