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IC80C32

CMOS SINGLE CHIP 8-BIT MICROCONTROLLER

Integrated Circuit Solution Inc 

IC80C52

IC80C32

Multiprocessor Communications

Modes 2 and 3 have a special provision for multiprocessor

communications. In these modes, nine data bits are

received, followed by a stop bit. The ninth bit goes into RB8;

then comes a stop bit. The port can be programmed such

that when the stop bit is received, the serial port interrupt

is activated only if RB8 = 1. This feature is enabled by

setting bit SM2 in SCON.

The following example shows how to use the serial interrupt

for multiprocessor communications. When the master

processor must transmit a block of data to one of several

slaves, it first sends out an address byte that identifies the

target slave. An address byte differs from a data byte in that

the ninth bit is 1 in an address byte and 0 in a data byte. With

SM2 = 1, no slave is interrupted by a data byte. An address

byte, however, interrupts all slaves, so that each slave can

examine the received byte and see if it is being addressed.

The addressed slave clears its SM2 bit and prepares to

receive the data bytes that follows. The slaves that are not

addressed set their SM2 bits and ignore the data bytes.

SM2 has no effect in Mode 0 but can be used to check the

validity of the stop bit in Mode 1. In a Mode 1 reception, if

SM2 = 1, the receive interrupt is not activated unless a valid

stop bit is received.

Baud Rates

The baud rate in Mode 0 is fixed as shown in the following

equation.

Mode 0 Baud Rate = Oscillator Frequency

12

The baud rate in Mode 2 depends on the value of the SMOD

bit in Special Function Register PCON. If SMOD = 0 (the

value on reset), the baud rate is 1/64 of the oscillator

frequency. If SMOD = 1, the baud rate is 1/32 of the

oscillator frequency, as shown in the following equation.

Using the Timer 1 to Generate Baud Rates

When Timer 1 is the baud rate generator, the baud rates in

Modes 1 and 3 are determined by the Timer 1 overflow rate

and the value of SMOD according to the following equation.

Mode 1, 3

Baud Rate

=

2SMOD

32

X (Timer 1 Overflow Rate)

The Timer 1 interrupt should be disabled in this application.

The Timer itself can be configured for either timer or

counter operation in any of its three running modes. In the

most typical applications, it is configured for timer operation

in auto-reload mode (high nibble of TMOD = 0010B). In this

case, the baud rate is given by the following formula.

Mode 1,3 =

Baud Rate

2SMOD

Oscillator Frequency

X

32

12x [256 – (TH1)]

Programmers can achieve very low baud rates with Timer

1 by leaving the Timer 1 interrupt enabled, configuring the

Timer to run as a 16-bit timer (high nibble of TMOD =

0001B), and using the Timer 1 interrupt to do a 16-bit

software reload.

Table 9 lists commonly used baud rates and how they can

be obtained from Timer 1.

Mode 2 Baud Rate = 2SMOD x (Oscillator Frequency)

64

In the IC80C51/31, the Timer 1 overflow rate da termines

th e baud rates in Modes 1 and 3. In the IC80C52/32, these

baud rates can be determined by Timer 1, Timer 2, or both

(one for transmit and the other for receive).

Integrated Circuit Solution Inc.

21

MC002-0B

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