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A8351601

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AMIC Technology 

A8351601 Series

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.

Oscillator Frequency

Mode 0 Baud Rate =

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.

2SMOD

Mode 2 Baud Rate =

X (Oscillator Frequency)

64

In the A8351601, the baud rates can be determined by

Timer 1, Timer 2, or both (one for transmit and the other for

receive).

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.

2SMOD

Mode 1,3 Baud Rate =

X (Timer 1 Overflow Rate)

32

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 3 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.

2SMOD

Oscillator Frequency

Mode 1,3 Baud Rate = 32 X 12 X [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.

(July, 2002, Version 1.0)

20

AMIC Technology, Inc.

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