AT89LP51 datasheet(59/113 Pages) ATMEL

59 / 113 page

3709C–MICRO–5/11

AT89LP51/52 - Preliminary

14.1

Multiprocessor Communications

Modes 2 and 3 have a special provision for multiprocessor communications. In these modes,

9data bits are received, followed by a stop bit. The 9th 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 9th 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. 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. See “Automatic Address Recognition” on page 61.

The SM2 bit 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.

14.2

Baud Rates

The baud rate in Mode 0 depends on the value of the SMOD1 bit in Special Function Register

PCON.7. If SMOD1 = 0 (the value on reset) and TB8 =0, the baud rate is 1/4 of the system fre-

quency in Fast mode. If SMOD1 = 1 and TB8 = 0, the baud rate is 1/2 of the system frequency,

as shown in the following equation:

:In Compatibility mode the baud rate is 1/6 of the system frequency, scaling to 1/3 when

SMOD1 = 1.

The baud rate in Mode 2 also depends on the value of the SMOD1 bit. If SMOD1 = 0, the baud

rate is 1/32 of the system frequency. If SMOD1 = 1, the baud rate is 1/16 of the system fre-

quency, as shown in the following equation:

14.2.1

Using Timer 1 to Generate Baud Rates

Setting TB8 = 1 in Mode 0 enables Timer 1 as the baud rate generator. When Timer 1 is the

baud rate generator for Mode 0, the baud rates are determined by the Timer 1 overflow rate and

the value of SMOD1 according to the following equation:

Mode 0 Baud Rate

TB8 = 0

SMOD1

--------------------

System Frequency

Mode 0 Baud Rate

TB8 = 0

SMOD1

--------------------

System Frequency

Mode 2 Baud Rate

SMOD1

--------------------

System Frequency

Mode 0 Baud Rate

TB8 = 1

SMOD1

--------------------

(Timer 1 Overflow Rate)

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