89C52 datasheet(21/50 Pages) ISSI

21 / 50 page

IS89C52

Integrated Silicon Solution, Inc. — 1-800-379-4774

MC013-1C

11/21/98

ISSI®

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

2SMOD

(Timer 1 Overflow Rate)

Baud 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

2SMOD

Oscillator Frequency

Baud Rate

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.

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

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.

Mode 2 Baud Rate =

2SMOD

x (Oscillator Frequency)

In the IS89C52, these baud rates can be determined by

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

receive).

89C52 датащи(PDF) 21 Page - Integrated Silicon Solution, Inc

89C52 датащи(HTML) 21 Page - Integrated Silicon Solution, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

датащи скачать

ссылки URL