STC89LE52RC datasheet(122/165 Pages) STC

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8.5 Buad Rates

The baud rate in Mode 0 is fixed:

SYSclk

Mode 0 Baud Rate =

The baud rate in Mode 2 depends on the value of bit SMOD in Special Function Register PCON. If SMOD =0

(which is the value on reset), the baud rate 1/

64 the System clock cycle. If SMOD = 1, the baud rate is

32 the

System clock cycle .

2SMOD

Mode 2 Baud Rate =

×(SYSclk)

(SYSclk)

In the STC89xx series, the baud rates in Modes 1 and 3 are determined by Timer1 or Timer 2 overflow rate.

The baud rate in Mode 1 and 3 are fixed:

Mode 1,3 Baud rate = (2

SMOD /32 ) x timer 1 overflow rate

= (2

SMOD /32 ) x timer 2 overflow rate

Timer 1 overflow rate = (SYSclk/12)/(256 - TH1);

Timer 2 overflow rate = SYSclk/(65536-(RCAP2H,RCAP2L))

When Timer 1 is used as the baud rate generator, the Timer 1 interrupt should be disabled in this application.

The Timer itself can be configured for either “timer” or “cormter” operation, and in any of its 3 running modes.

In the most typcial applications, it is configured for “timer” operation, in the auto-reload mode (high nibble of

TMOD = 0010B).

One can achieve very low baud rate with Timer 1 by leaving the Timer 1 interrupt enabled, and configuring the

Timer to run as a 16-bit timer (high nibble of TMOD = 0001B), and using the Timer 1 interrupt to do a l6-bit

software reload.

The following table lists various commonly used baud rates and how they can be obtained from Timer 1.

Baud Rate

OSC

SMOD

Timer 1

C/T Mode

Reload

Value

Mode 0 MAX: 1MHZ

Mode 2 MAX: 375K

Mode 1,3: 62.5K

19.2K

9.6K

4.8K

2.4K

1.2K

137.5

110

12MHZ

11.059MHZ

11.986MHZ

6MHZ

12MHZ

FFH

FDH

FAH

F4H

E8H

1DH

72H

FEEBH

Timer 1 Generated Commonly Used Baud Rates

When Timer 2 is used as the baud rate generator (either TCLK or RCLK in T2CON is '1'), the baud rate is as

follows,

Mode 1,3 Baud rate =

SMOD ×SYSclk

SYSclk

32×(65536-(RCAP2H,RCAP2L))

65536-(RCAP2H,RCAP2L))

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