Philips Semiconductors

Product specification

89C51/89C52/89C54/89C58

80C51 8-bit microcontroller family

4K/8K/16K/32K Flash

1999 Oct 27

13

When Timer 2 is in the baud rate generator mode, one should not try

to read or write TH2 and TL2. As a baud rate generator, Timer 2 is

incremented every state time (osc/2) or asynchronously from pin T2;

under these conditions, a read or write of TH2 or TL2 may not be

accurate. The RCAP2 registers may be read, but should not be

written to, because a write might overlap a reload and cause write

and/or reload errors. The timer should be turned off (clear TR2)

before accessing the Timer 2 or RCAP2 registers.

Table 4 shows commonly used baud rates and how they can be

obtained from Timer 2.

Summary Of Baud Rate Equations

Timer 2 is in baud rate generating mode. If Timer 2 is being clocked

through pin T2(P1.0) the baud rate is:

Baud Rate

+ Timer 2 Overflow Rate

16

If Timer 2 is being clocked internally , the baud rate is:

Baud Rate

+

f

OSC

[32

[65536

* (RCAP2H, RCAP2L)]]

To obtain the reload value for RCAP2H and RCAP2L, the above

equation can be rewritten as:

RCAP2H, RCAP2L

+ 65536 *

f

OSC

32

Baud Rate

Timer/Counter 2 Set-up

Except for the baud rate generator mode, the values given for T2CON

do not include the setting of the TR2 bit. Therefore, bit TR2 must be

set, separately, to turn the timer on. see Table 5 for set-up of Timer 2

as a timer. Also see Table 6 for set-up of Timer 2 as a counter.

Table 5.

Timer 2 as a Timer

T2CON

MODE

INTERNAL CONTROL

(Note 1)

EXTERNAL CONTROL

(Note 2)

16-bit Auto-Reload

00H

08H

16-bit Capture

01H

09H

Baud rate generator receive and transmit same baud rate

34H

36H

Receive only

24H

26H

Transmit only

14H

16H

Table 6.

Timer 2 as a Counter

TMOD

MODE

INTERNAL CONTROL

(Note 1)

EXTERNAL CONTROL

(Note 2)

16-bit

02H

0AH

Auto-Reload

03H

0BH

NOTES:

1. Capture/reload occurs only on timer/counter overflow.

2. Capture/reload occurs on timer/counter overflow and a 1-to-0 transition on T2EX (P1.1) pin except when Timer 2 is used in the baud rate

generator mode.