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P89C54UFBB

80C51 8-bit microcontroller family 4K/8K/16K/32K Flash

Philips Electronics 

Philips Semiconductors

80C51 8-bit microcontroller family

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

Product specification

89C51/89C52/89C54/89C58

Timer 1

Overflow

OSC

NOTE: OSC. Freq. is divided by 2, not 12.

÷2

C/T2 = 0

T2 Pin

C/T2 = 1

Transition

Detector

Control

TR2

TL2

(8-bits)

TH2

(8-bits)

÷2

“0”

“1”

“1”

“0”

SMOD

RCLK

RCAP2L

Reload

RCAP2H

“1”

“0”

÷ 16

RX Clock

TCLK

÷ 16

TX Clock

T2EX Pin

EXF2

Timer 2

Interrupt

Control

EXEN2

Note availability of additional external interrupt.

Figure 6. Timer 2 in Baud Rate Generator Mode

SU00068

Table 4. Timer 2 Generated Commonly Used

Baud Rates

Baud Rate

Osc Freq

Timer 2

RCAP2H

RCAP2L

375 k

12 MHz

FF

FF

9.6 k

12 MHz

FF

D9

2.8 k

12 MHz

FF

B2

2.4 k

12 MHz

FF

64

1.2 k

12 MHz

FE

C8

300

12 MHz

FB

1E

110

12 MHz

F2

AF

300

6 MHz

FD

8F

110

6 MHz

F9

57

Baud Rate Generator Mode

Bits TCLK and/or RCLK in T2CON (Table 4) allow the serial port

transmit and receive baud rates to be derived from either Timer 1 or

Timer 2. When TCLK= 0, Timer 1 is used as the serial port transmit

baud rate generator. When TCLK= 1, Timer 2 is used as the serial

port transmit baud rate generator. RCLK has the same effect for the

serial port receive baud rate. With these two bits, the serial port can

have different receive and transmit baud rates – one generated by

Timer 1, the other by Timer 2.

Figure 6 shows the Timer 2 in baud rate generation mode. The baud

rate generation mode is like the auto-reload mode, in that a rollover in

TH2 causes the Timer 2 registers to be reloaded with the 16-bit value

in registers RCAP2H and RCAP2L, which are preset by software.

The baud rates in modes 1 and 3 are determined by Timer 2’s

overflow rate given below:

Modes

1

and

3

Baud Rates

+ Timer

2

Overflow

16

Rate

The timer can be configured for either “timer” or “counter” operation.

In many applications, it is configured for “timer” operation (C/T2*=0).

Timer operation is different for Timer 2 when it is being used as a

baud rate generator.

Usually, as a timer it would increment every machine cycle (i.e., 1/12

the oscillator frequency). As a baud rate generator, it increments

every state time (i.e., 1/2 the oscillator frequency). Thus the baud

rate formula is as follows:

Modes 1 and 3 Baud Rates =

Oscillator Frequency

[32 [65536 * (RCAP2H, RCAP2L)]]

Where: (RCAP2H, RCAP2L) = The content of RCAP2H and

RCAP2L taken as a 16-bit unsigned integer.

The Timer 2 as a baud rate generator mode shown in Figure 6, is

valid only if RCLK and/or TCLK = 1 in T2CON register. Note that a

rollover in TH2 does not set TF2, and will not generate an interrupt.

Thus, the Timer 2 interrupt does not have to be disabled when

Timer 2 is in the baud rate generator mode. Also if the EXEN2

(T2 external enable flag) is set, a 1-to-0 transition in T2EX

(Timer/counter 2 trigger input) will set EXF2 (T2 external flag) but

will not cause a reload from (RCAP2H, RCAP2L) to (TH2,TL2).

Therefore when Timer 2 is in use as a baud rate generator, T2EX

can be used as an additional external interrupt, if needed.

1999 Oct 27

12

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