P80C31 datasheet(13/38 Pages) PHILIPS

13 / 38 page

Philips Semiconductors

Product specification

80C51/87C51/80C31

80C51 8-bit microcontroller family

4K/128 OTP/ROM/ROMless, low voltage (2.7V–5.5V),

low power, high speed (33 MHz)

2000 Jan 20

Baud Rate Generator Mode

Bits TCLK and/or RCLK in T2CON (Table 3) 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 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:

Oscillator Frequency

[32

[65536

* (RCAP2H, RCAP2L)]]

Modes 1 and 3 Baud Rates =

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.

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.

Table 4.

Timer 2 Generated Commonly Used

Baud Rates

Ba d Rate

Osc Freq

Timer 2

Baud Rate

Osc Freq

RCAP2H

RCAP2L

375K

12MHz

9.6K

12MHz

2.8K

12MHz

2.4K

12MHz

1.2K

12MHz

300

12MHz

110

12MHz

300

6MHz

110

6MHz

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

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

Baud Rate

OSC

[32

[65536

* (RCAP2H, RCAP2L)]]

Where fOSC= Oscillator Frequency

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

equation can be rewritten as:

RCAP2H, RCAP2L

+ 65536 *

OSC

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.

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