P89C668 datasheet(38/89 Pages) PHILIPS

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Philips Semiconductors

Product data

P89C660/P89C662/P89C664/

P89C668

80C51 8-bit Flash microcontroller family

16KB/32KB/64KB ISP/IAP Flash with 512B/1KB/2KB/8KB RAM

2002 Oct 28

OSC

C/T2 = 0

C/T2 = 1

TR2

Control

TL2

(8-bits)

TH2

(8-bits)

÷ 16

RCAP2L

RCAP2H

EXEN2

Control

EXF2

Timer 2

Interrupt

T2EX Pin

Transition

Detector

T2 Pin

Reload

÷ 2

“0”

“1”

RX Clock

÷ 16

TX Clock

“0”

“1”

“0”

“1”

Timer 1

Overflow

Note availability of additional external interrupt.

SMOD

RCLK

TCLK

SU01213

Figure 25. Timer 2 in Baud Rate Generator Mode

Table 10.

Timer 2 Commonly Used Generated

Baud Rates

Baud Rate

Timer 2

12 clock

mode

6 clock

mode

Osc Freq

RCAP2H

RCAP2L

375 k

750 k

12 MHz

9.6 k

19.2 k

12 MHz

2.8 k

5.6 k

12 MHz

2.4 k

4.8 k

12 MHz

1.2 k

2.4 k

12 MHz

300

600

12 MHz

110

220

12 MHz

300

600

6 MHz

110

220

6 MHz

Baud Rate Generator Mode

Bits TCLK and/or RCLK in T2CON (see Figure 20) 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.

Table 10 shows commonly used baud rates and how they can be

obtained from Timer 2.

Figure 25 shows Timer 2 in baud rate generation mode. The baud

rate generation mode is similar to 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.,

1/6 the oscillator frequency in 6 clock mode, 1/12 the oscillator

frequency in 12 clock mode). As a baud rate generator, it increments

at the oscillator frequency in 6 clock mode (fOSC/2 in 12 clock

mode). Thus the baud rate formula is as follows:

Oscillator Frequency

[n *

[65536

(RCAP2H, RCAP2L)]]

Modes 1 and 3 Baud Rates =

* n =

16 in 6 clock mode

32 in 12 clock mode

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 25, 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, 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.

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