P89V52X2FN datasheet(24/56 Pages) PHILIPS

24 / 56 page

P89V52X2_1

Preliminary data sheet

Rev. 01 — 7 June 2007

24 of 56

NXP Semiconductors

P89V52X2

80C51 with 256 B RAM, 192 B data EEPROM

used as the UART transmit baud rate generator. RCLK has the same effect for the UART

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

transmit baud rates – Timer 1 or Timer 2.

Figure 17 shows Timer 2 in baud rate generator mode:

The baud rate generation mode is like the auto-reload mode, when 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/16

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

baud rate formula is as follows:

Modes 1 and 3 Baud Rates =

(3)

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 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.

Fig 17. Timer 2 in Baud Rate Generator mode

002aaa526

TX/RX baud rate

timer 2

interrupt

OSC

÷2

T2 pin

C/T2 = 0

C/T2 = 1

TL2

(8-bits)

TH2

(8-bits)

control

TR2

EXF2

RCAP2L RCAP2H

control

EXEN2

transition

detector

T2EX pin

reload

OscillatorFrequency

65536

RCAP2H RCAP2L

()

–

()

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