13

AT89LS8252

0850C–MICRO–3/06

Baud Rate Generator

Timer 2 is selected as the baud rate generator by setting

TCLK and/or RCLK in T2CON (Table 2). Note that the baud

rates for transmit and receive can be different if Timer 2 is

used for the receiver or transmitter and Timer 1 is used for

the other function. Setting RCLK and/or TCLK puts Timer 2

into its baud rate generator mode, as shown in Figure 4.

The baud rate generator 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 according to the following equation.

The Timer can be configured for either timer or counter

operation. In most applications, it is configured for timer

operation (CP/T2 = 0). The timer operation is different for

Timer 2 when it is used as a baud rate generator. Normally,

as a timer, it increments every machine cycle (at 1/12 the

oscillator frequency). As a baud rate generator, however, it

increments every state time (at 1/2 the oscillator fre-

quency). The baud rate formula is given below.

where (RCAP2H, RCAP2L) is the content of RCAP2H and

RCAP2L taken as a 16 bit unsigned integer.

Timer 2 as a baud rate generator is shown in Figure 4. This

figure is valid only if RCLK or TCLK = 1 in T2CON. Note

that a rollover in TH2 does not set TF2 and will not gener-

ate an interrupt. Note too, that if EXEN2 is set, a 1-to-0

transition in T2EX will set EXF2 but will not cause a reload

from (RCAP2H, RCAP2L) to (TH2, TL2). Thus when Timer

2 is in use as a baud rate generator, T2EX can be used as

an extra external interrupt.

Note that when Timer 2 is running (TR2 = 1) as a timer in

the baud rate generator mode, TH2 or TL2 should not be

read from or written to. Under these conditions, the Timer is

incremented every state time, and the results of a read or

write 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.

Modes 1 and 3 Baud Rates

Timer 2 Overflow Rate

16

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=

Modes 1 and 3

Baud Rate

---------------------------------------

Oscillator Frequency

32

65536

RCAP2H,RCAP2L

()

–

[]

×

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=

Figure 5. Timer 2 in Clock-Out Mode