AT89S53_00 datasheet(13/33 Pages) ATMEL

13 / 33 page

AT89S53

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 Fig-

ure 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.

Programmable Clock Out

A 50% duty cycle clock can be programmed to come out on

P1.0, as shown in Figure 5. This pin, besides being a regu-

la r I / 0 pin , ha s tw o alte r n a t e f u nct i o n s. It ca n be

programmed to input the external clock for Timer/Counter 2

or to output a 50% duty cycle clock ranging from 61 Hz to 4

MHz at a 16 MHz operating frequency.

To configure the Timer/Counter 2 as a clock generator, bit

C/T2 (T2CON.1) must be cleared and bit T2OE (T2MOD.1)

must be set. Bit TR2 (T2CON.2) starts and stops the timer.

The clock-out frequency depends on the oscillator fre-

quency and the reload value of Timer 2 capture registers

(RCAP2H, RCAP2L), as shown in the following equation.

In the clock-out mode, Timer 2 rollovers will not generate

an interrupt. This behavior is similar to when Timer 2 is

used as a baud-rate generator. It is possible to use Timer 2

as a baud-rate generator and a clock generator simulta-

neously. Note, however, that the baud-rate and clock-out

frequencies cannot be determined independently from one

another since they both use RCAP2H and RCAP2L.

Modes 1 and 3 Baud Rates

Timer 2 Overflow Rate

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

Modes 1 and 3

Baud Rate

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

Oscillator Frequency

65536

RCAP2H,RCAP2L

()

–

[]

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

Clock-Out Frequency

Oscillator Frequency

65536

RCAP2H,RCAP2L

()

–

[]

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

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