AT89S852-12PC View Datasheet(PDF) - Atmel Corporation

Part Name

Description

Manufacturer

AT89S852-12PC

8 bit Microcontroller with 8K bytes Flash

Atmel Corporation 

AT89S8252

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 regis-

ters to be reloaded with the 16 bit value in registers

RCAP2H and RCAP2L, which are preset by software.

The baud rates in Modes l and 3 are determined by Timer

2’s overflow rate according to the following equation.

Modes1 and 3 Baud Rates

=

Timer 2 OverflowRate

16

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. Nor-

mally, 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 oscilla-

tor frequency). The baud rate formula is given below.

Modes1 and 3

Baud Rate

=

Oscillator Frequency

32 x [65536 − (RCAP2H, RCAP2L)]

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

generate an interrupt. Note too, that if EXEN2 is set, a l-to-

0 transition in T2EX will set EXF2 but will not cause a re-

load 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.

Figure 5. Timer 2 in Clock-Out Mode

13

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