AT89LP51 datasheet(55/113 Pages) ATMEL

55 / 113 page

3709C–MICRO–5/11

AT89LP51/52 - Preliminary

Figure 12-5. Timer 2 Diagram: Auto-Reload Mode (DCEN = 1)

The timer overflow/underflow rate for up-down counting mode is the same as for up counting

mode, provided that the count direction does not change. Changes to the count direction may

result in longer or shorter periods between time-outs.

12.4

Baud Rate Generator

Timer 2 is selected as the baud rate generator by setting TCLK and/or RCLK in T2CON (Table

12-3). 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 12-6.

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 UART 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 con-

figured for timer operation (CP/T2 = 0). The baud rate formulas are 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 12-6. 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 inter-

rupt. 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 gen-

erator, T2EX can be used as an extra external interrupt. Also note that the Baud Rate and

Frequency Generator modes may be used simultaneously.

÷TPS

Modes 1 and 3 Baud Rates

Timer 2 Overflow Rate

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

Modes 1, 3

Baud Rate

System Frequency

TPS

()

65536

RCAP2H,RCAP2L

()

–

[]

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