AT89 datasheet(16/41 Pages) ATMEL

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AT89S8252

0401F–MICRO–11/03

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 regular I/0 pin, has two alternate functions. It can be pro-

grammed to input the external clock for Timer/Counter 2 or to output a 50% duty cycle

clock ranging from 61 Hz to 4 MHz (for 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 frequency 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 simultaneously. Note, however, that the

baud-rate and clock-out frequencies cannot be determined independently from one

another since they both use RCAP2H and RCAP2L.

Clock Out Frequency

Oscillator Frequency

65536

RCAP2H,RCAP2L

()

–

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AT89 Datasheet(PDF) 16 Page - ATMEL Corporation