AT89S53_00 datasheet(17/33 Pages) ATMEL

17 / 33 page

AT89S53

Timer 2 interrupt is generated by the logical OR of bits TF2

and EXF2 in register T2CON. Neither of these flags is

cleared by hardware when the service routine is vectored

to. In fact, the service routine may have to determine

whether it was TF2 or EXF2 that generated the interrupt,

and that bit will have to be cleared in software.

The Timer 0 and Timer 1 flags, TF0 and TF1, are set at

S5P2 of the cycle in which the timers overflow. The values

are then polled by the circuitry in the next cycle. However,

the Timer 2 flag, TF2, is set at S2P2 and is polled in the

same cycle in which the timer overflows.

Oscillator Characteristics

XTAL1 and XTAL2 are the input and output, respectively,

of an inverting amplifier that can be configured for use as

an on-chip oscillator, as shown in Figure 11. Either a quartz

crystal or ceramic resonator may be used. To drive the

device from an external clock source, XTAL2 should be left

unconnected while XTAL1 is driven, as shown in Figure 12.

There are no requirements on the duty cycle of the external

clock signal, since the input to the internal clocking circuitry

is through a divide-by-two flip-flop, but minimum and maxi-

mum voltage high and low time specifications must be

observed.

Figure 11. Oscillator Connections

Note:

C1, C2 = 30 pF ± 10 pF for Crystals

= 40 pF ± 10 pF for Ceramic Resonators

Figure 12. External Clock Drive Configuration

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