Datasheet

12
2487D–MICRO–6/08
AT89S51
Figure 10-1. Interrupt Sources
11. 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 11-1. 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 11-2. There are no
requirements on the duty cycle of the external clock signal, since the input to the internal clock-
ing circuitry is through a divide-by-two flip-flop, but minimum and maximum voltage high and low
time specifications must be observed.
Figure 11-1. Oscillator Connections
Note: C1, C2 = 30 pF ± 10 pF for Crystals
= 40 pF ± 10 pF for Ceramic Resonators
IE1
IE0
1
1
0
0
TF1
TF0
INT1
INT0
TI
RI
C2
XTAL2
GND
XTAL1
C1