Oscillator
Characteristics
XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier which
can be configured for use as an on-chip oscillator, as shown in Figure 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 2.
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
maximum voltage high and low time specifications must be observed.
Figure 1. Oscillator Connections
Note:
C1, C2 = 30 pF 10 pF for Crystals
= 40 pF 10 pF for Ceramic Resonators
Figure 2. External Clock Drive Configuration
X2 Mode Description The clock for the entire circuit and peripherals is normally divided by 2 before being
used by the CPU core and peripherals. This allows any cyclic ratio (duty cycle) to be
accepted on XTAL1 input. In X2 mode, as this divider is bypassed, the signals on
XTAL1 must have a cyclic ratio (duty cycle) between 40% to 60%. Figure 3 shows the
clock generation block diagram.
Figure 3. Clock Generation Block Diagram
X2 Mode
(XTAL1)/2
÷
2
XTAL1
FXTAL
State Machine: 6 Clock Cycles
CPU Control
FOSC
4
AT89C2051x2
3285B–MICRO–10/03
ATMEL [ ATMEL ]
