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 7. 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 8.
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.
is restored to its normal operating level and must be held
active long enough to allow the oscillator to restart and
stabilize.
Figure 7.
Oscillator Connections
C2
XTAL2
C1
XTAL1
Idle Mode
In idle mode, the CPU puts itself to sleep while all the on-
chip peripherals remain active. The mode is invoked by
software. The content of the on-chip RAM and all the spe-
cial functions registers remain unchanged during this
mode. The idle mode can be terminated by any enabled
interrupt or by a hardware reset.
Note that when idle mode is terminated by a hardware
reset, the device normally resumes program execution
from where it left off, up to two machine cycles before the
internal reset algorithm takes control. On-chip hardware
inhibits access to internal RAM in this event, but access to
the port pins is not inhibited. To eliminate the possibility of
an unexpected write to a port pin when idle mode is termi-
nated by a reset, the instruction following the one that
invokes idle mode should not write to a port pin or to exter-
nal memory.
GND
Note:
C1, C2 = 30 pF
±
10 pF for Crystals
= 40 pF
±
10 pF for Ceramic Resonators
Figure 8.
External Clock Drive Configuration
NC
XTAL2
Power-down Mode
In the power-down mode, the oscillator is stopped, and the
instruction that invokes power-down is the last instruction
executed. The on-chip RAM and Special Function Regis-
ters retain their values until the power-down mode is
terminated. The only exit from power-down is a hardware
reset. Reset redefines the SFRs but does not change the
on-chip RAM. The reset should not be activated before V
CC
EXTERNAL
OSCILLATOR
SIGNAL
XTAL1
GND
Status of External Pins During Idle and Powe-down Modes
Mode
Idle
Idle
Power-down
Power-down
Program Memory
Internal
External
Internal
External
ALE
1
1
0
0
PSEN
1
1
0
0
PORT0
Data
Float
Data
Float
PORT1
Data
Data
Data
Data
PORT2
Data
Address
Data
Data
PORT3
Data
Data
Data
Data
12
AT89C52
ATMEL [ ATMEL CORPORATION ]
