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 Fig-
ure 8. 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 mini-
mum and maximum voltage high and low time specifica-
tions must be observed.
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 re-
set, the device normally resumes program execution from
where it left off, up to two machine cycles before the inter-
nal reset algorithm takes control. On-chip hardware inhib-
its 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 in-
vokes idle mode should not write to a port pin or to external
memory.
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 termi-
nated. 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
is restored to its normal operating level and must be held
active long enough to allow the oscillator to restart and
stabilize.
XTAL2
XTAL1
GND
NC
EXTERNAL
OSCILLATOR
SIGNAL
Figure 8. External Clock Drive Configuration
C2
XTAL2
GND
XTAL1
C1
Figure 7. Oscillator Connections
Notes: C1, C2 = 30 pF
±
10 pF for Crystals
= 40 pF
±
10 pF for Ceramic Resonators
Status of External Pins During Idle and Power Down
Mode
Program Memory
ALE
PSEN
PORT0
PORT1
PORT2
PORT3
Idle
Internal
1
1
Data
Data
Data
Data
Idle
External
1
1
Float
Data
Address
Data
Power Down
Internal
0
0
Data
Data
Data
Data
Power Down
External
0
0
Float
Data
Data
Data
AT89C52
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