AT89S51-33AC ATMEL [ATMEL Corporation], AT89S51-33AC Datasheet - Page 13

• Manufacturer Part Number: AT89S51-33AC
• Description: 8-bit Microcontroller with 4K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

12. Idle Mode
13. Power-down Mode
2487C–MICRO–03/05
Figure 11-2. External Clock Drive Configuration
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 special function regis-
ters 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 pro-
gram 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 terminated by a reset, the instruction following the one that invokes
idle mode should not write to a port pin or to external memory.
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 Registers retain their
values until the Power-down mode is terminated. Exit from Power-down mode can be initiated
either by a hardware reset or by activation of an enabled external interrupt (INT0 or INT1). Reset
redefines the SFRs but does not change the on-chip RAM. The reset should not be activated
before V
the oscillator to restart and stabilize.
Table 13-1.
Mode
Idle
Idle
Power-down
Power-down
CC
is restored to its normal operating level and must be held active long enough to allow
Status of External Pins During Idle and Power-down Modes
Program Memory
Internal
External
Internal
External
OSCILLATOR
EXTERNAL
SIGNAL
ALE
NC
1
0
0
1
PSEN
1
1
0
0
XTAL2
XTAL1
GND
PORT0
Data
Float
Data
Float
PORT1
Data
Data
Data
Data
Address
PORT2
Data
Data
Data
AT89S51
PORT3
Data
Data
Data
Data
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