at89c51ac3-s3sim ATMEL Corporation, at89c51ac3-s3sim Datasheet - Page 32

• Manufacturer Part Number: at89c51ac3-s3sim
• Description: At89c51ac3 Enhanced 8-bit Microcontroller With 64kb Flash Memory
• Manufacturer: ATMEL Corporation

Entering Power-Down Mode
Exiting Power-Down Mode
Figure 19. Power-Down Exit Waveform Using INT1:0#
32
AT89C51AC3
INT1:0#
OSC
Active phase
To enter Power-Down mode, set PD bit in PCON register. The AT89C51AC3 enters the
Power-Down mode upon execution of the instruction that sets PD bit. The instruction
that sets PD bit is the last instruction executed.
Note:
There are two ways to exit the Power-Down mode:
1. Generate an enabled external interrupt.
Note:
Note:
2. Generate a reset.
Note:
Note:
–
–
If VCC was reduced during the Power-Down mode, do not exit Power-Down mode until
VCC is restored to the normal operating level.
The AT89C51AC3 provides capability to exit from Power-Down using INT0#,
INT1#.
Hardware clears PD bit in PCON register which starts the oscillator and
restores the clocks to the CPU and peripherals. Using INTx# input,
execution resumes when the input is released (see Figure 19). Execution
resumes with the interrupt service routine. Upon completion of the interrupt
service routine, program execution resumes with the instruction immediately
following the instruction that activated Power-Down mode.
The external interrupt used to exit Power-Down mode must be configured as level sensi-
tive (INT0# and INT1#) and must be assigned the highest priority. In addition, the
duration of the interrupt must be long enough to allow the oscillator to stabilize. The exe-
cution will only resume when the interrupt is deasserted.
Exit from power-down by external interrupt does not affect the
content.
A logic high on the RST pin clears PD bit in PCON register directly and
asynchronously. This starts the oscillator and restores the clock to the CPU
and peripherals. Program execution momentarily resumes with the
instruction immediately following the instruction that activated Power-Down
mode and may continue for a number of clock cycles before the internal
reset algorithm takes control. Reset initializes the AT89C51AC3 and vectors
the CPU to address 0000h.
During the time that execution resumes, the internal RAM cannot be accessed; however,
it is possible for the Port pins to be accessed. To avoid unexpected outputs at the Port
pins, the instruction immediately following the instruction that activated the Power-Down
mode should not write to a Port pin or to the external RAM.
Exit from power-down by reset redefines all the
RAM content.
Power-down phase
Oscillator restart phase
SFRs
, but does not affect the internal
Active phase
SFRs
4383D–8051–02/08
nor the internal RAM