at89c51rd2-smsum ATMEL Corporation, at89c51rd2-smsum Datasheet - Page 81

• Manufacturer Part Number: at89c51rd2-smsum
• Description: 8-bit Flash Microcontroller
• Manufacturer: ATMEL Corporation

18.3
18.3.1
18.3.2
Figure 18-1. Power-Down Exit Waveform Using INT1:0#
4235J–8051–01/08
Power-Down Mode
Entering Power-Down Mode
Exiting Power-Down Mode
INT1:0#
OSC
The Power-Down mode places the AT89C51RD2/ED2 in a very low power state. Power-Down
mode stops the oscillator, freezes all clock at known states. The CPU status prior to entering
Power-Down mode is preserved, i.e., the program counter, program status word register retain
their data for the duration of Power-Down mode. In addition, the
served. The status of the Port pins during Power-Down mode is detailed in Table 18-1.
Note:
To enter Power-Down mode, set PD bit in PCON register. The AT89C51RD2/ED2 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 three ways to exit the Power-Down mode:
Note:
Note:
Active phase
1. Generate an enabled external interrupt.
2. Generate a reset.
– The AT89C51RD2/ED2 provides capability to exit from Power-Down using INT0#,
– A logic high on the RST pin clears PD bit in PCON register directly and
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 18-1). 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.
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
VCC may be reduced to as low as V
pation. Take care, however, that VDD is not reduced until Power-Down mode is invoked.
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 external interrupt used to exit Power-Down mode must be configured as level sensitive
(INT0# and INT1#) and must be assigned the highest priority. In addition, the duration of the inter-
rupt must be long enough to allow the oscillator to stabilize. The execution will only resume when
the interrupt is deasserted.
Exit from power-down by external interrupt does not affect the
Power-down phase
Oscillator restart phase
RET
during Power-Down mode to further reduce power dissi-
AT89C51RD2/ED2
SFRs
SFR
Active phase
and RAM contents are pre-
nor the internal RAM content.
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