user manual

8XC251SA, SB, SP, SQ USER’S MANUAL

12-6

12.4.1 Entering Powerdown Mode

To enter powerdown mode, set the PCON register PD bit. The 8XC251Sx enters the power-down

mode upon execution of the instruction that sets the PD bit. The instruction that sets the PD bit is

the last instruction executed.

12.4.2 Exiting Powerdown Mode

CAUTION

If V

was reduced during the powerdown mode, do not exit powerdown until

is restored to the normal operating level.

There are two ways to exit the powerdown mode:

• Generate an enabled external interrupt. Hardware clears the PD bit in the PCON register

which starts the oscillator and restores the clocks to the CPU and peripherals. 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 powerdown mode.

NOTE

To enable an external interrupt, set the IE register EX0 and/or EX1 bit[s]. The

external interrupt used to exit powerdown mode must be configured as level

sensitive and must be assigned the highest priority. In addition, the duration of

the interrupt must be of sufficient length to allow the oscillator to stabilize.

• Generate a reset. See section 11.4, “Reset.” A logic high on the RST pin clears the PD bit in

the PCON register directly and asynchronously. This starts the oscillator and restores the

clocks to the CPU and peripherals. Program execution momentarily resumes with the

instruction immediately following the instruction that activated powerdown and may

continue for a number of clock cycles before the internal reset algorithm takes control.

Reset initializes the 8XC251Sx and vectors the CPU to address FF:0000H.

NOTE

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 powerdown mode should not write to a port pin or to the

external RAM.