Calculator User Manual

Power-Down Mode
5-36
5.8 Power-Down Mode
The ’C2xx has a power-down mode that allows the ’C2xx core to enter a dor-
mant state and use less power than during normal operation. Executing an
IDLE instruction initiates power-down mode. When the IDLE instruction
executes, the program counter is incremented once, and then all CPU activi-
ties are halted. While the ’C2xx is in power-down mode, all of its internal con-
tents are maintained. The content of all on-chip RAM remains unchanged. The
peripheral circuits continue to operate, allowing the serial ports and the timer
to take the CPU out of the power-down state. The CLKOUT1 pin remains ac-
tive if bit 0 of the CLK register is set to 0.
The methods for terminating power-down mode depend on whether the pow-
er-down was initiated under normal circumstances or as part of a HOLD opera-
tion. The following subsections describe the differences.
5.8.1 Normal Termination of Power-Down Mode
If power-down has been initiated, any hardware interrupt (internal or external)
takes the processor out of the IDLE state. If you use reset or NMI
, the CPU will
immediately execute the corresponding interrupt service routine. In addition,
if you use reset, registers will assume their reset values.
For a maskable hardware interrupt to wake the processor, it must be un-
masked by the interrupt mask register (IMR bit = 1). However, if the interrupt
is unmasked and is then requested, the processor will leave the IDLE state re-
gardless of the value of the INTM bit (bit 9 of status register ST0). The value
of the INTM bit will only determine the action of the CPU
after power-down has
been terminated:
INTM = 0. The interrupt is enabled, and the CPU executes the correspond-
ing interrupt service routine.
INTM = 1. The interrupt is disabled, and the CPU continues with the
instruction after IDLE.
If you do not want the CPU to follow an interrupt service routine before continu-
ing with the interrupted program sequence:
Do not use reset or NMI to bring the processor out of power-down.
Make sure your program globally disables maskable interrupts (sets INTM
to 1) before IDLE is executed.