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CHAPTER 3 CPU
3.7.4
Watch Mode
This section describes the operations of watch mode.
s Operation of watch Mode
r Changing to watch mode
Watch mode stops the clocks that clock the CPU and the main peripheral functions. You can go
to watch mode only from subclock mode (in which the main clock oscillation is stopped).
Prior to going to watch mode, registers are saved and the contents of RAM are held. All chip
functions other than watch prescaler (timeclock interrupt), external interrupt circuit, and certain
functions that run off of the subclock stop. Accordingly, data can be held with extremely small
power consumption.
Writing "1" to the timeclock bit in the standby control register (STBC: TMD) changes the CPU to
watch mode.
This can be done, however, only when the system clock select bit of the system clock control
register (SYCC: SCS) is "0" (subclock mode active).
When you go to watch mode, external pin states are held if the pin state specification bit in the
standby control register (STBC: SPL) is "0" If SPL is "1" external pins go the high-impedance
state. (Pins with a pull-up resistor (optional) go to the "H" level)
If an interrupt request is generated when "1" is written to the TMD bit, the write to the bit is
ignored, and the CPU continues the instruction execution without change to watch mode. (The
CPU does not assume watch mode even after completion of the interrupt processing.)
r Wake-up from watch mode
A reset, a timeclock interrupt or an external interrupt wakes up CPU from watch mode.
If a reset occurs during watch mode on a product with power-on reset, the reset operation starts
after the main clock oscillation stabilization delay time.
Products without power-on reset do not require for the oscillation stabilization delay time after a
reset in watch mode.
The reset initializes pin states.
If an interrupt request with an interrupt level higher than "11" occurs from a watch prescaler or
an external interrupt circuit during watch mode, the CPU wakes up from watch mode, regardless
of the interrupt enable flag (CCR:I) and interrupt level bits ((CCR: IL1, IL0) in the CPU. Only
timeclock or external interrupt requests can occur during watch mode because most of the
peripheral functions except watch prescaler are stopped.
After wake-up from stop mode, the normal interrupt operation is performed. If the interrupt
request is accepted, the CPU executes interrupt processing. If the interrupt request is not
accepted, the CPU continues execution from the subsequent instruction following the instruction
executed immediately before entering watch mode.
Some peripheral functions restart from mid-operation when the CPU wakes up from watch
mode. The first interval time from the interval timer function, for example, is indeterminate.
Therefore,initialize all peripheral functions after wake-up from watch mode.