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CHAPTER 3 CPU
3.7.4
Watch Mode
This section describes the operations of watch mode.
I Operation of watch mode
G 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 (watch prescaler 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 watch 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 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.)
G Wake-up from watch mode
A reset, a watch prescaler interrupt or an external interrupt wakes up CPU from watch mode.
If a reset occurs during watch mode on a product, the reset operation starts after the main clock oscillation
stabilization wait time.
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 watch prescaler 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.