
CHAPTER 21 STANDBY FUNCTION
User
’
s Manual U15862EJ3V0UD
666
21.6 Subclock Operation Mode
21.6.1 Setting and operation status
The subclock operation mode is set when the CK3 bit of the processor clock control register (PCC) is set to 1 in the
normal operation mode.
When the subclock operation mode is set, the internal system clock is changed from the main clock to the
subclock.
When the MCK bit of the PCC register is set to 1, the operation of the main clock oscillator is stopped. As a result,
the system operates only with the subclock. However, watchdog timer 1 stops counting when subclock operation is
started (CLS bit of PCC register = 1). (Watchdog timer 1 retains the value before the subclock operation mode was
set.)
Table 21-8 shows the operation status in subclock operation mode.
In the subclock operation mode, the current consumption can be reduced to a level lower than in the normal
operation mode because the subclock is used as the internal system clock. In addition, the current consumption can
be further reduced to the level of the STOP mode by stopping the operation of the main system clock oscillator.
Caution
When manipulating the CK3 bit of the PCC register, do not change the set values of the CK2 to
CK0 bits (using a bit manipulation instruction to manipulate the bit is recommended).
For details, refer to 6.3 (1) Processor clock control register (PCC).
21.6.2 Releasing subclock operation mode
The subclock operation mode is released by RESET pin input when the CK3 bit of the PCC register is cleared to 0.
If the main clock is stopped (MCK bit of PCC register = 1), set the MCK bit of the PCC register to 1, secure the
oscillation stabilization time of the main clock by software, and clear the CK3 bit of the PCC register to 0.
The normal operation mode is restored when the subclock operation mode is released.
Caution
When manipulating the CK3 bit of the PCC register, do not change the set values of the CK2 to
CK0 bits (using a bit manipulation instruction to manipulate the bit is recommended).
For details, refer to 6.3 (1) Processor clock control register (PCC).