
CHAPTER 24 STANDBY FUNCTION
User’s Manual U17790EJ2V0UD
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24.7 Subclock Operation Mode
24.7.1 Setting and operation status
The subclock operation mode is set by setting the PCC.CK3 bit 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.
Check whether the clock has been switched by using the PCC.CLS bit.
When the PCC.MCK bit is set to 1, the operation of the main clock oscillator is stopped. As a result, the system
operates only on the subclock.
In the subclock operation mode, the power 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 power consumption can
be further reduced to the level of the STOP mode by stopping the operation of the main clock oscillator.
Table 24-10 shows the operating status in subclock operation mode.
Cautions 1. When manipulating the CK3 bit, do not change the set values of the PCC.CK2 to PCC.CK0 bits
(using a bit manipulation instruction to manipulate the bit is recommended). For details of
the PCC register, see 6.3 (1) Processor clock control register (PCC).
2. If the following conditions are not satisfied, change the CK2 to CK0 bits so that the conditions
are satisfied and set the subclock operation mode.
Internal system clock (fCLK) > Subclock (fXT = 32.768 kHz)
× 4
Remark
Internal system clock (fCLK): Clock generated from main clock (fXX) in accordance with the settings of the
CK2 to CK0 bits
24.7.2 Releasing subclock operation mode
The subclock operation mode is released by a reset signal (reset by RESET pin input, WDT2RES signal, low-
voltage detector (LVI), or clock monitor (CLM)) when the CK3 bit is cleared to 0.
If the main clock is stopped (MCK bit = 1), set the MCK bit to 1, secure the oscillation stabilization time of the main
clock by software, and clear the CK3 bit to 0.
The normal operation mode is restored when the subclock operation mode is released.
Caution
When manipulating the CK3 bit, 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 of the PCC register, see 6.3 (1) Processor clock control register (PCC).