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User’s Manual U19780EJ2V0UD
537
CHAPTER 20 STANDBY FUNCTION
20.1 Standby Function and Configuration
20.1.1 Standby function
The standby function
is mounted onto all 78K0/Kx2-A microcontroller products.
The standby function is designed to reduce the operating current of the system. The following two modes are
available.
(1) HALT
mode
HALT instruction execution sets the HALT mode. In the HALT mode, the CPU operation clock is stopped. If the
high-speed system clock oscillator, internal high-speed oscillator, internal low-speed oscillator, or subsystem
clock oscillator is operating before the HALT mode is set, oscillation of each clock continues. In this mode, the
operating current is not decreased as much as in the STOP mode, but the HALT mode is effective for restarting
operation immediately upon interrupt request generation and carrying out intermittent operations frequently.
Note
The 78K0/KB2-A is not provided with a subsystem clock oscillator.
(2) STOP
mode
STOP instruction execution sets the STOP mode. In the STOP mode, the high-speed system clock oscillator and
internal high-speed oscillator stop, stopping the whole system, thereby considerably reducing the CPU operating
current.
Because this mode can be cleared by an interrupt request, it enables intermittent operations to be carried out.
However, because a wait time is required to secure the oscillation stabilization time after the STOP mode is
released when the X1 clock is selected, select the HALT mode if it is necessary to start processing immediately
upon interrupt request generation.
In either of these two modes, all the contents of registers, flags and data memory just before the standby mode is
set are held. The I/O port output latches and output buffer statuses are also held.
Cautions 1. The STOP mode can be used only when the CPU is operating on the main system clock. The
subsystem clock oscillation cannot be stopped. The HALT mode can be used when the CPU
is operating on either the main system clock or the subsystem clock.
2. When shifting to the STOP mode, be sure to stop the peripheral hardware operation
operating with main system clock before executing STOP instruction.
3. To reduce the operating current of the A/D converter block, execute the STOP instruction
after performing the following processing:
•
Stop the A/D conversion operation:
Clear bit 7 (ADCS) and bit 0 (ADCE) of the A/D
converter mode register (ADM) to 0
•
Stop the A/D converter voltage booster: Clear bit 1 (VRGV) of the analog reference
voltage control register (ADVRC) to 0.