Chapter 18 Standby Function; Standby Function And Configuration; Standby Function - NEC 78K0/KD1 Series User Manual

18.1 Standby Function and Configuration

18.1.1 Standby function

Table 18-1. Relationship Between HALT Mode, STOP Mode, and Clock
HALT mode
STOP mode
The standby function is designed to reduce the power consumption 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, but the
system clock oscillator continues oscillating. In this mode, power consumption 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.
(2) STOP mode
STOP instruction execution sets the STOP mode. In the STOP mode, the X1 input clock oscillator stops,
stopping the whole system, thereby considerably reducing the CPU power consumption.
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, 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. STOP mode can be used only when operating on the X1 input clock or Ring-OSC clock.
HALT mode can be used when operating on the X1 input clock, Ring-OSC clock, or
subsystem clock. However, when the STOP instruction is executed during Ring-OSC clock
operation, the X1 oscillator stops, but Ring-OSC oscillator does not stop.
2. When shifting to the STOP mode, be sure to stop the peripheral hardware operation before
executing STOP instruction.
3. The following sequence is recommended for power consumption reduction of the A/D
converter when the standby function is used: First clear bit 7 (ADCS) of the A/D converter
mode register (ADM) to 0 to stop the A/D conversion operation, and then execute the HALT or
STOP instruction.
4. Ring-OSC clock oscillation cannot be stopped in the STOP mode. However, when the Ring-
OSC clock is used as the CPU clock, the CPU operation is stopped for 17/f
mode is released.

CHAPTER 18 STANDBY FUNCTION

X1 Input Clock Ring-OSC Clock
Oscillation continues Oscillation continues
Oscillation stopped Oscillation continues
Preliminary User's Manual U16315EJ1V0UD
Subsystem Clock
Oscillation continues Operation stopped
Oscillation continues Operation stopped
CPU Clock
(s) after STOP
R
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