
390
μPD780948, μPD78F0948, μPD780949, μPD78F0949
Chapter 23 Standby Function
23.1
Standby Function and Configuration
23.1.1 Standby function
The standby function is designed to decrease power consumption of the system. The following two
modes are available.
(1) HALT mode
HALT instruction execution sets the HALT mode. The HALT mode is intended to stop the CPU operation
clock. System clock oscillator continues oscillation. In this mode, current consumption cannot be
decreased as in the STOP mode. The HALT mode is valid to restart immediately upon interrupt request
and to carry out intermittent operations such as watch applications.
(2) STOP mode
STOP instruction execution sets the STOP mode. In the STOP mode, the main system clock oscillator
stops and the whole system stops. CPU current consumption can be considerably decreased.
Data memory low-voltage hold (down to V
DD
= 4.0 V) is possible. Thus, the STOP mode is effective to
hold data memory contents with ultra-low current consumption. Because this mode can be cleared upon
interrupt request, it enables intermittent operations to be carried out.
However, because a wait time is necessary to secure an oscillation stabilization time after the STOP mode
is cleared, select the HALT mode if it is necessary to start processing immediately upon interrupt request.
In any mode, all the contents of the register, flag, and data memory just before standby mode setting
are held. The input/output port output latch and output buffer statuses are also held.
Cautions:
1. The STOP mode can be used only when the system operates with the main system
clock (subsystem clock oscillation cannot be stopped). The HALT mode can be used
with either the main system clock or the subsystem clock.
2. When proceeding to the STOP mode, be sure to stop the peripheral hardware
operation and execute the 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 (CS) to 0 to stop
the A/D conversion operation, and then execute the HALT or STOP instruction.