NEC UPD789026 Series, User Manual

Page: 167 / 213

User's Manual U11919EJ3V0UM00
167
CHAPTER 11 STANDBY FUNCTION
11.1 Standby Function and Configuration
11.1.1 Standby function
The standby function is to reduce the power consumption of the system and can be effected in the following two
modes:
(1) HALT mode
This mode is set when the HALT instruction is executed. The HALT mode stops the operation clock of the
CPU. The system clock oscillation circuit continues oscillating. This mode does not reduce the current
drain as much as the STOP mode, but is useful for resuming processing immediately when an interrupt
request is generated, or for intermittent operations.
(2) STOP mode
This mode is set when the STOP instruction is executed. The STOP mode stops the system clock
oscillation circuit and stops the entire system. The current drain of the CPU can be substantially reduced in
this mode.
Data memory can be retained at low voltages (V
DD
= 1.8 V min.). Therefore, this mode is useful for
retaining the contents of the data memory at an extremely low current.
The STOP mode can be released by an interrupt request, so that this mode can be used for intermittent
operation. However, some time is required until the system clock oscillation circuit settles after the STOP
mode has been released. If processing must be resumed immediately by using an interrupt request,
therefore, use the HALT mode.
In both modes, the previous contents of the registers, flags, and data memory before setting standby mode are
all retained. In addition, the statuses of the output latch of the I/O ports and output buffer are also retained.
Caution To set the STOP mode, be sure to stop the operations of the peripheral hardware, and then
execute the STOP instruction.
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