NEC PD754144 - Manual (Page 215)

215 User’s Manual U10676EJ3V0UM
CHAPTER 8 STANDBY FUNCTION
The
µPD754244 possesses a standby function that reduces the power consumption of the system. This standby
function can be implemented in the following two modes.
• STOP mode
• HALT mode
The functions of the STOP and HALT modes are as follows.
(1) STOP mode
In this mode, the system clock oscillator is stopped and therefore the entire system is stopped. The power
consumption of the CPU is substantially reduced.
Moreover, the contents of the data memory can be retained at a low voltage (V DD = 1.8 V MIN.). This mode
is therefore useful for retaining the data memory contents with an extremely low current consumption.
The STOP mode of the µPD754244 can be released by an interrupt request; therefore, the microcontroller
can operate intermittently. However, because a certain wait time is required for stabilizing the oscillation of
the clock oscillator when the STOP mode has been released, use the HALT mode if processing must be started
immediately after the standby mode has been released by an interrupt request.
(2) HALT mode
In this mode, the operating clock of the CPU is stopped. Oscillation of the system clock oscillator continues.
This mode does not reduce the power consumption as much as the STOP mode, but it is useful when
processing must be resumed immediately when an interrupt request is issued, or for an intermittent operation
such as a watch operation.
In either mode, all the contents of the registers, flags, and data memory immediately before the standby mode
is set are retained. Moreover, the contents of the output latches and output buffers of the I/O ports are also retained;
therefore, the statuses of the I/O ports are processed in advance so that the current consumption of the overall system
can be minimized.
The following page describes the points to be noted in using the standby mode.
Cautions 1. You can operate the µPD754144 efficiently with a low current consumption at a low voltage
by selecting the standby mode and CPU clock. In any case, however, the time described in
6.2.3 Setting CPU clock is required until the operation is started with the new clock when
the clock has been changed by manipulating the control register. To use the clock switching
function and standby mode in combination, therefore, set the standby mode after the time
required for switching has elapsed.
2. To use the standby mode, process so that the current consumption of the I/O ports is
minimized.
Especially, do not leave input ports open; be sure to input either a low or high level.