UPD784938 Datasheet, PDF (653/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 25 STANDBY FUNCTION
25.6 Check Items when STOP Mode/IDLE Mode is Used
Check items required to reduce the current consumption when STOP mode/IDLE mode is used are shown below.
(1) Is the output level of each output pin appropriate?
The appropriate output level for each pin varies according to the next-stage circuit. You should select the output level
that minimizes the current consumption.
• If high level is output when the input impedance of the next-stage circuit is low, a current will flow from the power
supply to the port, resulting in an increased current consumption. This applies when the next-stage circuit is a CMOS
IC, etc. When the power supply is off, the input impedance of a CMOS IC is low. In order to suppress the current
consumption, or to prevent an adverse effect on the reliability of the CMOS IC, low level should be output. If a high
level is output, latchup may result when power is turned on again.
• Depending on the next-stage circuit, inputting low level may increase the current consumption. In this case, high-
level or high-impedance output should be used to reduce the current consumption.
• If the next-stage circuit is a CMOS IC, the current consumption of the CMOS IC may increase if the output is made
high-impedance when power is supplied to it (the CMOS IC may also be overheated and damaged). In this case
you should output an appropriate level, or pull the output high or low with a resistor.
The method of setting the output level depends on the port mode.
• When a port is in control mode, the output level is determined by the status of the on-chip hardware, and therefore
the on-chip hardware status must be taken into consideration when setting the output level.
• In port mode, the output level can be set by writing to the port output latch and port mode register by software.
When a port is in control mode, its output level can be set easily by changing to port mode.
Preliminary User’s Manual U13987EJ1V0UM00
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