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CHAPTER 24 STANDBY FUNCTION
User’s Manual U12697EJ3V0UM
24.6 Check Items When Using STOP or IDLE Mode
The checks required to decrease the current consumption when using the STOP mode or IDLE mode are described
below.
(1) Is the output level of each output pin appropriate?
The appropriate output level of each pin differs with the circuit in the next stage. Select the output level so that
the current consumption is minimized.
•
If a high level is output when the input impedance of the circuit in the next stage is low, current flows from
the power source to the port, and the current consumption increases. This occurs when the circuit in the next
stage is, for example, a CMOS IC. When the power supply is turned off, the input impedance of a CMOS
IC becomes low. To suppress the current consumption and not negatively affect the reliability of the CMOS
IC, output a low level. If a high level is output, latch-up results when the power supply is applied again.
•
Depending on the circuit in the next stage, the current consumption sometimes increases when a low level
is input. In this case, output a high level or high impedance to eliminate the current consumption.
•
When the circuit in the next stage is a CMOS IC, if the output is high impedance when power is supplied to
the CMOS IC, the current consumption of the CMOS IC sometimes increases (in this case, the CMOS IC
overheats and is sometimes destroyed). In this case, output a suitable level or pull-up or pull-down resistors.
The setting method for the output level differs with the port mode.
•
Since the output level is determined by the state of the internal hardware when the port is in the control mode,
the output level must be set while considering the state of the internal hardware.
•
The output level can be set by writing to the output latch of the port and the port mode register by the software
when in the port mode.
When the port enters the control mode, the port mode is changed by simply setting the output level.
(2) Is the input level to each input pin appropriate?
Set the voltage level input to each pin within the range from the V
SS
voltage to the V
DD
voltage. If a voltage outside
of this range is applied, not only does the current consumption increase, but the reliability of the
µ
PD784225 is
negatively affected.
In addition, do not increase the middle voltage.
(3) Are internal pullup resistors needed?
Unnecessary pull-up resistors increase the current consumption and are another cause of device latch-up. Set
the pull-up resistors to the mode in which they are used only in the required parts.
When the parts needing pull-up resistors and the parts not needing them are mixed together, externally connect
the pull-up resistors where they are needed and set the mode in which the internal pull-up resistors are not used.
