MC81F4x16
In the left case, Tr. base current flows from port to GND.
To avoid power consumption, there should be low output to
the port.
In the left case, much current flows from port to
GND.
Figure 27-7 Application Example of Unused Output Port
Note :
In the STOP operation, the power dissipation associated with the oscillator and the internal
hardware is lowered; however, the power dissipation associated with the pin interface
(depending on the external circuitry and program) is not directly determined by the hardware
operation of the STOP feature. This point should be little current flows when the input level is
stable at the power voltage level (VDD/VSS); however, when the input level becomes higher
than the power voltage level (by approximately 0.3V), a current begins to flow. Therefore, if
cutting off the output transistor at an I/O port puts the pin signal into the high impedance
state, a current flow across the ports input transistor, requiring it to fix the level by pull-up or
other means.
It should be set properly in order that current flow through port doesn't exist.
First consider the port setting to input mode. Be sure that there is circuit. In input mode, the pin
impedance viewing from external MCU is very high that the current doesn‟t flow. But input voltage
level should be VSS or VDD. Be careful that if unspecified voltage, i.e. if uncertain voltage level (not VSS
or VDD) is applied to input pin, there can be little current (max. 1mA at around 2V) flow.
If it is not appropriate to set as an input mode, then set to output mode considering there is no current
flow. The port setting to High or Low is decided by considering its relationship with external circuit. For
example, if there is external pull-up resistor then it is set to output mode, i.e. to High, and if there is
external pull-down register, it is set to low.
October 19, 2009 Ver.1.35
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