Recommendation for Hardware Setup 32-Bit FR81S Family
Document No. 002-09375 Rev. *B
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4 Port Input / Unused Pins / Latch-up
How to connect input port pins and how to proceed with unused pins
4.1
Port Input / Unused Pins
It is strongly recommended not to leave Input Pins unconnected. In this case those pins can enter a so-called floating
state. This can cause a high ICC current, which is adverse to low power modes. Also damage of the MCU can happen.
Use the internal pull-up/down resistors, if the port provides such function. If not, use external pull-up or pull-down
resistors to define the input-level. If both solutions are not possible, set the Port Pin to Output.
Never connect a potential divider with almost same resistor values.
Figure 19. Principle using of input circuit to avoid latch-up or leak current
Be careful with connection of input pins to other devices, which can go into High-Z states. Always use pull-up or
pull-down resistors in this case.
Outputs from external circuits should always be connected via a serial resistor to a MCU input pin.
Debouncing and decoupling capacitors should always be chosen as smallest as possible. Please refer to Latch-up
consideration (switch).
All pins are set to input Hi-Z after its power-on default. Therefore set unused pins to input with internal pull-up/down
resistor, or provide them with pull-up or pull-down resistors.
