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FR460 Hardware Set Up
Chapter 3 Port Input / Unused Pins / Latch-up
All pins are set to input HiZ 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.
Do not connect any input ports directly to VDD or VSS (GND)! Always use pull up or
down resistors (2k ... 4k Ohms). If available it is possible to use the internal pull up or
pull down resistors as well. Please note the value internal resistors can be 20k ... 100k
depends on the device and temperature.
3.2 Latch-up consideration (switch)
Be careful with external switches to VDD or ground together with debouncing capacitors
connected to port pins.
A usual configuration is shown in the following schematic:
R
is a pull-down resistor and C
a debouncing capacitor. If the switch SW is open, a "0" is
PD
BD
read from the port pin Pxy. If the switch is closed the input changes to "1".
From the physical aspect, it has to be considered, that the switch is often placed in distance to
the MCU by cable, wire, or circuit path. The longer the circuit path is the higher will be its
inductivity L
(and capacity C
).
X
X
An equivalent circuit diagram looks like the following illustration:
By closing the switch SW at time t
the following voltage can be measured at point (A):
0
MCU-AN-300033-E-V15
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© Fujitsu Microelectronics Europe GmbH
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