Fujitsu ALL FR 460 Series Application Note page 18

These overshoots may cause an internal latch-up on the port pin, because the internal
clamping diode connected to VDD becomes conductive. Similar is the effect, if the switch SW
is opened. In this case there are under shoots on the port pin.
The frequency of the oscillation can be calculated by
The inductivity L is the unknown value and depends on the PCB, its routing, and the wire
X
lengths.
There are two counter measurements to prevent from latch-up.
One solution is to decrease the capacity of the debouncing capacitor. This increases the
oscillation frequency, and the over-all energy of the overshoots is smaller.
This solution has two disadvantages: First the debouncing effect decreases and second, there
is no guarantee, that the latch-up condition is eliminated.
A better solution is to use a series resistor at the port pin like in the following schematic:
MCU-AN-300033-E-V15
FR460 Hardware Set Up
Chapter 3 Port Input / Unused Pins / Latch-up
=
f
OSC
"big" capacity
1
.
2 L C
X DB
"small" capacity
- 18 - © Fujitsu Microelectronics Europe GmbH