Cypress FR81S CY91F52 Series Recommendation For Hardware Setup page 23

Recommendation for Hardware Setup 32-Bit FR81S Family
A better solution is to use a series resistor at the port pin like in the following schematic:
Figure 27. Series resistor
The series resistor RS reduces the amplitude of the oscillation and decreases the voltage offset at first. The resistor
must not be chosen too high, so that the port pin input voltage VP is within the positive CMOS/TTL/Automotive level.
Figure 28. Reduction of the signal on the pin due to the series resistor
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Document No. 002-09375 Rev. *B 23