7220 Datasheet, PDF (234/303 Pages) Renesas Technology Corp – 8-BIT SINGLE-CHIP MICROCOMPUTER

APPENDIX
6.4 Countermeasures against noise
6.4.5 Setup for I/O ports
Setup I/O ports using hardware and software as follows:
<Hardware>
qConnect a resistor of 100 Ω or more to an I/O port in series.
<Software>
qAs for an input port, read data several times by a program for checking whether input levels are equal
or not.
qAs for an output port, since the output data may reverse because of noise, rewrite data to its data register
at fixed periods.
qRewrite data to direction registers and pull-up control registers (only the product having it) at fixed
periods.
When a direction register is set for input port again at fixed periods, a several-nanosecond short pulse may
be output from this port. If this is undesirable, connect a capacitor to this port to remove the noise pulse.
N.G.
XIN
XOUT
VSS
Fig. 6.4.10 Setup for I/O ports
XIN
XOUT
VSS
O.K.
7220 Group User’s Manual
6-15