Renesas 4514 - Manual (Page 191)

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APPENDIX
3.4 Notes on noise
4513/4514 Group User’s Manual
3.4 Notes on noise
Countermeasures against noise are described below.
The following countermeasures are effective against
noise in theory, however, it is necessary not only to
take measures as follows but to evaluate before actual
use.
3.4.1 Shortest wiring length
The wiring on a printed circuit board can function
a s a n a n t e n n a w h i c h f e e d s n o i s e i n t o t h e
microcomputer.
The shorter the total wiring length (by mm unit), the
less the possibility of noise insertion into a
microcomputer.
(1) Package
Select the smallest possible package to make
the total wiring length short.
● Reason
The wiring length depends on a microcom-
puter package. Use of a small package,
for example QFP and not DIP, makes the
total wiring length short to reduce influence
of noise.
Fig. 3.4.1 Selection of packages
(2) Wiring for RESET input pin
Make the length of wiring which is connected
to the RESET input pin as short as possible.
Especially, connect a capacitor across the
RESET input pin and the V
SS pin with the
shortest possible wiring.
● Reason
In order to reset a microcomputer correctly,
1 machine cycle or more of the width of a
pulse input into the RESET pin is required.
If noise having a shorter pulse width than
this is input to the RESET input pin, the
reset is released before the internal state
of the microcomputer is completely initialized.
This may cause a program runaway.
DIP
SDIP
SOP
QFP
Fig. 3.4.2 Wiring for the RESET input pin
RESET
Reset
circuit
Noise
V
SS
V
SS
Reset
circuit
V SS
RESET
V SS
N.G.
O.K.