4552 Datasheet, PDF (60/145 Pages) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

4552 Group
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 fol-
lows but to evaluate before actual use.
1. Shortest wiring length
(2) Wiring for clock input/output pins
• Make the length of wiring which is connected to clock I/O pins as
short as possible.
• Make the length of wiring across the grounding lead of a capaci-
tor which is connected to an oscillator and the VSS pin of a
microcomputer as short as possible.
• Separate the VSS pattern only for oscillation from other VSS pat-
terns.
(1) Wiring for RESET pin
Make the length of wiring which is connected to the RESET pin as
short as possible. Especially, connect a capacitor across the
RESET pin and the VSS 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 mi-
crocomputer is completely initialized.
This may cause a program runaway.
Noise
<Reason>
If noise enters clock I/O pins, clock waveforms may be deformed.
This may cause a program failure or program runaway. Also, if a
potential difference is caused by the noise between the VSS level of
a microcomputer and the VSS level of an oscillator, the correct
clock will not be input in the microcomputer.
Noise
XIN
XOUT
VSS
XIN
XOUT
VSS
Reset
circuit
VSS
N.G.
RESET
VSS
N.G.
Fig. 55 Wiring for clock I/O pins
O.K.
Reset
circuit
VSS
O.K.
Fig. 54 Wiring for the RESET pin
RESET
VSS
Rev.3.02 Dec 22, 2006 page 60 of 142
REJ03B0023-0302