M37544 Datasheet, PDF (41/68 Pages) Renesas Technology Corp – SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

7544 Group
NOTES ON USE
Countermeasures against noise
1. Shortest wiring length
(1) Package
Select the smallest possible package to make the total wiring
length short.
<Reason>
The wiring length depends on a microcomputer package. Use of a
small package, for example QFP and not DIP, makes the total wir-
ing length short to reduce influence of noise.
(3) 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 (within 20 mm) across the grounding
lead of a capacitor 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.
<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.
DIP
SDIP
SOP
QFP
Fig. 49 Selection of packages
(2) 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
(within 20mm).
<Reason>
The width of a pulse input into the RESET pin is determined by the
timing necessary conditions. If noise having a shorter pulse width
than the standard is input to the RESET pin, the reset is released
before the internal state of the microcomputer is completely initial-
ized. This may cause a program runaway.
Noise
XIN
XOUT
VSS
XIN
XOUT
VSS
N.G.
Fig. 51 Wiring for clock I/O pins
O.K.
(4) Wiring to CNVss pin
Connect the CNVss pin to the Vss pin with the shortest possible
wiring.
<Reason>
The processor mode of a microcomputer is influenced by a poten-
tial at the CNVss pin. If a potential difference is caused by the
noise between pins CNVss and Vss, the processor mode may be-
come unstable. This may cause a microcomputer malfunction or a
program runaway.
Noise
Noise
Reset
circuit
VSS
N.G.
RESET
VSS
Reset
circuit
VSS
RESET
VSS
CNVSS
VSS
N.G.
Fig. 52 Wiring for CNVss pin
CNVSS
VSS
O.K.
O.K.
Fig. 50 Wiring for the RESET pin
Rev.1.04 2004.06.08 page 41 of 66
REJ03B0012-0104Z