m3819 Renesas Electronics Corporation., m3819 Datasheet - Page 188

• Manufacturer Part Number: m3819
• Description: Mitsubishi 8-bit Single-chip Microcomputer 740 Family / 38000 Series
• Manufacturer: Renesas Electronics Corporation.

3. APPENDIX
3.2 Countermeasures against 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.
Fig. 3.2.1 Wiring for the RESET pin
174
3.2.1 Shortest wiring length
The wiring on a printed circuit board can be as an antenna which feeds noise into the microcomputer.
The shorter the total wiring length (by mm unit), the less the possibility of noise insertion into a microcomputer.
(1) Wiring for the 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 V
Reason
The reset works to initialize a microcomputer.
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 initialized. This may cause a program runaway.
(2) Wiring for clock input/output pins
Reason
A microcomputer's operation synchronizes with a clock generated by the oscillator (circuit). 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 V
an oscillator, the correct clock will not be input in the microcomputer.
Make the length of wiring which is connected to clock I/O pins as short as possible.
Make the length of wiring (within 20mm) across the grounding lead of a capacitor which is connected to an oscillator
Separate the V
and the V
SS
pin of a microcomputer as short as possible.
SS
Reset
circuit
pattern only for oscillation from other V
V
SS
SS
pin with the shortest possible wiring (within 20mm).
Noise
3819 Group USER’S MANUAL
3.2 Countermeasures against noise
RESET
V
SS
SS
patterns.
SS
level of a microcomputer and the V
Reset
circuit
V
SS
MITSUBISHI MICROCOMPUTER
V
RESET
SS
3819 Group
SS
level of