m37905 Renesas Electronics Corporation., m37905 Datasheet - Page 539

• Manufacturer Part Number: m37905
• Description: Mitsubishi 16-bit Single-chip Microcomputer 7700 Family / 7900 Series
• Manufacturer: Renesas Electronics Corporation.

APPENDIX
Appendix 7. Countermeasure against noise
20-108
4. Oscillator protection
The oscillator, which generates the basic clock for the microcomputer operations, must be protected from
the affect of other signals.
(1) Distance oscillator from signal lines with large current flows
(2) Distance oscillator from signal lines with frequent potential level changes
(3) Oscillator protection using Vss pattern
Install the microcomputer, especially the oscillator, as far as possible from signal lines which handle
currents larger than the microcomputer current value tolerance.
Reason: The microcomputer is used in
Reason: Signal lines with frequently changing
Print a Vss pattern on the bottom (soldering
side) of a double-sided printed circuit board,
under the oscillator mount position.
Connect the Vss pattern to the Vss pin of the
microcomputer with the shortest possible wiring,
separating it from other Vss patterns.
Install an oscillator and its wiring pattern away from signal lines where potential levels change
Do not cross these signal lines over the clock-related or noise-sensitive signal lines.
frequently.
systems which contain signal lines
f o r c o n t r o l l i n g m o t o r s , L E D s ,
thermal heads, etc. Noise occurs
due to mutual inductance when a
large current flows through the signal
lines.
potential levels may affect other
signal lines at a rising or falling edge.
In particular, if the lines cross over
a clock-related signal line, clock
waveforms may be deformed, which
causes a microcomputer malfunction
or a program runaway.
7905 Group User’s Manual Rev.1.0
Fig. 8 Wiring for signal lines where large current
Fig. 9 Wiring for signal lines where potential levels
Fig. 10 Vss pattern underneath mounted oscillator
Separate Vss lines for oscillation and supply.
Mounted pattern
example of
oscillator unit.
Mutual inductance
flows
frequently change
Do not cross.
changing potential levels
I/O pin for signal with frequently
Large current
An example of Vss pattern on the
underside of an oscillator.
M
X
X
V
M37905
OUT
IN
SS
Vss
X
X
M37905
IN
OUT
X
X
Vss
M37905
OUT
IN