Renesas 7700 FAMILY User Manual page 513

APPENDIX
Appendix 8. Examples of noise immunity improvement
(2) Wiring for clock input/output pins
Make the length of wiring connected to the clock input/output pins as short as possible.
Make the length of wiring between the grounding lead of the capacitor, which is connected to
the oscillator and Vss pin of the microcomputer, as short as possible (within 20 mm).
Separate the Vss pattern for oscillation from all other Vss patterns. (Refer to Figure 14.)
Reason: The microcomputer's operation
synchronizes with a clock generated
by the oscillation circuit.
If noise enters clock I/O pins, clock
waveforms may be deformed. This
may cause a malfunction or a
program runaway.
Also, if the noise causes a potential
difference between the Vss level
of the microcomputer and the Vss
level of an oscillator, the correct
clock will not be input in the
microcomputer.
(3) Wiring for CNVss pin
Connect CNVss pin to Vss pin with the shortest possible wiring.
Reason: T h e p r o c e s s o r m o d e o f t h e
microcomputer is influenced by a
potential at CNVss pin when
C N V s s a n d V s s p i n s a r e
connected.
If the noise causes a potential
difference between CNVss and
Vss pins, the processor mode may
become unstable. This may cause
a microcomputer malfunction or
a program runaway.
20–62
Not acceptable
Fig. 6. Wiring for clock input/output pins
M37751
CNVss
Not Acceptable
Fig. 7. Wiring for CNVss pin
7751 Group User's Manual
Noise
M37751
X
IN
X
OUT
Vss
Acceptable
Noise
Vss
Acceptable
M37751
X
IN
X
OUT
Vss
M37751
CNVss
Vss