7470_N Datasheet, PDF (284/354 Pages) Renesas Technology Corp – 8-BIT SINGLE-CHIP MICROCOMPUTER 740 FAMILY / 7470 SERIES

APPLICATION
2.9 Countermeasures against noise
(2) Wiring for clock input/output pins
7470 7471 7477 7478
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q Make the length of wiring which is connected to clock I/O pins as short as possible.
q Make the length of wiring (within 20mm) across the grounding lead of a capacitor which is connected
to an oscillator and the VSS pin of a microcomputer as short as possible.
q Separate the VSS pattern only for oscillation from other VSS patterns.
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 malfunction 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
N.G.
XIN
XOUT
VSS
O.K.
Fig. 2.9.2 Wiring for clock I/O pins
An example of VSS patterns on the
underside of a printed circuit board
Oscillator wiring
pattern example
XIN
XOUT
VSS
Separate the VSS line for oscillation from other VSS lines
7470/7471/7477/7478 GROUP USER’S MANUAL
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