4559 Datasheet, PDF (73/148 Pages) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

4559 Group
(2) Connection of bypass capacitor across VSS line
and VDD line
Connect an approximately 0.1 µF bypass capacitor across the
VSS line and the VDD line as follows:
• Connect a bypass capacitor across the VSS pin and the VDD pin
at equal length.
• Connect a bypass capacitor across the VSS pin and the VDD pin
with the shortest possible wiring.
• Use lines with a larger diameter than other signal lines for VSS
line and VDD line.
• Connect the power source wiring via a bypass capacitor to the
VSS pin and the VDD pin.
(2) Installing oscillator away from signal lines where potential
levels change frequently
Install an oscillator and a connecting pattern of an oscillator
away from signal lines where potential levels change
frequently. Also, do not cross such signal lines over the
clock lines or the signal lines which are sensitive to noise.
• Reason
Signal lines where potential levels change frequently (such as
the CNTR pin signal line) may affect other lines at signal
rising edge or falling edge. If such lines cross over a clock line,
clock waveforms may be deformed, which causes a
microcomputer failure or a program runaway.
VDD
VDD
Do not cross
CNTR
VSS
VSS
XIN
XOUT
VSS
N.G.
O.K.
N.G.
Fig 77. Bypass capacitor across the VSS line and the
VDD line
Fig 79. Wiring to a signal line where potential levels
change frequently
(3) Oscillator concerns
Take care to prevent an oscillator that generates clocks for a
microcomputer operation from being affected by other signals.
(1) Keeping oscillator away from large current signal lines
Install a microcomputer (and especially an oscillator) as far
as possible from signal lines where a current larger than the
tolerance of current value flows.
(3) Oscillator protection using VSS pattern
As for a two-sided printed circuit board, print a VSS pattern
on the underside (soldering side) of the position (on the
component side) where an oscillator is mounted.
Connect the VSS pattern to the microcomputer VSS pin with
the shortest possible wiring.
Besides, separate this VSS pattern from other VSS patterns.
• Reason
In the system using a microcomputer, there are signal lines for
controlling motors, LEDs, and thermal heads or others. When
a large current flows through those signal lines, strong noise
occurs because of mutual inductance.
Mutual inductance
M
Microcomputer
Large
current
XIN
XOUT
VSS
GND
Oscillator wiring
pattern example
An example of VSS patterns on the
underside of a printed circuit board
XIN
XOUT
VSS
Separate the VSS line for oscillation from other VSS lines
Fig 80. VSS pattern on the underside of an oscillator
Fig 78. Wiring for a large current signal line
Rev.1.04 Aug 23, 2007 Page 73 of 146
REJ03B0188-0104