4559 Datasheet, PDF (72/148 Pages) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

4559 Group
NOTES ON 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.
Noise
(1) Shortest wiring length
The wiring on a printed circuit board can function 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 RESET input pin
Make the length of wiring which is connected to the RESET
input pin as short as possible.
Especially, connect a capacitor across the RESET input pin
and the VSS pin with the shortest possible wiring.
• Reason
In order to reset a microcomputer correctly, 1 machine cycle or
more of the width of a pulse input into the RESET pin is
required.
If noise having a shorter pulse width than this is input to the
RESET input pin, the reset is released before the internal state
of the microcomputer is completely initialized.
This may cause a program runaway.
Noise
Reset
circuit
VSS
N.G.
RESET
VSS
Reset
circuit
RESET
XIN
XOUT
VSS
N.G.
XIN
XOUT
VSS
O.K.
Fig 75. Wiring for clock I/O pins
• Reason
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 VSS level of a microcomputer and the VSS level of an
oscillator, the correct clock will not be input in the
microcomputer.
(3) Wiring to CNVSS pin
Connect an approximately 5 kΩ resistor to the VPP pin and
also to the GND pattern supplied to the VSS pin with
shortest possible wiring.
• Reason
The CNVSS pin is the power source input pin for the built-in
QzROM. When programming in the built-in QzROM, the
impedance of the CNVSS pin is low to allow the electric
current for writing flow into the QzROM. Because of this,
noise can enter easily. If noise enters the CNVSS pin, abnormal
instruction codes or data are read from the built-in QzROM,
which may cause a program runaway.
VSS
VSS
O.K.
Fig 74. Wiring for the RESET input pin
(2) Wiring for clock input/output pins
• Make the length of wiring which is connected to clock I/O
pins as short as possible.
• Make the length of wiring across the grounding lead of a
capacitor which is connected to an oscillator and the VSS
pin of a microcomputer as short as possible.
• Separate the VSS pattern only for oscillation from other
VSS patterns.
(Note) T h e sh o rte st
CNVss
about 5kΩ
VSS
(Note) ^{T h e sh o rte st}
Note: This indicates pin.
Fig 76. Wiring for CNVSS pin
Rev.1.04 Aug 23, 2007 Page 72 of 146
REJ03B0188-0104