4571 Datasheet, PDF (59/126 Pages) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

4571 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.
1. Shortest wiring length
(1) Wiring for RESET pin
Make the length of wiring which is connected to the RESET pin
as short as possible. Especially, connect a capacitor across the
RESET 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.
(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.
<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.
Noise
Noise
Reset
circuit
VSS
N.G.
RESET
VSS
Reset
circuit
VSS
RESET
VSS
O.K.
Fig 62. Wiring for the RESET pin
XIN
XOUT
VSS
XIN
XOUT
VSS
N.G.
Fig 63. Wiring for clock I/O pins
O.K.
(3) Port K Wiring
Do not leave port K open. Always connect it to the VDD pin or
VSS pin using the thickest wire at the shortest distance.
When port K is used for key matrix, connect it to the VDD pin
through a pull-up resistor.
In that case too, place a pull-up resistor close to port K and
connect it to port K or the VDD pin using the thickest wire at the
shortest distance as above.
<Reason>
Port K is also used as the power source input pin (VPP pin) for
the built-in QzROM.
When programming to the QzROM, the impedance of port K is
low so that the electric writing current will flow into the
QzROM. This allows noise to enter easily. If noise enters from
port K, abnormal instruction codes or data are read from the
QzROM, which may cause a program runaway.
(Note)
VDD
VSS
Shortest
distance
K
(Note)
VDD
(Note)
VSS
Shortest
distance
K
(Note)
(Note)
VDD
Shortest
distance
VSS
A pull-up resistor
Rev.1.02 May 25, 2007 Page 59 of 124
REJ03B0179-0102
K
(Note)
Note: This indicates pin.
Shortest
distance
Fig 64. Wiring for port K