4507_M Datasheet, PDF (212/216 Pages) Renesas Technology Corp – 4-BIT CISC SINGLE-CHIP MICROCOMPUTER 4500 SERIES

4507 Group
APPENDIX
3.4 Notes on noise
(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.
q 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.
N.G.
Do not cross
CNTR
XIN
XOUT
VSS
Fig. 3.4.9 Wiring to a signal line where potential
levels change frequently
(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.
3.4.5 Setup for I/O ports
Setup I/O ports using hardware and software as
follows:
<Hardware>
• Connect a resistor of 100 Ω or more to an I/O port
in series.
<Software>
• As for an input port, read data several times by
a program for checking whether input levels are
equal or not.
• As for an output port or an I/O port, since the
output data may reverse because of noise, rewrite
data to its output latch at fixed periods.
• Rewrite data to pull-up control registers at fixed
periods.
3.4.6 Providing of watchdog timer function by
software
If a microcomputer runs away because of noise or
others, it can be detected by a software watchdog
timer and the microcomputer can be reset to normal
operation. This is equal to or more effective than
program runaway detection by a hardware watchdog
timer. The following shows an example of a watchdog
timer provided by software.
In the following example, to reset a microcomputer
to normal operation, the main routine detects errors
of the interrupt processing routine and the interrupt
processing routine detects errors of the main routine.
This example assumes that interrupt processing is
repeated multiple times in a single main routine
processing.
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
Fig. 3.4.10 VSS pattern on the underside of an
oscillator
Rev.2.01 Feb 04, 2005
REJ09B0195-0201
3-46