4509 Datasheet, PDF (69/142 Pages) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

4509 Group
4. Oscillator concerns
Take care to prevent an oscillator that generates clocks for a micro-
computer 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 pos-
sible 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 con-
trolling motors, LEDs, and thermal heads or others. When a large
current flows through those signal lines, strong noise occurs be-
cause of mutual inductance.
(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 pro-
gram runaway.
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. 71 Vss pattern on the underside of an oscillator
Mutual inductance
M
Microcomputer
Large
current
XIN
XOUT
VSS
GND
Fig. 69 Wiring for a large current signal line
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 re-
verse because of noise, rewrite data to its port latch at fixed
periods.
• Rewrite data to pull-up control registers at fixed periods.
N.G.
Do not cross
CNTR
XIN
XOUT
VSS
Fig. 70 Wiring to a signal line where potential levels change fre-
quently
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 fol-
lowing shows an example of a watchdog timer provided by software.
In the following example, to reset a microcomputer to normal opera-
tion, 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.
Rev.1.02 2006.12.22 page 69 of 140
REJ03B0147-0102