4571 Datasheet, PDF (61/126 Pages) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

4571 Group
4. 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.
(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.
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
<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.
Fig 69. Vss pattern on the underside of an oscillator
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.
Mutual inductance
M
Microcomputer
Large
current
XIN
XOUT
VSS
GND
Fig 67. Wiring for a large current signal line
N.G.
Do not cross
CNTR
XIN
XOUT
VSS
<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 port latch at fixed
periods.
• Rewrite data to pull-up control registers at fixed periods.
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.
Fig 68. Wiring to a signal line where potential levels
change frequently
Rev.1.02 May 25, 2007 Page 61 of 124
REJ03B0179-0102