M37544 Datasheet, PDF (43/68 Pages) Renesas Technology Corp – SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

7544 Group
4. Oscillator concerns
So that the product obtains the stabilized operation clock on the
user system and its condition, contact the resonator manufacturer
and select the resonator and oscillation circuit constants.
Be careful especially when range of voltage and temperature is
wide.
Take care to prevent an oscillator that generates clocks for a mi-
crocomputer 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 toler-
ance of current value flows.
<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 lev-
els 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 wave-
forms may be deformed, which causes a microcomputer failure or
a program runaway.
(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.
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. 57 Vss pattern on the underside of an oscillator
➀ Keeping oscillator away from large current signal lines
Mutual inductance
M
Microcomputer
Large
current
XIN
XOUT
VSS
GND
➁ Installing oscillator away from signal lines where potential lev-
els change frequently
N.G.
Do not cross
CNTR
XIN
XOUT
VSS
Fig. 56 Wiring for a large current signal line/Writing of signal
lines where potential levels change frequently
Rev.1.04 2004.06.08 page 43 of 66
REJ03B0012-0104Z