SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
MITSUBISHI MICROCOMPUTERS
38C1 Group
43
NOTES ON USE
VL3 pin
When LCD drive control circuit is not used, connect VL3 to VCC.
Countermeasures against noise
(1) Shortest wiring length
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
(within 20mm).
q Reason
The width of a pulse input into the RESET pin is determined by
the timing necessary conditions. If noise having a shorter pulse
width than the standard is input to the RESET pin, the reset is
released before the internal state of the microcomputer is com-
pletely initialized. This may cause a program runaway.
Fig. 45 Wiring for clock I/O pins
(2) Connection of bypass capacitor across VSS line and VCC line
In order to stabilize the system operation and avoid the latch-up,
connect an approximately 0.1
F bypass capacitor across the VSS
line and the VCC line as follows:
Connect a bypass capacitor across the VSS pin and the VCC pin
at equal length.
Connect a bypass capacitor across the VSS pin and the VCC pin
with the shortest possible wiring.
Use lines with a larger diameter than other signal lines for VSS
line and VCC line.
Connect the power source wiring via a bypass capacitor to the
VSS pin and the VCC pin.
Fig. 44 Wiring for the RESET pin
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 (within 20 mm) 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.
q Reason
If noise enters clock I/O pins, clock waveforms may be de-
formed. 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 oscil-
lator, the correct clock will not be input in the microcomputer.
Fig. 46 Bypass capacitor across the VSS line and the VCC line
RESET
Reset
circuit
Noise
VSS
Reset
circuit
VSS
RESET
VSS
N.G.
O.K.
Noise
XIN
XOUT
VSS
XIN
XOUT
VSS
N.G.
O.K.
VSS
VCC
VSS
VCC
N.G.
O.K.