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7540 Group User
’
s Manual
2-12
APPLICATION
2.1 I/O port
2.1.7 Termination of unused pins
(1) Terminate unused pins
I/O ports :
Set the I/O ports for the input mode and connect them to V
CC
or V
SS
through each resistor of
1 k
to 10 k
.
Ports that permit the selecting of a built-in pull-up resistor can also use this resistor. Set the I/
O ports for the output mode and open them at
“
L
”
or
“
H
”
.
When opening them in the output mode, the input mode of the initial status remains until the
mode of the ports is switched over to the output mode by the program after reset. Thus, the
potential at these pins is undefined and the power source current may increase in the input
mode. With regard to an effects on the system, thoroughly perform system evaluation on the user
side.
Since the direction register setup may be changed because of a program runaway or noise, set
direction registers by program periodically to increase the reliability of program.
(2) Termination remarks
Input ports and I/O ports :
Do not open in the input mode.
G
Reason
The power source current may increase depending on the first-stage circuit.
An effect due to noise may be easily produced as compared with proper termination
and
shown on the above.
I/O ports :
When setting for the input mode, do not connect to V
CC
or V
SS
directly.
G
Reason
If the direction register setup changes for the output mode because of a program runaway or
noise, a short circuit may occur between a port and V
CC
(or V
SS
).
I/O ports :
When setting for the input mode, do not connect multiple ports in a lump to V
CC
or V
SS
through
a resistor.
G
Reason
If the direction register setup changes for the output mode because of a program runaway or
noise, a short circuit may occur between ports.
At the termination of unused pins, perform wiring at the shortest possible distance (20 mm or less)
from microcomputer pins.