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APPENDIX
3874 Group User’s Manual
3.3 Notes on use
3.3.13 Notes on input and output pins
(1)
Notes in stand-by state
In stand-by state*1 for low-power dissipation, do not make input levels of an input port and an I/O
port “undefined”, especially for I/O ports of the N-channel open-drain.
Pull-up (connect the port to VCC) or pull-down (connect the port to VSS) these ports through a
resistor.
When determining a resistance value, note the following points:
External circuit
Variation of output levels during the ordinary operation
When using built-in pull-up resistor, note on varied current values:
When setting as an input port : Fix its input level
When setting as an output port : Prevent current from flowing out to external
q Reason
Even when setting as an output port with its direction register, in the following state :
P-channel......when the content of the port latch is “0”
N-channel......when the content of the port latch is “1”
the transistor becomes the OFF state, which causes the ports to be the high-impedance state.
Note that the level becomes “undefined” depending on external circuits.
Accordingly, the potential which is input to the input buffer in a microcomputer is unstable in the
state that input levels of a input port and an I/O port are “undefined”. This may cause power
source current.
*1 stand-by state : the stop mode by executing the STP instruction
the wait mode by executing the WIT instruction
(2)
Modifying output data with bit managing instruction
When the port latch of an I/O port is modified with the bit managing instruction*2, the value of the
unspecified bit may be changed.
q Reason
The bit managing instructions are read-modify-write form instructions for reading and writing data
by a byte unit. Accordingly, when these instructions are executed on a bit of the port latch of an
I/O port, the following is executed to all bits of the port latch.
As for a bit which is set for an input port :
The pin state is read in the CPU, and is written to this bit after bit managing.
As for a bit which is set for an output port :
The bit value of the port latch is read in the CPU, and is written to this bit after bit managing.
Note the following :
Even when a port which is set as an output port is changed for an input port, its port latch holds
the output data.
As for a bit of the port latch which is set for an input port, its value may be changed even when
not specified with a bit managing instruction in case where the pin state differs from its port latch
contents.
*2 bit managing instructions : SEB, and CLB instructions