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Rev.2.00
Aug 28, 2006
page 10 of 148
7641 Group
REJ09B0336-0200
APPLICATION
2.1 I/O port
2.1.5 Notes on I/O port
(1) Notes in standby state
In standby state1 for low-power dissipation, do not make input levels of an I/O port “undefined”.
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
The potential which is input to the input buffer in a microcomputer is unstable in the state that input
levels of an I/O port are “undefined”. This may cause power source current.
1 standby state: stop mode by executing STP instruction
wait mode by executing 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 instruction2, 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 bit which is set for input port:
The pin state is read in the CPU, and is written to this bit after bit managing.
As for bit which is set for output port:
The bit value 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 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
(3) Pull-up control
When using port P2, which includes a pull-up resistor, as an output port, its port pull-up control is
invalidated, that is, pull-up cannot be enabled.
q Reason
Pull-up/pull-down control is valid only when each direction register is set to the input mode.