![](http://datasheet.mmic.net.cn/230000/7540_datasheet_15567442/7540_86.png)
7540 Group User
’
s Manual
APPLICATION
2-11
2.1 I/O port
2.1.6 Notes on input and output ports
Notes on using input and output ports are described below.
(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
”
.
Pull-up (connect the port to V
CC
) or pull-down (connect the port to V
SS
) 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 a 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.
G
Reason
The output 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.
G
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
(3) Usage for the 32-pin version
Fix the P3
5
, P3
6
pull-up control bit of the pull-up control register to
“
1
”
.
Keep the P3
6
/INT
1
input level selection bit of the port P1P3 control register
“
0
”
(initial state).