7540 datasheet(87/366 Pages) RENESAS

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2-11

REJ09B0018-0200Z

Rev.2.00

Jun 21, 2004

7540 Group

APPLICATION

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 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 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.

q 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.

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

(3) Usage for the 32-pin version

➀ Fix the P35, P36 pull-up control bit of the pull-up control register to “1”.

➁ Keep the P36/INT1 input level selection bit of the port P1P3 control register “0” (initial state).

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