7540 datasheet(295/366 Pages) RENESAS

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3-73

REJ09B0018-0200Z

Rev.2.00

Jun 21, 2004

7540 Group

APPENDIX

3.3 Notes on use

3.3.1 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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