38C3_M datasheet(73/224 Pages) RENESAS

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2-8

38C3 Group User’s Manual

APPLICATION

2.1 I/O port

2.1.4 Notes on I/O port

(1)

Notes in standby state

In standby 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 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

l Reason

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 standby state: stop mode by executing STP instruction

wait mode by executing WIT instruction

(2)

Modifying port latch of I/O port 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.

l 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/Pull-down control

When each port which has built-in pull-up/pull-down resistor (P0, P1, P2, P4, P50, P52–P57, P6, P7,

P8) is set to output port, pull-up/pull-down control of corresponding port become invalid. (Pull-up/Pull-

down cannot be set.)

l Reason

Pull-up control is valid only when each direction register is set to the input mode.

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