7546 datasheet(85/95 Pages) RENESAS

85 / 95 page

Rev.1.21

Nov 15, 2006

page 85 of 93

REJ03B0160-0121

7546 Group

NOTES ON PERIPHERAL FUNCTIONS

Notes on I/O Ports

1.Port direction register

• Set bits 6 and 7 of port P2 direction register to “1”.

• Set bits 5 and 6 of port P3 direction register to “1”.

2.Interrupt edge selection register

Set bit 2 of interrupt edge selection register to “1”.

3.Be sure to set bit 1 of port P1P3 control register (address 1716)

to “0”.

4. Port P0P3 drive capacity control register

The number of LED drive port (drive capacity is HIGH) is 8.

5. Pull-up control register

When using each port which built in pull-up resistor as an output

port, the pull-up control bit of corresponding port becomes invalid,

and pull-up resistor is not connected.

Pull-up control is effective only when each direction register is set

to the input mode.

6. 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 val-

ues:

• When setting as an input port : Fix its input level

• When setting as an output port : Prevent current from flowing out

to external.

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 mi-

crocomputer is unstable in the state that input levels of an 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

7. Modifying output data with bit managing instruction

When the port latch of an I/O port is modified with the bit manag-

ing instruction*2, the value of the unspecified bit may be changed.

The bit managing instructions are read-modify-write form instruc-

tions 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 man-

aging instruction in case where the pin state differs from its port

latch contents.

*2 bit managing instructions : SEB, and CLB instructions

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