7477 datasheet(63/354 Pages) RENESAS

63 / 354 page

HARDWARE

1.10 I/O pins

7470/7471/7477/7478 GROUP USER’S MANUAL

1-46

(3) Fix of a port input level in stand-by state

Fix input levels of an input and an I/O port for getting effect of low-power dissipation in stand-by

state∗, especially for the I/O ports of the N-channel open-drain.

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, make sure the following:

q External circuit

q Variation of output levels during the ordinary operation

∗: “Stand-by state”: The stop mode by execution of the STP instruction or the wait mode by execution

of the WIT instruction:

Reason

Even when setting as an output port with its direction register, in the following state:

• N-channel ....... when the content of the port latch is “1”

the transistor becomes the OFF state, which causes the ports to be the high-impedance state. Make

sure 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 an input and an I/O port are “undefined.” This may cause power source current.

7477 Datasheet(PDF) 63 Page - Renesas Technology Corp