
7721 Group User’s Manual
6–7
INPUT / OUT PUT PINS
6.3 Examples of handling unused pins
When unusing an I/O pin, some handling is necessary for the pin. Examples of handling unused pins are
described below.
The following are just examples. The user shall modify them according to the user’s actual application and
test them.
Table 6.3.1 Examples of handling unused pins
Pin name
P4
3
to P4
7
, P5 to P10
6.3 Examples of handling unused pins
Handling example
Set these pins to the input mode and connect each
pin to Vcc or Vss via a resistor; or set these pins to
the output mode and leave them open
(Notes 1 and 2).
Leave them open.
Connect these pins to Vcc via a resistor (pull-up)
(Note 2).
Connect this pin to Vcc or Vss.
Connect this pin to Vcc.
Connect these pins to Vss.
BLE, BHE, ALE,
φ
1
, ST0, ST1
HOLD, RDY
CNVss
AVcc
AVss, V
REF
Notes 1:
When leaving these pins open after they are set to the output mode, note the following: these pins
function as input ports from reset until they are switched to the output mode by software. Therefore,
voltage levels of these pins are undefined and the power source current may increase while these
pins function as input ports. After reset, immediately set these ports to the output mode.
Software reliability can be enhanced by setting the contents of the above ports’ direction registers
periodically. This is because these contents may be changed by noise, a program runaway which
occurs owing to noise, etc.
2:
For unused pins, use the shortest possible wiring (within 20 mm from the microcomputer’s pins).
3:
This applies when a clock externally generated is input to the X
IN
pin.
Fig. 6.3.1 Examples of handling unused pins
P4
3
–P4
7
, P5–P10
M
ST0
ST1
BLE
BHE
ALE
1
X
OUT
AV
SS
V
REF
HOLD
RDY
AV
CC
CNV
SS
8
M
V
CC
V
SS
G
When setting ports to input mode
G
When setting ports to output mode
Left open
Left open
Left open
AV
SS
V
REF
HOLD
RDY
AV
CC
CNV
SS
8
V
CC
V
SS
Left open
P4
3
–P4
7
, P5–P10
Left open
ST0
ST1
BLE
BHE
ALE
1
X
OUT
8
CNV
SS
can be connected to V
CC
, too.