
HARDWARE
7540 Group User’s Manual
1-17
FUNCTIONAL DESCRIPTION
I/O Ports
[Direction registers] PiD
The I/O ports have direction registers which determine the input/
output direction of each pin. Each bit in a direction register corre-
sponds to one pin, and each pin can be set to be input or output.
When “1” is set to the bit corresponding to a pin, this pin becomes
an output port. When “0” is set to the bit, the pin becomes an in-
put port.
When data is read from a pin set to output, not the value of the pin
itself but the value of port latch is read. Pins set to input are float-
ing, and permit reading pin values.
If a pin set to input is written to, only the port latch is written to and
the pin remains floating.
[Pull-up control register] PULL
By setting the pull-up control register (address 0016
16
), ports P0
and P3 can exert pull-up control by program. However, pins set to
output are disconnected from this control and cannot exert pull-up
control.
Note: P2
6
/A
N6
, P2
7
/AN
7
, P3
5
and P3
6
do not exist for the 32-pin
version.
Accordingly, the following settings are required;
Set direction registers of ports P2
6
and P2
7
to output.
Set direction registers of ports P3
5
and P3
6
to output.
[Port P1P3 control register] P1P3C
By setting the port P1P3 control register (address 0017
16
), a
CMOS input level or a TTL input level can be selected for ports
P1
0
, P1
2
, P1
3
, P3
6
, and P3
7
by program.
Fig. 16 Structure of port P1P3 control register
Fig. 15 Structure of pull-up control register
Port P1P3 control register
(P1P3C: address 0017
16
, initial value: 00
16
)
b7 b0
P3
7
/INT
0
input level selection bit
0 : CMOS level
1 : TTL level
P3
6
/INT
1
input level selection bit
0 : CMOS level
1 : TTL level
P1
0
,P1
2
,P1
3
input level selection bit
0 : CMOS level
1 : TTL level
Not used
Note:
Keep setting the P3
6
/INT
1
input level selection bit
to
“
0
”
(initial value) for 32-pin version.
Pull-up control register
(PULL: address 0016
16
, initial value: 00
16
)
P0
0
pull-up control bit
b7 b0
0 : Pull-up Off
1 : Pull-up On
Note :
Pins set to output ports are disconnected from pull-up control.
P3
7
pull-up control bit
P3
5
, P3
6
pull-up control bit
P3
4
pull-up control bit
P3
0
–
P3
3
pull-up control bit
P0
4
–
P0
7
pull-up control bit
P0
2
, P0
3
pull-up control bit
P0
1
pull-up control bit