NEC switch - Manual (Page 72)

CHAPTER 4 PORT FUNCTIONS
User’s Manual U12978EJ3V0UD
72
4.4 Port Function Operation
The operation of a port differs depending on whether the port is set to the input or output mode, as described
below.
4.4.1 Writing to I/O port
(1) In output mode
A value can be written to the output latch of a port by using a transfer instruction. The contents of the output
latch can be output from the pins of the port.
The data once written to the output latch is retained until new data is written to the output latch.
(2) In input mode
A value can be written to the output latch by using a transfer instruction. However, the status of the port pin is
not changed because the output buffer is off.
The data once written to the output latch is retained until new data is written to the output latch.
Caution A 1-bit memory manipulation instruction is executed to manipulate 1 bit of a port.
However, this instruction accesses the port in 8-bit units. When this instruction is
executed to manipulate a bit of an input/output port, therefore, the contents of the output
latch of the pin that is set to the input mode and not subject to manipulation become
undefined.
4.4.2 Reading from I/O port
(1) In output mode
The status of a pin can be read by using a transfer instruction. The contents of the output latch are not
changed.
(2) In input mode
The status of a pin can be read by using a transfer instruction. The contents of the output latch are not
changed.
4.4.3 Arithmetic operation of I/O port
(1) In output mode
An arithmetic operation can be performed on the contents of the output latch. The result of the operation is
written to the output latch. The contents of the output latch are output from the port pins.
The data once written to the output latch is retained until new data is written to the output latch.
(2) In input mode
The contents of the output latch become undefined. However, the status of the pin is not changed because the
output buffer is off.
Caution A 1-bit memory manipulation instruction is executed to manipulate 1 bit of a port.
However, this instruction accesses the port in 8-bit units. When this instruction is
executed to manipulate a bit of an input/output port, therefore, the contents of the output
latch of the pin that is set to the input mode and not subject to manipulation become
undefined.