Operation Of Port Functions; Writing To I/O Port; Reading From I/O Port; Arithmetic Operation Of I/O Port - NEC mPD789101 User Manual

5.4 Operation of Port Functions

The operation of a port differs depending on whether the port is set in input or output mode, as described below.

5.4.1 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 in the input mode and not subject to manipulation become undefined.

5.4.2 Reading from I/O port

(1) In output mode
The contents of the output latch 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.
Caution Be sure that port 5 is read with V
value may not be read correctly.

5.4.3 Arithmetic operation of I/O port

(1) In output mode
An arithmetic operation can be performed with 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 in the input mode and not subject to manipulation become undefined.
CHAPTER 5 PORT FUNCTIONS
= 3.5 to 5.5 V. When V
DD
User's Manual U13045EJ2V0UM00
is lower than 3.5 V, the input
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85