Port Function Operations; Writing To I/O Port; Reading From I/O Port; Operations On I/O Port - NEC mPD780208 Subseries User Manual

4.4 Port Function Operations

Port operations differ depending on whether the input or output mode is set, as shown below.

4.4.1 Writing to I/O port

(1) Output mode
A value is written to the output latch by a transfer instruction, and the output latch contents are output from
the pin.
Once data is written to the output latch, it is retained until data is written to the output latch again.
(2) Input mode
A value is written to the output latch by a transfer instruction, but since the output buffer is off, the pin status
does not change.
Once data is written to the output latch, it is retained until data is written to the output latch again.
Caution In the case of a 1-bit memory manipulation instruction, although a single bit is manipulated,
the port is accessed in 8-bit units. Therefore, on a port with a mixture of input and output
pins, the output latch contents for pins specified as input are undefined except for the
manipulated bit.

4.4.2 Reading from I/O port

(1) Output mode
The output latch contents are read by a transfer instruction. The output latch contents do not change.
(2) Input mode
The pin status is read by a transfer instruction. The output latch contents do not change.

4.4.3 Operations on I/O port

(1) Output mode
An operation is performed on the output latch contents, and the result is written to the output latch. The output
latch contents are output from the pins.
Once data is written to the output latch, it is retained until data is written to the output latch again.
(2) Input mode
The output latch contents become undefined. However, the pin status does not change because the output
buffer is turned off.
Caution In the case of a 1-bit memory manipulation instruction, although a single bit is manipulated,
the port is accessed in 8-bit units. Therefore, on a port with a mixture of input and output
pins, the output latch contents for pins specified as input are undefined except for the
manipulated bit.
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CHAPTER 4 PORT FUNCTIONS
User's Manual U11302EJ4V0UM