Port Function Operations; Writing To I/O Port; Reading From I/O Port; Operations On I/O Port - NEC 78K0 User Manual

4.4

Port Function Operations

Port operations differ depending on whether the input or output mode is set, as shown below.
Caution In the case of a 1-bit memory manipulation instruction, although a single bit is manipulated, the
port is accessed as an 8-bit unit. Therefore, on a port with a mixture of input and output pins,
the output latch contents for pins specified as input are undefined, even for bits other than the
manipulated bit.
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.
The data of the output latch is cleared by reset.
(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.
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.
The data of the output latch is cleared by reset.
(2) Input mode
The pin level is read and an operation is performed on its contents. The result of the operation is written to the
output latch, but since the output buffer is off, the pin status does not change.
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CHAPTER 4 PORT FUNCTIONS
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User's Manual U15947EJ2V0UD