UPD784938 Datasheet, PDF (122/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 6 PORT FUNCTIONS
(2) When set as an input port
The port pin level can be loaded into an accumulator by means of a transfer instruction, etc. In this case, too, writes can
be performed to the output latch, and data transferred from the accumulator by a transfer instruction, etc., is stored in all
output latches irrespective of the port input/output specification. However, since the output buffer of a bit specified as an
input port is high-impedance, the data is not output to the port pin (when a bit specified as input is switched to an output
port, the output latch contents are output to the port pin). Also, the contents of the output latch of a bit specified as an input
port cannot be loaded into an accumulator.
Figure 6-5. Port Specified as Input Port
WRPORT
Internal
bus
Output
latch
RDIN
P0n
n = 0 to 7
Caution A bit manipulation instruction manipulates one bit as the result, but accesses the port in 8-bit units.
Therefore, if a bit manipulation instruction is used on a port with a mixture of input and output pins, the
contents of the output latch of pins specified as inputs will be undefined (excluding bits manipulated with
a SET1 or CLR1 instruction, etc.). Particular care is required when there are bits which are switched
between input and output.
Caution is also required when manipulating the port with other 8-bit arithmetic instructions.
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Preliminary User’s Manual U13987EJ1V0UM00