UPD784938 Datasheet, PDF (177/733 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. 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 6-57. Port Specified as Input Port
WRPORT
Internal
bus
Output
latch
RDIN
P6n
n = 4 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,
or port mode and control mode, the contents of the output latch of pins specified as inputs or pins
specified as in the control mode 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.
(3) When used as control pins
Cannot be manipulated or tested by software.
Preliminary User’s Manual U13987EJ1V0UM00
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