S1C6S2N7 TECHNICAL HARDWARE
EPSON
I-33
CHAPTER 4: PERIPHERAL CIRCUITS AND OPERATION (I/O Ports)
I/O port data (0F6H)
I/O port data can be read and output data can be written
through the port.
When writing data
When 1 is written:
High level
When 0 is written:
Low level
When an I/O port is set to the output mode, the written
data is output from the I/O port pin unchanged. When 1
is written as the port data, the port pin goes high (VDD),
and when 0 is written, the level goes low (VSS). Port data
can also be written in the input mode.
When reading data
When 1 is read:
High level
When 0 is read:
Low level
The pin voltage level of the I/O port is read. When the I/O
port is in the input mode the voltage level being input to
the port pin can be read; in the output mode the output
voltage level can be read. When the pin voltage is high
(VDD) the port data read is 1, and when the pin voltage is
low (VSS) the data is 0. Also, the built-in pull-down resis-
tance functions during reading, so the I/O port pin is
pulled down.
-
When the I/O port is set to the output mode and a low-imped-
ance load is connected to the port pin, the data written to the
register may differ from the data read.
-
When the I/O port is set to the input mode and a low-level
voltage (Vss) is input by the built-in pull-down resistance, an
erroneous input results if the time constant of the capacitive
load of the input line and the built- in pull-down resistance load
read, the time that the input line is pulled down is equivalent to
0.5 cycles of the CPU system clock. Hence, the electric poten-
tial of the pins must settle within 0.5 cycles. If this condition
cannot be met, some measure must be devised, such as
arranging a pull-down resistance externally, or performing
multiple read-outs.
P00–P03
Note