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參數資料
| 型號： | MTA85802S-04I/SS |
| 元件分類： | 微控制器/微處理器 |
| 英文描述： | 8-BIT, OTPROM, 4 MHz, RISC MICROCONTROLLER, PDSO20 |
| 封裝： | 0.209 INCH, PLASTIC, SSOP-20 |
| 文件頁數： | 9/72頁 |
| 文件大小： | 760K |
| 代理商： | MTA85802S-04I/SS |
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1995 Microchip Technology Inc.
DS40115C-page 17
MTA85XXX
5.8.4
BIDIRECTIONAL I/O PORTS
Some instructions operate internally as read followed
by write operations. The BCF and BSF instructions, for
example, read the entire port into the CPU, execute the
bit operation, and re-output the result. Caution must be
used when these instructions are applied to a port
where one or more pins are used as input/outputs.
For example, a BSF operation on bit5 of PORTB will
cause all eight bits of PORTB to be read into the CPU.
Then the BSF operation takes place on bit5 and
PORTB is re-output to the output latches. If another bit
of PORTB is used as a bidirectional I/O pin (say bit0)
and it is defined as an input at this time, the input signal
present on the pin itself would be read into the CPU and
re-written to the data latch of this particular pin, over-
writing the previous content. As long as the pin stays in
the input mode, no problem occurs. However, if bit0 is
switched into output mode later on, the content of the
data latch may now be unknown.
A pin actively outputting a '0' or '1' should not be driven
from external devices at the same time in order to
change the level on this pin ("wired-or", "wired-and").
The resulting high output currents may damage the
chip.
5.8.5
SUCCESSIVE OPERATIONS ON I/O
PORTS
The actual write to an I/O port will happen at the end of
an instruction cycle, whereas for reading, the data must
be valid at the beginning of the instruction cycle
(Figure 5-8). Therefore, care must be exercised if a
write followed by a read operation is carried out on the
same I/O port. The sequence of instructions should be
such to allow
the pin voltage to stabilize (load
dependent) before the next instruction which causes
that file to be read into the CPU is executed. Otherwise,
the previous state of that pin may be read into the CPU
rather than the new state. When in doubt, it is better to
separate these instructions with a NOP or an other
instruction not accessing this I/O port.
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