ST10F273 datasheet(66/179 Pages) STMICROELECTRONICS

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Parallel ports

ST10F273

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If the alternate input function of a pin is used, the direction of the pin must be programmed

for input (DPx.y=‘0’) if an external device is driving the pin. The input direction is the default

after reset. If no external device is connected to the pin, however, one can also set the

direction for this pin to output. In this case, the pin reflects the state of the port output latch.

Thus, the alternate input function reads the value stored in the port output latch. This can be

used for testing purposes to allow a software trigger of an alternate input function by writing

to the port output latch.

On most of the port lines, the user software is responsible for setting the proper direction

when using an alternate input or output function of a pin.

This is done by setting or clearing the direction control bit DPx.y of the pin before enabling

the alternate function.

There are port lines, however, where the direction of the port line is switched automatically.

For instance, in the multiplexed external bus modes of PORT0, the direction must be

switched several times for an instruction fetch in order to output the addresses and to input

the data.

Obviously, this cannot be done through instructions. In these cases, the direction of the port

line is switched automatically by hardware if the alternate function of such a pin is enabled.

To determine the appropriate level of the port output latches check how the alternate data

output is combined with the respective port latch output.

There is one basic structure for all port lines with only an alternate input function. Port lines

with only an alternate output function, however, have different structures due to the way the

direction of the pin is switched and depending on whether the pin is accessible by the user

software or not in the alternate function mode.

All port lines that are not used for these alternate functions may be used as general purpose

I/O lines.

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