8574_info_en.txt

# PCF8574(A).

It is an I/O port expander with control via the i2c bus. This chip contains only one internal register,
which is its physical I/O port. That is, if you transfer a byte to it, it will immediately be set to its pins.

If you read a byte from it, it will reflect the logical states on its pins.

When applying a logical "0" to the output, the chip pulls its pin(s) to ground (GND),
When applying a log. "1" to the output, the chip connects its pin(s) to +power (Vdd) through a current source(!) of 100 μA.
Therefore, you cannot get a "real" log. "1" at the output!
This is done in order to configure the PCF8574 pin for input/output without additional(!) registers.

You just need to write a log. "1" (set the state of the IC pin to Vdd) and can simply short it to GND.
The current source will not allow the output stage of the I/O expander to "burn out".

If the pin of the microcircuit is pulled to the ground, then the ground potential is on it, and a logical 0 is read.
If the pin of the microcircuit is pulled to Vdd, then a logical 1 is read.

That is, to configure the pin as DI, it is necessary that a logical "1" is written in the corresponding register bit.

The load must be connected between the pin of the microcircuit and GND!

To configure the pin as DO, nothing is needed!

If the load connected to the pin of the microcircuit switches to the "1" state at a current less (!) than 100 μA, then everything is fine.
Write "1" to the desired bit, the load to "1". Write "0" and the load switches to "0".

But if you have a LED/LED load or something else that requires a current of more than 100 µA, then you need to write logical 0 to the corresponding register bit.
That is, turn on the load with a logical zero!!! The current, in this case, goes to the load along the path: Vdd -> load -> microcircuit output -> internal microcircuit transistor -> GND.