The TI datasheet for the PCF8575 specifically denotes the ability to drive LEDs. But I see little (to nothing) about inputs. How do I tell if I need a pullup resistor on the pins when using them as an input with a dry contact (relay or switch contact; not the output of another chip)?
They are designed the same as the Intel port pins When the pins are set high then an input can pull it low..
This device is basically based on the 8255 parallel port chip..
NXP also has this same chip. Their data sheet clears up the mystery. It's "quasi-bidirectional". When you read the register, you are reading tbe state of the pins. When you write to the register, you are setting the port pins.
If you cross out all of the 'output' related sentences above there are only two left, and neither indicate if a pull up or pull down resistor is required for a reliable signal.