Peter Dannegger wrote:

"E.g. I use lots of 74HC595 cascaded and every shift register correspondend with a byte in the 8051 bit space."

The other obvious advantage of the '595 is that it's about one fourth the price of the I2C parts.

Of course, you do lose the interrupt and the bidirectional I/O you get with the '8574. Driving '595s also requires more pins, as you need to clock both the shift register and the output register. Finally, a lot of micros have built-in I2C masters, which beats the heck outta bit-banging a couple of pins.

It's a trade-off. It's engineering. It's how it is.

-a