Can you give an indication of the intensity of the processor usage?

well, go figure
if you use loops the usage will be 100% during transmission (if you want max speed, you might as well loop).
If you use a timer or PCA and interrupt for each bit (high and low) (4 highs/lows every 2.5us, for full speed) you will, in that interrupt need to check if the external chip is holding the clock (wait) and otherwise transmit data/clock.

Since there is no way in hades you can run full speed, make darn sure that you get a RTC chip that is specified to NEVER update the time while being read or you will need to read 2/3 times and ensure 2 identical reads.

My soft IIC implemetations have often beed awfully slow, just to make the interrupt happen rarely enough not to affect processing too much.

The saving grace is that the IIC spec is so that if a clock comes every 3us or once a year, it still works.

Erik

NOTE TO OTHERS: a soft IIC master is fairly simple, if you can hnadle the time, a soft slave IIC is a nightmare.