On another note: a 6ms ISR will likely give you heaps of trouble, not only with IIC.

I am sure you will agree with the he general statement "keep your ISRs short and sweet".

The issue with lenghty ISRs is that they tend to interfere with other actions e.g. a 6ms ISR for e.g. EI0 may create trouble if the UART is running 19k2 or faster and you do not use a double buffered derivative.

Another aspect, that I have seen in a few cases is that a long ISR actually interfered with the main in such a way that the main got overrun with "ISR results" after a while because it simply did not have enough non-ISR time to process them.

Ok, maybe i was "too generic" but I would never allow a 6ms ISR in any of my products simply because the effects of a long/bad ISR tend to show up "once amonth in a released product"

The darn thing about ISRs is that there is no way in hades to verify that they do not cause "rarely occuring problems" and thus, while a long ISR may be OK there is no way to verify that it is.

I have found ISR length problems that showed up units less than once a month in any one of 100 units, thus I always "globalize" the statement I made.

If you, Michael, have a means of verifying (not testing - that is worthless) that a lenghty ISR will not cause problems in a given design Please, pretty please, post it.

Erik