> I am also thinking of the possibility that the basic
> compiler I use may mask interrupts when performing certain
> "special" instructions (or rather macros) such as writing to
> a LCD display module. This I could find by browsing in the
> assembler file...

It is quite likely that interrupts are being turned off in
some routines.

Quickest way to find out is to set up a timer interrupt at
the byte rate you expect from the I2C port and just toggle a bit.
Any jitter in the width of the pulses will show you just how bad
the latency is.

You could also toggle a 'debug' bit as part of your I2C service
routine. I found that setting a bit at the start, and clearing
it just before I returned, gave me a great deal of insight into
how well my various ISR routines were handling things, and helped
me add additionally polling points as required.

Robert

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