Problems with ADC?

I am experiencing a very strange problem, and I wonder if anyone has a similar experience.

In a application I am using ADC input and port pin outputs on the same port (Port C on a MEGA328).

I have a boolean function, let us call it test, and a pin defined thus



Now if I set



the processor misbehaves, consistent with it resetting (I do not have watchdog or brownout detect) but I cannot say for sure that that is what is happening.

However, if I put



everything behaves as it should.

Looking at the assembler, after optimisation timings should be about the same. The difference between these two pieces of code is that the one that does not work uses read-modify-write of the C port (IN and OUT), whereas the one that does work uses direct port pin manipulation (CBI and SBI). But I honestly don't see why that would make a difference unless ADC detection gets upset, and I don't see why it should.

There is nothing logically wrong with the code generated as far as I can see.

Has anyone experienced anything similar?

Thanks for info, Rolf. That gives me confidence.

I don't need to worry about read-modify-write. I am happy with atomic instructions. I know how to generate them so that is fine. I thought about disable interrupts, but felt that properly documented the if-then-else construct was cleaner and shorter (in asm).

In fact I tend to use this type of construct in interrupt routines anyway, because although the source is longer, the optimised asm is shorter and runs almost 3 x as fast in my case!

Thanks again