Let's have an AVR with ADC, say ATMega128. Let's have written an ADC ISR which re-triggers the ADC, and let's initialise the ADC for single-conversion and enable the interrupt. That should yield us a continuously running AD conversion, isn't it.
Now if we want to exchange some variables between main and the ISR, a well-behaving main won't temporarily disable the interrupts globally (through sei/cli), but only the ADC interrupt.
How to do that properly?