You just can't recurse and the same function can't be called from isr and from main context at the same time. Basically, nothing is re-entrable. Otherwise it's

You just can't recurse and the same function can't be called from isr and from main context at the same time. Basically, nothing is re-entrable. Otherwise it's quite fine. To be sure, you shouldn't be doing those things anyway if you have 64B of RAM and 512 words or ROM. :D