If I grab correctly what you write, you have an input signal that can assume any value between 0 Volt and 14 Volt, and you are interested in knowing when the input value is "exactly" zero Volt or goes above 11 Volt.
In this case I can suggest you to use analog comparators instead of ADC. Basically, comparator has two pins: one of them can be used as reference and, when the voltage on the other pin gets higher/lower than the reference, you can trigger an interrupt. Clean and elegant solution with very quick detection.
You need an MPU with two comparators, for example have a look at the ATMEL XMega MPU series (ALL models have at least 2 comparators).