Robin2:
Yes with the onboard ADC...
Imagine @george29 did not mention how input data is acquired (he did not so it should be easy to imagine). Also imagine @george29's input data is not coming from the on-board ADC but is coming from another microcontroller and the two communicate over radio. Also imagine the PID loop is designed to run every 12 seconds (which should be easy because that is your example).
What could go wrong if fresh data is collected at the same rate as the PID loop is updated?