The literature states that a SR flip flop is a sequential device and that sequential devices are those whose output depends on its current inputs and prior state. However, that doesn't make sense to me: flip flops are the basic building blocks of some memories and I wonder how useful a memory can be if its next state depends on its prior state. That would be a very bizarre kind of storage! You have to have the ability to write anything you need to and, in fact, that is exactly what a flip flop allows you to do. So, my question is: am I missing something (conceptually speaking) or in fact a flip flop does not comply with the definition of sequential logic? Thank you very much in advance!
The output of a FF depends on not only the present values of its inputs, but also on the previous values of those inputs. This is the definition of a sequential circuit.
In other words, there are combinations of input values for which the output can be in either of two states — which one is determined by the past history of the input values. Internal feedback gives the circuit "memory".
What is the current output state of this FlipFlop?
_______ 0-|S Q|-? 0-|R | |_______|
It depends on the previous state!