Classification of Designs
COMBINATIONAL
Output only depends on the instantaneous input values.
IDEAL: Output responds instantly.
REALITY: Propagation delay, spurious outputs ("glitches") that must eventually settle.
=> No feedback path exists from the outputs to the inputs.*
SEQUENTIAL
Circuit contains "memory" (store) so the output may depend on the sequence of previous inputs (since power-up!)
REALITY: Output depends on the duration of the inputs.
=> There must be at least one feedback path from the outputs to the inputs.
* Is this really true? Why?