Circuits That Remember
Combinational circuits are memoryless — the same inputs always give the same outputs. But a computer must remember: a register holds a value, a counter tracks a count. Sequential logic adds state — the circuit's output depends not only on current inputs but on stored information about the past.
Memory in hardware comes from feedback: an output is wired back to an input, so the circuit can hold a value. The simplest memory element is a latch, which can store one bit. Left alone, it keeps its stored bit; with the right inputs, it can be set to 1 or reset to 0.
Uncontrolled feedback is chaotic, so real designs are synchronous: governed by a clock, a signal that ticks at a fixed rate. State updates happen only at a clock edge (the rising or falling transition), so the whole system advances in lock-step, one well-defined step per tick. Between edges, combinational logic settles; at each edge, the new state is captured. This discipline is what makes complex digital systems predictable.
The general shape of a sequential system is a finite state machine (FSM): a set of states, plus rules that, given the current state and inputs, determine the next state and the outputs. A traffic light, a vending machine, and a CPU's control unit are all finite state machines. The state register (built from flip-flops, next lesson) holds the current state; combinational logic computes the next state; the clock advances it.
Common pitfall: thinking a sequential circuit responds to its inputs continuously, like combinational logic. In a synchronous design, state changes only at clock edges — inputs are sampled and the state advances one step per tick. Reasoning about a sequential circuit means reasoning tick by tick, not moment by moment.
A finite state machine drawn as labeled state circles with transition arrows; an accent "current state" token hops from state to state on each clock-edge pulse, showing state advancing one step per tick.