sequential logic

Sequential logic is circuitry that remembers. Unlike combinational logic, whose output depends only on the inputs right now, a sequential circuit's output depends on the inputs and on its stored history — its current state. Think of a turnstile counter at a stadium: pressing the button does not just light up a momentary signal, it advances a running tally that persists. The very same input applied at two different moments can produce two different outputs, because the circuit carries memory of what came before. This memory is what lets hardware do anything that unfolds over time: count, sequence steps, run a program.

Where does memory come from in a circuit of gates? From feedback — connecting an output back to an input so a value can sustain itself. The simplest memory element built this way is a latch (a pair of cross-coupled gates that hold a bit), and the disciplined version is the edge-triggered flip-flop, which captures its input only at a clock edge and holds it steady until the next edge. A sequential circuit is therefore built from two parts: combinational logic that computes the next state and the outputs from the current state and inputs, and a bank of flip-flops that store the state and update all together on the clock tick. This is the standard finite-state-machine structure.

Why it matters: sequential logic is what turns a calculator of instantaneous functions into a machine with behaviour over time. Counters, registers, shift registers, controllers, and ultimately the whole processor are sequential. The honest discipline that makes it reliable is synchronous design: a single clock decides exactly when every flip-flop updates, so the combinational logic between flip-flops is given a fixed window to settle and is only sampled once it is stable. Without that clock-imposed order, feedback and gate delays would let circuits race and produce unpredictable results.