clocked

A clocked circuit or device is one whose operation is paced by an external electronic timing signal, usually a square wave or pulse train generated by a crystal oscillator or phase-locked loop. Instead of operating freely at its own speed, the circuit samples inputs, executes logic operations, or transfers data only when the clock signal permits. This synchronization is fundamental to virtually all digital electronics, from microprocessors to shift registers to state machines.

The clock signal typically runs at a fixed frequency measured in megahertz or gigahertz. A processor clocked at 100 MHz receives 100 million clock pulses per second; each pulse (or cycle) represents one opportunity for the circuit to perform an operation. Not every gate or function acts on every clock pulse; logic conditions and enable signals determine what actually happens at each tick. The clock edges (rising or falling transitions) are what matter: circuits sample their inputs and update their outputs synchronously with these edges, creating predictable timing relationships throughout the system.

Clock speed fundamentally limits circuit performance. A faster clock allows more operations per second, but it also tightens timing margins: signals must settle and propagate through logic before the next clock edge arrives. If a signal does not settle in time, setup time or hold time violations occur, causing metastability or incorrect data capture. Designers must therefore balance clock frequency against propagation delays, parasitic capacitances, and signal routing.

Clocked versus asynchronous

The opposite approach is asynchronous logic, in which circuits operate without a global clock and handshake via request and acknowledge signals instead. Asynchronous circuits can be faster in localized regions and consume less power when idle, but they are far harder to design and verify. Most practical digital systems are clocked because synchronous design is simpler: a single global timebase makes it straightforward to reason about causality, state transitions, and timing closure.

In manufacturing and test, a device is often said to be clocked at a particular speed to denote its qualified operating frequency. A microcontroller might be rated to run at 1 MHz, 8 MHz, or 16 MHz depending on its process node and supply voltage. Running a clocked device above its specified frequency risks timing violations and data corruption; running it below specification is safe but wasteful. Adaptive clock throttling, common in modern processors, adjusts clock speed dynamically to manage power and heat.