Clock Domains and Timing References
Every digital system runs on clocks; their stability, and safe crossing between independent clock domains, underpin correct real-time timing.
Clocks Set the Beat
Digital systems are paced by clocks: periodic signals that step logic forward. The quality of these clocks, their frequency accuracy, stability, and low jitter, sets the precision of everything timed by them, from sampling instants to network schedules. A real-time system is only as good as the timing references it is built on.
Clock Quality
- Accuracy: how close the frequency is to nominal, set by the reference source
- Stability: how the frequency drifts with temperature, aging, and supply
- Phase noise and jitter: short-term variation in edge timing
- Resolution: the smallest time interval the clock can distinguish
Reference Sources
Timing references range from ordinary crystal oscillators, adequate for most digital work, to temperature-compensated and oven-controlled crystals for better stability, to atomic references and GPS-disciplined oscillators for absolute accuracy. High-precision data acquisition and deterministic networking often distribute a single high-quality reference to all units so that every sample and every scheduled action shares one stable timebase.
Clock Domain Crossing
Large systems contain multiple clock domains, regions clocked by different, unsynchronized sources. Passing a signal from one domain to another is a known hazard: sampling a signal that changes near a clock edge can produce metastability, where a flip-flop settles to an undefined value. Safe crossing uses synchronizers, multi-stage flip-flops for single bits, and handshake or first-in-first-out structures for multi-bit data, to move information between domains without corruption.
Why It Matters for Control
Clock choices ripple through the whole real-time design. A jittery sampling clock corrupts data as surely as a poor anti-alias filter; an inaccurate clock makes a nominal loop period drift; unsynchronized domains make timestamps from different units incomparable. In an FPGA-based controller, timing determinism is guaranteed relative to the clock, so a stable, well-characterized reference is what turns the hardware's structural determinism into real, trustworthy timing accuracy.