Clockskew

Clock skew is the difference in timing between clock signals or clock values observed at two points in a system. It is a key consideration in both hardware timing and distributed timekeeping, where precise synchronization is essential for correct operation.

In digital electronics, clock skew arises from unequal routing delays and clock distribution network characteristics. Variations in path length, routing, and loading, as well as process, voltage, and temperature changes, can cause the clock edge to arrive at different registers at different times. Skew can be positive or negative relative to a reference. If skew is too large, data captured by downstream registers may violate setup or hold times, leading to timing violations or metastability. Designers mitigate skew by balancing clock trees, using deskew circuits, matched paths, and sometimes phase-locked loops or delay compensation.

In distributed systems, clock skew refers to differences among the real times kept by clocks on separate machines, due to oscillator drift and variable network latency. Skew degrades timestamp accuracy, event ordering, and coordination in distributed protocols. Synchronization protocols such as NTP (Network Time Protocol) and PTP (Precision Time Protocol) estimate offsets and skew and discipline local clocks toward a reference. Techniques include hardware timestamping, clock discipline algorithms, and using a common reference clock or more accurate time sources to reduce skew.

Measurement and modeling vary by domain. Hardware uses timing analysis to bound acceptable skew within a timing budget, while networks measure offset and skew through timestamp exchanges. Clock skew, whether in circuitry or in networks, is a fundamental factor shaping system reliability and performance, with domain-specific strategies to minimize or compensate for its effects.