What are Clock Skew and Clock Jitter in VLSI designs?
Clock skew is the spatial difference in clock arrival times at different sequential components due to physical routing differences. Clock jitter is the temporal variation of a clock edge from its ideal position due to power noise or thermal factors. Both degrade timing margins.
Spatial vs Temporal Clock Variations
Clock distribution networks are the heartbeat of synchronous digital systems. However, ideal clock distribution is impossible in real silicon due to physical layout variations and environmental factors.
Clock Skew: Spatial Variation
Clock skew (Tskew) is the difference in clock signal arrival times between two related flip-flops (launch and capture). Skew can be positive (clock arrives at the capture flip-flop later than the launch flip-flop) or negative (clock arrives at capture earlier).
While positive skew can sometimes help satisfy setup timing constraints, negative skew reduces setup margins. In all cases, skew increases the risk of hold violations.
Clock Jitter: Temporal Variation
Clock jitter represents the dynamic, cycle-to-cycle variation in the clock period at a single point. It is caused by internal thermal noise, phase-locked loop (PLL) inaccuracies, and power supply variations. Jitter must be accounted for as timing uncertainty during Static Timing Analysis (STA).
Mitigation Techniques
- Symmetrical CTS Trees: Constructing balanced H-Tree or clock-mesh topologies during physical design.
- Shielding: Adding ground shielding wires adjacent to clock lines to prevent cross-coupling capacitance.
- Decoupling Capacitors: Deploying dense decaps near clock buffers to limit supply noise and reduce jitter.
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