Clock Net Shielding

Clock nets are among the most critical signals in a digital design. The clock tree is carefully constructed to minimize skew, often aiming for zero skew across all endpoints. However, this delicate balance can be disrupted by a phenomenon known as crosstalk.

Crosstalk

Crosstalk occurs when a signal on one wire (the aggressor) unintentionally interferes with a neighboring wire (the victim) due to capacitive coupling. In high-speed circuits, this interference can cause:

• Signal distortion
• Noise and glitches
• Timing violations (e.g., hold or setup failures)
• Overall deterioration of the clock tree’s integrity

Shielding Clock Nets

To mitigate crosstalk, clock nets are shielded — meaning they are surrounded by non-switching metal lines (usually tied to $V_{dd}$ or $V_{ss}$).

Shielding helps to:

• Reduce coupling capacitance between adjacent wires
• Protect victim nets from noise caused by nearby switching activity
• Maintain clean clock edges and predictable timing

Effects of Unshielded Aggressors

If a switching aggressor net is routed near an unshielded victim net, the strong capacitive coupling can cause two major problems:

1. Glitch: A sudden voltage drop or spike on the victim line due to energy transfer from the aggressor. This can lead to incorrect data being latched into memory or flip-flops, resulting in functional errors.
2. Delta Delay: A change in the timing of signal propagation due to induced noise — this can shift arrival times and potentially violate setup or hold constraints.

How Shielding Helps

Shielding works by placing $V_{dd}$ or $V_{ss}$ wires adjacent to critical nets (like clocks). These non-switching shields act as buffers, absorbing or blocking unwanted coupling from aggressor lines.

• Since the shields do not toggle, they do not transfer switching noise.
• This ensures that the victim net remains stable, preserving signal integrity and timing accuracy.