Virtual Leaf Drivers for Clock Grid Skew Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional clock distribution circuitry in integrated circuits faces challenges in minimizing clock signal skew across the chip, which limits operating frequency due to varying propagation delays and load differences, especially in complex designs like microprocessors where clock tree buffers are limited.
Innovation Solution
The introduction of virtual leaf driver nodes that split the output of leaf buffers into multiple signal paths of equal length to connect to distributed points on the clock grid, ensuring consistent propagation delay and reducing skew between virtual driver nodes, while tailoring transistor strengths based on load and edge type to manage power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional buffer trees with constant number of tiers are used, then skew is minimized, but the number of buffers is limited and cannot drive heavy loads at any point on the chip
Solution Approach 1:
The clock distribution network is segmented into two independent parts: a buffer tree that provides synchronized clock signals to multiple drive points, and a mesh grid that distributes these signals throughout the chip. This segmentation allows the buffer tree to focus on skew minimization while the mesh grid provides adaptability for driving heavy loads at any location.
Solution Approach 2:
The mesh grid acts as an intermediary between the buffer tree output and the clock consumers. Instead of directly connecting buffer outputs to all clock consumers, the mesh grid intermediate structure provides multiple paths for signal distribution, enabling heavy load driving capability while maintaining the skew-minimizing buffer tree architecture.
2Device complexity
If H-tree routing pattern is used, then number of buffers and conductor run length are held constant, but clock skew varies depending on distance from nearest driven point
Solution Approach 1:
The solution transitions from a two-dimensional H-tree planar routing to a three-dimensional structure by overlaying a mesh grid on top of the buffer tree. This adds a distribution dimension that allows clock signals to reach all areas of the chip with uniform skew characteristics, regardless of their distance from buffer outputs.
Solution Approach 2:
The mesh grid is designed to provide equipotential clock distribution, where all points on the grid receive clock signals with essentially the same skew relative to their respective drive points. This creates a uniform clock environment across the entire chip, eliminating the distance-dependent skew variation inherent in H-tree routing.
3Power
If leaf buffers are directly connected to clock grid, then drive capability is provided, but propagation delay causes edge lag that differs between consumers at different points
Solution Approach 1:
Different regions of the mesh grid are driven by multiple virtual leaf driver nodes with appropriately sized transistors tailored to local load conditions. This local optimization ensures that each region receives sufficient drive capability while maintaining consistent propagation delays, as each virtual driver is positioned and sized to serve its specific area efficiently.
4Adaptability or versatility
If more buffers are added to drive heavy loads, then load capability increases, but skew minimization becomes more difficult
Solution Approach 1:
Multiple buffer tree outputs are merged into a unified mesh grid distribution system. This combination allows the system to leverage the skew-minimizing properties of the buffer tree architecture while simultaneously utilizing the mesh grid's ability to distribute signals uniformly across the entire chip, achieving both skew control and heavy load driving capability.
Data Source
AI summary
Various embodiments of methods and systems for providing virtual leaf driver nodes in a clock tree to drive a clock grid of an integrated circuit are disclosed. An integrated circuit may include a large number of clocked elements such as registers, flip-flops, etc. whose operation is synchronized by one or more clocks. For example, an operation performed by circuitry on one side of the die may need to occur at precisely the same time as another operation performed by circuitry on the other side of the die. In order to assure synchronicity of these events, a clock grid may be provided in the IC that is driven by virtual leaf driver nodes. The clock tree driving the clock grid may include a tier of leaf buffers. The output of a leaf buffer may be split, and the branches of the output connected to separate points on the clock grid.