Clock design is a pivotal aspect of digital design, ensuring the synchronization of various components within an integrated circuit (IC). This blog delves into advanced techniques in clock design, focusing on reducing timing violations, minimizing duty cycle distortion (DCD), and implementing efficient clock gating. These methods are crucial for achieving high performance and low power consumption in modern ICs. We will explore specific cases and the latest cutting-edge technologies in the field.

Clock tree synthesis is a vital process that ensures the even distribution of the clock signal to all sequential elements in a design. One effective method to reduce timing violations is enabling global routing during CTS. By default, a virtual router is used for fast prediction of wire patterns without layer assignment, but enabling global routing can provide more accurate timing results by considering routing resources and avoiding global congestion.

In a design experiment, enabling global routing during CTS reduced the setup slack from -46.1ps to 9ps, decreased launch path latency from 247.7ps to 222.3ps, and improved skew from 75.2ps to 31.3ps. Despite a slight increase in buffer and inverter count, the overall power consumption increased marginally, indicating a beneficial trade-off for improved timing performance​.

Duty cycle distortion occurs when there is a propagation delay difference between the low-to-high and high-to-low transitions of the clock signal. This issue becomes more pronounced in designs at 7nm and below, where deep clock distributions can accumulate significant DCD due to manufacturing process variations, electrical noise, and transistor aging.

A cutting-edge solution to address DCD is Infinisim’s ClockEdge, which offers SPICE-accurate, high-capacity clock analysis. ClockEdge simulates the entire circuit using full interconnect parasitics, identifying nets with duty cycle, minimum pulse width, and rail-to-rail failures. This tool enables designers to achieve high verification coverage and confidently sign-off on complex clock domains​.

ClockEdge was used to simulate a clock domain with over 4 million gates, delivering SPICE-accurate results overnight. The tool identified DCD issues that traditional static timing analysis (STA) missed, providing insights for optimizing the clock network and improving overall design robustness​​.

Sequential clock gating offers significant power savings compared to combinational clock gating by analyzing temporal signal stability over multiple clock cycles. This technique reduces unnecessary clocking of registers, potentially saving up to 30% in power consumption in wide datapaths.

Manual implementation of sequential clock gating is complex and error-prone, requiring precise tracking of data values over multiple cycles. Automated tools like Calypto’s PowerPro can infer and verify sequential clock gating logic, ensuring correctness and improving design efficiency​.

A design team implemented sequential clock gating in a large SoC, reducing clock power consumption significantly. The automated tool provided a seamless integration into the existing design flow, verifying the inserted gating logic and ensuring reliable operation​​.

Modern SoCs often contain multiple clock domains, requiring careful handling of signals crossing these domains to avoid metastability, glitches, and data coherency issues. Hierarchical CDC verification methodologies, such as bottom-up and top-down approaches, ensure the correctness of asynchronous boundaries early in the design cycle.

Tools like Maestro technology by Movellus offer advanced solutions for handling multi-rate communication without clock-domain-crossing FIFOs, reducing retiming flops, and improving overall PPA (Performance, Power, Area). These solutions simplify the implementation of complex clock networks, enhancing design robustness and efficiency​.

Advanced clock design techniques, including CTS optimization, DCD minimization, efficient clock gating, and hierarchical CDC verification, are essential for developing high-performance and low-power ICs. By leveraging cutting-edge tools and methodologies, designers can achieve significant improvements in timing, power, and area, ensuring the robustness and reliability of modern digital designs.