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How do you use clock gating and buffering techniques for STA?

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Clock tree synthesis (CTS) is a crucial step in the design of integrated circuits that use synchronous logic. It involves creating a network of clock buffers and wires that distribute the clock signal to all the flip-flops and other timing elements in the circuit. CTS affects the performance, power consumption, and reliability of the circuit, so it must be done carefully and efficiently. In this article, we will explore some of the challenges and techniques of CTS in the context of static timing analysis (STA), which is a method of verifying the timing requirements of the circuit.

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1 What is STA?

STA is a way of checking whether the circuit meets the timing constraints imposed by the clock frequency, the logic delays, and the setup and hold times of the flip-flops. STA analyzes the worst-case scenarios of the circuit, such as the longest and shortest paths, the maximum and minimum delays, and the effects of process variations and environmental conditions. STA helps to identify and fix any timing violations that could cause functional errors or performance degradation in the circuit.

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Swati Jindal

Design Realization | Physical Design | Signoff | 3DIC STCO
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The most critical input for STA is your timing constraints. Unless those are correct and optimized your results are not reliable. Constraints like input and output delay which are dependent on logic outside your design, or clock constraints which define the performance of the design and the scenarios in which you are conducting STA are critical to determine whether the design would meet the expected performance criteria or not.

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2 Why is CTS important for STA?

CTS is important for STA because it determines the clock skew and clock latency of the circuit. Clock skew is the difference in arrival times of the clock signal at different flip-flops. Clock latency is the delay from the clock source to the flip-flops. Both clock skew and clock latency affect the timing margins and the critical paths of the circuit. Ideally, CTS should minimize clock skew and clock latency, while maintaining a balanced and robust clock tree that can handle variations and noise.

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Vivek Kandhagatla

ASIC Design Engineer | Physical Design Engineer
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By routing the clock and data signals in opposite directions, you can minimize or eliminate clock-to-data skew. This can be beneficial in synchronous systems where maintaining proper timing relationships between clock and data is crucial at the cost of performance.

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Austin Maxson

ASIC Physical Design Engineer and Technical Fellow
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Useful skew in the clock tree to eliminate timing violations is also a critical technique to take advantage of in more advanced nodes. This is when you skew the clock on purpose to fix timing violations. It can eliminate the need for tools to insert additional delay later on in the data paths for hold fixing or increased drive strengths and lower voltage thresholds in the data path for setup violations. This reduces power and congestion.

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3 How do you use clock gating for STA?

Clock gating is a technique of reducing power consumption by disabling the clock signal to some parts of the circuit when they are not needed. It can also improve the timing margins by removing some unnecessary clock transitions that could cause glitches or extra delays. However, clock gating introduces several challenges for CTS and STA, such as ensuring that the gated clock signal meets the setup and hold times of the flip-flops, accounting for the delay and power overhead of the clock gating logic, and avoiding creating clock domains with different frequencies or phases. To address these challenges, CTS and STA tools should support features such as generating and optimizing clock gating cells and logic, analyzing and verifying the gated clock signal and its impact on timing, and reporting and debugging any clock gating issues or violations.

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4 How do you use clock buffering for STA?

Clock buffering is a technique used to improve the quality of the clock signal by inserting buffers or inverters along the clock wires. This can help reduce wire resistance and capacitance, amplify the signal strength, and reduce noise susceptibility, as well as adjusting the signal phase and duty cycle, and balancing the clock skew and latency across the clock tree. However, this method also has drawbacks such as increasing power consumption and heat dissipation, introducing additional delay and jitter, and occupying more area and routing resources. Therefore, CTS and STA tools must consider the benefits versus costs of clock buffering when choosing the optimal number, size, and location of buffers; minimizing insertion delay and jitter; maximizing power efficiency and reliability; and resolving any conflicts or congestion with other signals or components.

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Austin Maxson

ASIC Physical Design Engineer and Technical Fellow
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Using inverters in the clock tree allows you to maintain more consistent duty cycle. If the rising edge transition time through a buffer is different than the falling edge, then the duty cycle will be affected with each buffer. With inverters you are balancing out this difference by inverting the signal such that the first inverter sees a rising edge and the second inverter sees a falling edge input. The variation of delays across timing conditions (process, voltage, temperature) can have a big impact on timing closure. The drive strength and voltage threshold of the cells used in the clock path along with the clock net lengths have to be chosen with this in mind.

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Jeffrey Peery

Physical Design Engineer at Microsoft
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RC segments should also be accounted for and effort should be made to keep the RC the same between clock repeaters. This helps to minimize variation across different PVTs. Also, for long distributions, repeater sizes/VT should be the same. This is essentially the problem with using buffers. There will always be a small inverter followed by a large inverter, which will introduce pulse shaping when you (always) have process variations.

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Swati Jindal

Design Realization | Physical Design | Signoff | 3DIC STCO
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Avoid saying clock buffering, it's better to say clock repeaters. We use inverters more commonly for CTS so clock buffering throws in doubts.

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How do you use clock gating and buffering techniques for STA?

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1 What is STA?

2 Why is CTS important for STA?

3 How do you use clock gating for STA?

4 How do you use clock buffering for STA?

Static Timing Analysis

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