Clock Tree Synthesis

Today, we take look on the CTS in very briefly manner. We will discuss more on it in upcoming posts and here, we go …………..

 

Clock tree synthesis is the process of connecting the clock pin of sequential cells to the clock source in order to balance clock skew. The concept of CTS is the automation insertion of buffer and inverter along the clock path of design in order to balance the clock skew to all clock inputs. The goal of CTS is to minimize setup and hold and logical DRC violation.

 

Figure: Clock Tree Synthesis

Figure: CTS  Block-level

Prior to the CTS clock network of the design are marked as ideal. So, the tool doesn’t try to optimize the clock paths while doing data path optimization at post-placement stage.

The performance of the chip is directly depending on clock frequency. Clock nets need to be routed with very great precision. That’s why we have to route clock net first and then signal nets. The reason behind this, we will discuss later. While routing the clock net, we need to take care of some factors such as RC values, length and width of the net, crosstalk effect on the net, and NDR rules. Even ideal clock network also not good i.e. zero skew value, otherwise, clocked the sequential cells at the same time lead to more power consumption.

In CTS, there are algorithms associated with clock net building. Also, every EDA company has different approaches to build the clock tree in a different way. EDA companies were worked on an existing one and build a new algorithm. But, no one wants to release their algorithm. There are basic setups of CTS, which are common in those algorithms, like building clock, route clock network and subsequently clock path optimization.

There are a number of topics under CTS such as clock terminology, local and global skew, NDR Rules, crosstalk and many more. We will discuss one by one and also, try to answer some question which will come while moving deeper into these topics.

 

At this note of discussion, we wrapped up the introduction of CTS and we will meet some new CTS topic. Thank you. Have a great day!!