The Effect of Clock Skew in Timing Analysis

As we have seen Timing Analysis of path in Part I and Part II, the delay of the cell causes timing violation. Along with it, the arrival of clock at different point of time at the clock pins of the sequential cell also causes the timing violation. Today we are going to understand how skew will affect the setup and hold violation of the timing path.

  

What is clock skew???

The difference between in arrival times of a pair clock signal. The clock skew in the design depends on the – how the clock tree is built at the CTS Stage. The clock skew is also depending on die size, process and environmental variation, i.e. PVT variation, RC delay, and clock loading. Both data propagation delay and clock skew are contributing to the timing calculation.

 It has two-part –

Positive Clock Skew: The skew that occurs when the clock reaches the captured cell later than the launch cell is called positive clock skew.

Negative Clock Skew: The skew that occurs when the clock reaches the captured cell earlier than the launch cell is called negative clock skew.

 

Effect of Positive Clock Skew on Setup Slack

The positive clock skew in the timing path helps to meet setup slack by offering more time to capture the data for the capture flip flop.

But,

It reduces the hold slack of second flip flop, FF2, i.e. it violated the current timing path.

It also reduces the setup slack for subsequent timing path (FF2 → FF3).

 

Effect of Negative Clock Skew on Hold Slack

The negative clock skew in the timing path helps to meet hold slack by offering delayed clock edge of launch flip flop so that it delaying in the launching data and subsequently data change.

But,

It reduces the setup slack of the current timing path (FF1 → FF2).

It also reduces the hold slack for subsequent timing path (FF2 → FF3).

 

 

We will learn more about Skew Balancing and the concept of Useful Skew under the heading of CTS.

 

At this note of discussion, we wrapped this topic and we will meet soon with a new topic of STA. Thank You, Have a nice day!!!