In the Last post , we had discuss about the Parameters required for good CTS . In this post we will discuss about the Various Clock tree structures widely used in the industry, which having its own merit & demerit.
Lets discuss different Clock Tree Structure one by one
Conventional CTS/Single point CTS:
Single point CTS is the default choice for most of the designers having lower frequency & lesser no of sinks. As name suggested having single clock source which distribute clock to every corner of design.
In Single point CTS the point of divergence lie at the clock source, so it shared very large uncommon clock path, more susceptible to OCV variation. In this clock gates are stratergically placed near the source, saving large amount of dynamic power
Advantages:
• Simplicity of Implementation
• Better clock gating, reducing Power dissipation
Disadvantages:
• Higher Insertion delay
• More uncommon clock path, more prone to OCV variation
• Difficult to achieve lower skew, due to asymmetric distribution of sinks.
So, conventional CTS is not a good choice for high frequency signals, having high no of sinks (all over core region)
Clock Mesh Structure
As the name suggests it create a dense mesh of shorted wires which is being driven by mesh drivers to distribute clock in every corner of the design.
In the mesh structure, there will be a network of pre-mesh drivers to drive the clock signal from clock port to input of mesh drivers. The output of all the mesh drivers will be shorted using a metal mesh, which will carry the clock signal across the block using horizontal and vertical metal stripes.
As we can see from Figure 2, mesh drivers are connected to mesh net as multi-driven net. The benefit of the mesh net is that it smoothes out the arrival time differences from the multiple mesh drivers that drive it. If the mesh net is dense enough it require only few stages of clock network (2-3 stages) to route all the sequential element with the clock, which makes the uncommon clock path is very less (path of divergence less), so more prone to OCV variation. In clock Mesh structure power dissipation is going to be high as clock gates cells are inserted after the mesh net, thereby implying clock mesh is always on & switching continuously (as clock gating done at local level only).
Advantages:
• Lower Skew
• Highly tolerant to On-Chip Variation
• Possible to achieve lower insertion delay
Disadvantage:
• More power dissipation (Dynamic)
• More routing resources required for creating mesh
• Difficult to implement
Multi-Source CTS:
Multi-Source CTS is a hybrid approach, between Conventional CTS & Clock Tree Mesh. It involves a global distribution network in form of a sparse mesh or an H-tree with tap points strategically inserted at different locations. These tap points are followed by a local clock tree distribution to route clock from these tap points to the Sequential cell clock end-pins
Advantages:
• More common clock path then Single point CTS , so more prone to OCV variation then Single Point CTS
• Less routing resources required then Mesh based clock Tree
• Less power Dissipation then Mesh based Clock Tree
• Less insertion delay as compared to Conventional Clock Tree
• Lesser Skew
In last , have tried to conclude all the design metric with different variants of Clock Tree
Lets discuss different Clock Tree Structure one by one
Conventional CTS/Single point CTS:
Single point CTS is the default choice for most of the designers having lower frequency & lesser no of sinks. As name suggested having single clock source which distribute clock to every corner of design.
 |
| Figure 1 : Single Point CTS |
In Single point CTS the point of divergence lie at the clock source, so it shared very large uncommon clock path, more susceptible to OCV variation. In this clock gates are stratergically placed near the source, saving large amount of dynamic power
Advantages:
• Simplicity of Implementation
• Better clock gating, reducing Power dissipation
Disadvantages:
• Higher Insertion delay
• More uncommon clock path, more prone to OCV variation
• Difficult to achieve lower skew, due to asymmetric distribution of sinks.
So, conventional CTS is not a good choice for high frequency signals, having high no of sinks (all over core region)
Clock Mesh Structure
As the name suggests it create a dense mesh of shorted wires which is being driven by mesh drivers to distribute clock in every corner of the design.
In the mesh structure, there will be a network of pre-mesh drivers to drive the clock signal from clock port to input of mesh drivers. The output of all the mesh drivers will be shorted using a metal mesh, which will carry the clock signal across the block using horizontal and vertical metal stripes.
 |
| Figure 2: Clock Mesh |
As we can see from Figure 2, mesh drivers are connected to mesh net as multi-driven net. The benefit of the mesh net is that it smoothes out the arrival time differences from the multiple mesh drivers that drive it. If the mesh net is dense enough it require only few stages of clock network (2-3 stages) to route all the sequential element with the clock, which makes the uncommon clock path is very less (path of divergence less), so more prone to OCV variation. In clock Mesh structure power dissipation is going to be high as clock gates cells are inserted after the mesh net, thereby implying clock mesh is always on & switching continuously (as clock gating done at local level only).
Advantages:
• Lower Skew
• Highly tolerant to On-Chip Variation
• Possible to achieve lower insertion delay
Disadvantage:
• More power dissipation (Dynamic)
• More routing resources required for creating mesh
• Difficult to implement
Multi-Source CTS:
Multi-Source CTS is a hybrid approach, between Conventional CTS & Clock Tree Mesh. It involves a global distribution network in form of a sparse mesh or an H-tree with tap points strategically inserted at different locations. These tap points are followed by a local clock tree distribution to route clock from these tap points to the Sequential cell clock end-pins
Advantages:
• More common clock path then Single point CTS , so more prone to OCV variation then Single Point CTS
• Less routing resources required then Mesh based clock Tree
• Less power Dissipation then Mesh based Clock Tree
• Less insertion delay as compared to Conventional Clock Tree
• Lesser Skew
In last , have tried to conclude all the design metric with different variants of Clock Tree
Design Metric
|
Single Point CTS
|
Mesh based CTS
|
Multi-Source CTS
|
Power
|
Low
|
High
|
Moderate
|
Performance
|
Low
|
High
|
Moderate
|
Area/Routing Resource
|
Low
|
High
|
Moderate
|
Impact of OCV
|
High
|
Low
|
Moderate
|
Let me know , in case you have any queries.Stay Blessed !!
