CTS

by kamalnadh

                                   CTS
        In this stage we built the clock tree by using inverters and buffers.In the chip clock signal is essential to the flip flops,to give the clock signal from clock source we built the clock tree.It is the process of balancing the clock skew and minimizing insertion delay in order to meet timing and power.

 Before discussing about CTS let see few definitions in CTS.

Aggressor:- A net which creates the effect on nearer net(victim).

Victim:- A net which receives the effect from nearer net(aggressor).

Shielding:- Placing ground net in between aggressor and victim net then voltage discharge on ground net.This will reduce cross-talk.

Cross-talk:- It is the undesirable electric interaction between two or more physical adjust nets due to the capacitance cross coupling.

CTO(clock tree optimization):- It improves the clock skew and clock insertion delay by applying additional optimization. CTO is performed during clock_opt process.

Inputs required for cts:-

• Detailed placement database.
• Target for latency and skew if specified.
• Buffers or inverters for building the clock tree.
• Clock tree DRC's.

Checks before cts:-

• Completed placement.
• Power ground nets - pre routed.
• Acceptable congestion.
• Estimated timing.
• HFNS.

Process:-

      It is a process of inserting buffers and inverters along with the clock path to balance the delay to all clock inputs. Before CTS we treat clock as ideal.If we did't built the clock the skew and insertion delay will increases.This will effects the chip performance.To overcome this we are constructing the clock tree by using inverters and buffers.Below fig shows the structure of before CTS and after CTS.

    Fig:-Before CTS

Fig:-After CTS

Clock tree optimization (CTO):-

It improves the clock skew and clock insertion delay by applying additional optimization. CTO is performed during clock_opt process.The CTO techniques are listed below

Delay insertion:- It will improve hold time.

Buffering:- It will improve setup time.

Buffer relocation:- Reduce skew and insertion delay.

Level adjustment:- Adjust a level of clock pins to a higher or lower

Gate sizing:- It may decrease the delay.

To fix max transition add buffers and to fix max capacitance decrease the net length,cloning.

NDR's:-
Non default rules are applied to reduce the cross-talk and electromigration.NDR's like double spacing,double width shielding.
By applying double spacing cross-talk will reduce.
By applying double width we can avoid electromigration.

Cross-talk:- 

          It is the undesirable electric interaction between two or more physical adjust nets due to the capacitance cross coupling.when two nets are in parallel the electric field of one net is effects the other net which is nearer to it.This called cross-talk effect.

The above fig explain the cross-talk effect.There are two nets running parallel and spacing between the nets are less so capacitance is more.The input of aggressor net inverter is falling signal and the output is raising edge.The input of victim net inverter is raising signal and the output is falling signal.When the rising edge signal is entering into the aggressor net 2nd inverter the charge will stores in the capacitance due to electrical interference at the same time the charge stored in the capacitance will discharge on falling signal of victim net 2nd inverter input.Due to this the falling signal will effect.Here 1st net is creating the effect so that is aggressor and second net receiving effect so that is victim.

Cross-talk reducing techniques:-
a) Increase the spacing between the aggressor and victim nets.
b) Shielding.
c) Maintain the stable supply.
d) Increase the drive strength of cell.
e) Layer jumping.
f) Victim net width increasing then resistance decreases.
g) Guard ring.
h) Cell sizing (up sizing).

CTS exceptions:-

Stop pins:- These are the end point of clock tree and used in calculation and optimization of DRC and clock tree timing(skew,insertion delay).

Float pins:- It's like stop pins with additional delay.

Exclude pins:- It is used only in calculation and optimization of DRC's.It not target skew and insertion delay.


Checklist after cts:-

• Skew report.
• Clock tree report.
• Timing report for setup and hold.
• Power and area report.

CTS goals:-

• Balancing the skew.
• Minimizing insertion delay.
• Minimizing power dissipation.
• Meet the logical DRC's.