ASIC-System on Chip-VLSI Design: Clock Tree Synthesis (CTS)

The goal of CTS is to minimize skew and insertion delay. Clock is not propagated before CTS as shown in Figure (1).

Figure (1) Ideal clock before CTS

After CTS hold slack should improve. Clock tree begins at .sdc defined clock source and ends at stop pins of flop. There are two types of stop pins known as ignore pins and sync pins. ‘Don’t touch’ circuits and pins in front end (logic synthesis) are treated as ‘ignore’ circuits or pins at back end (physical synthesis). ‘Ignore’ pins are ignored for timing analysis. If clock is divided then separate skew analysis is necessary.

Global skew achieves zero skew between two synchronous pins without considering logic relationship.

Local skew achieves zero skew between two synchronous pins while considering logic relationship.

If clock is skewed intentionally to improve setup slack then it is known as useful skew.

Rigidity is the term coined in Astro to indicate the relaxation of constraints. Higher the rigidity tighter is the constraints.

In Clock Tree Optimization (CTO) clock can be shielded so that noise is not coupled to other signals. But shielding increases area by 12 to 15%. Since the clock signal is global in nature the same metal layer used for power routing is used for clock also. CTO is achieved by buffer sizing, gate sizing, buffer relocation, level adjustment and HFN synthesis. We try to improve setup slack in pre-placement, in placement and post placement optimization before CTS stages while neglecting hold slack. In post placement optimization after CTS hold slack is improved. As a result of CTS lot of buffers are added. Generally for 100k gates around 650 buffers are added.

Global skew report is shown below.

**********************************************************************
*
* Clock Tree Skew Reports
*
* Tool : Astro
* Version : V-2004.06 for IA.32 -- Jul 12, 2004
* Design : sam_cts
* Date : Sat May 19 16:09:20 2007
*
**********************************************************************

======== Clock Global Skew Report =============================

Clock: clock
Pin: clock
Net: clock

Operating Condition = worst
The clock global skew = 2.884
The longest path delay = 4.206
The shortest path delay = 1.322

The longest path delay end pin: \mac21\/mult1\/mult_out_reg[2]/CP
The shortest path delay end pin: \mac22\/adder1\/add_out_reg[3]/CP

The Longest Path:
====================================================================
Pin Cap Fanout Trans Incr Arri Master/Net
--------------------------------------------------------------------
clock 0.275 1 0.000 0.000 r clock
U1118/CCLK 0.000 0.000 0.000 r pc3c01
U1118/CP 3.536 467 1.503 1.124 1.124 r n174
\mac21\/mult1\/mult_out_reg[2]/CP
4.585 3.082 4.206 r sdnrq1
[clock delay] 4.206
====================================================================

The Shortest Path:
====================================================================
Pin Cap Fanout Trans Incr Arri Master/Net
--------------------------------------------------------------------
clock 0.275 1 0.000 0.000 r clock
U1118/CCLK 0.000 0.000 0.000 r pc3c01
U1118/CP 3.536 467 1.503 1.124 1.124 r n174
\mac22\/adder1\/add_out_reg[3]/CP
1.701 0.198 1.322 r sdnrq1
[clock delay] 1.322
====================================================================