21 October 2007

Timing Analysis in Physical Design

Timing analysis at back end requires knowledge of all clock related constraints provided at front end. When .sdc file given to physical design tool (like Astro) its first object is to remove all Wire Load Models (WLM) which are used for front end timing analysis. In backend there is no term called as wire load model. Actual delays are calculated based on the RC value of metal layers. All RC values like sidewall, junction and fringe capacitances are stored as Table Look Up (TLU) format in technology file.

In backend design hold violation has higher priority compared to setup violation because hold violation is related to data path of the design. Setup violation can be eliminated by slowing down the clock.

Placement and routing goal is always to meet timing constraints provided by the .sdc file. If latency and uncertainty are not set for clock at front end then at backend doing Clock Tree Synthesis (CTS) is not possible.

Cell delay and net delay are stored as look up table.

Cell delay consists of transition, timing arcs and capacitances while net delay is constituted by RCs only. Cell delays are available in libraries

. Net delays are specified in technology files. (In front end it is in WLM). Cell delays are fixed. Net delays are not fixed and they depend on interconnect length and width. Net delay parameters Rnet and Cnet are available as Table Look Up (TLU) provided by the vendor.

There is one more set of file TLU+ which account for Ultra Deep Sub Micron (UDSM) effects. UDSM effects are not included in TLU file. A mapping file maps TLU to TLU+. UDSM effects like Optical Proximity Correction (OPC), Resumption Enhanced Technology (RET) and Litho Compliance Check (LCC) are not taken care by Astro. For the placement stage virtual RC (based on Manhattan distance) Layout Parasitic Extraction (LPE) mode is used. For CTS real R and virtual C is used and for routing Real RC is used.

Clock definition given to SAMM in front end design flow is generated as .sdc file from Design Compiler is given below. It includes clock frequency, rise and fall time, setup and hold, skew and insertion delay.

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# Created by Design Compiler write_sdc on Fri May 11 18:35:45 2007
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create_clock -period 4.85 -waveform {0 2.425} [get_ports {clock}]
set_clock_transition -rise 0.04 [get_clocks {clock}]
set_clock_transition -fall 0.04 [get_clocks {clock}]
set_clock_uncertainty 0.485 -setup [get_clocks {clock}]
set_clock_uncertainty 0.27 -hold [get_clocks {clock}]
set_clock_latency 0.45 [get_clocks {clock}]
set_clock_latency -source 0.45 [get_clocks {clock}]


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