At which stage should WLM and SDF be used ?

[jitendravlsi]

wlm vs sdf

In Physical Design flow, upto which stage we use wlm(wire load model) and at which stage we use sdf(standard delay format)?


the questions may be silly but please try to clarify my doubts


thanks

 

[yeeore]

wlm vs sdf

If you use physical synthesis (topographical mode in DC[SNPS] or PLE in RC[CDNS] or smth like this) you should not use wlm at all.
You should load your floorplan in synthesis tool before synthesis from RTL.
But if you use old approach: wlm synthesis and topo creation with real RC and delays, I think you should use .SDF file for back-annotation of your netlist with delays from your topo for simulation, for example.

 

[jitendravlsi]

Re: wlm vs sdf

Sorry dear

I didn't get your point. what are you trying to say?

please explain in simple way.........


by the way thanks for reply.

 

[johanlo]

Re: wlm vs sdf

In the old days DC was unware of cell placement, and wire load model (WLM) was used to estimate the netdelay. The WLM is based on the net fanout, and each standard cell library contained some WLMs to choose from.

Now DC is placement aware, running the tool in topographical mode. You have to specify information like die size, placement blockages, macrocell placement etc. to take full advantage of this feature. I belive Synopsys clamims within 5% accuracy from IC Compiler. DC in topographical mode uses real RC data from the process node, and calculates the delay by adding the most common contributions to netdelay.

 

[ivlsi]

you should use .SDF file for back-annotation of your netlist

Does SDF include internal delays over the cells or just interconnect delays?

If it doesn't include internal delays over the cells, then where they are taken from?

Thank you!

- - - Updated - - -

wire load model (WLM) was used to estimate the netdelay

Using WLM, transition delays over the cells are estimated, not net-delays. Transition delays are affected by fan-outs.
Net delays are considered just after back-annotation.

- - - Updated - - -

each standard cell library contained some WLMs to choose from

Why several WLMs are required?

 

[rca]

SDF could used or not the cell delay.
But I never used SDF in backend tool, only for netlist simulation.