What delays are calculated using WLM model?

[ivlsi]

Hi All,

What delays are calculated using WLM model - Transition or Propagation?

Thank you

 

[sharath666]

Re: WLM: Transition or Propagation Delay

Propagation

 

[RCircuit]

Re: WLM: Transition or Propagation Delay

You need input transition to look up the propagation, am I right? So transition should be calculated as well.

 

[ivlsi]

Re: WLM: Transition or Propagation Delay

So, how transition delay is calculated by tools? WLM? NLDM? etc?

 

[Nanda San]

Re: WLM: Transition or Propagation Delay

NLDM models are dumb with respect to timing. Mainly used in crude synthesis .
use should use ccs models for timing instead.

Wire load model will give you the net delays and gate delays from which the tool would derive the stage delays...

Transition delay or slew is the time taken by a signal to rise from 10 % to 90 %( 80%) of its maximum value more specifically the rise time

Propagation delay on the other hand would depend on the input transition time or slew rate and the output load

 

[ivlsi]

Re: WLM: Transition or Propagation Delay

So, where the tools take the transition delays from? How do they calculate them?

 

[Nanda San]

Re: WLM: Transition or Propagation Delay

The tool shall pick up the transition delay values from the .lib model

 

[ivlsi]

Re: WLM: Transition or Propagation Delay

The tool shall pick up the transition delay values from the .lib model

And what about transition delays over the nets?

 

[pradeeppiskala]

Re: WLM: Transition or Propagation Delay

Input and output pins are usually constrained with transition value and output cap. With this as input transition of net, delay of net is calculated based on R and C (WLM) and also output transition at end of net (connecting to cell) is calculated by tool (PT /ETS). The output transition of net connecting to cell (input transition at cell input) and out cap of cell is used get cell delay from libs which has delays value in a table format for different trans vs cap.

Using WLM, both transition on output of net and delay of net is calculated

 

[ivlsi]

Re: WLM: Transition or Propagation Delay

So, do WLM include information on both propagation and transition delays? As far as I know they have a single value only for each of Length-Fanout-Resistance-Capacitace combination. Am I wrong?

 

[birdy123]

Best article for all basic and doubt... and even after that if you have any question .. Feel free to ask.

Wire Load Model

 

[ivlsi]

There in nothing regarding the transition delay there...

 

[pradeeppiskala]

Re: WLM: Transition or Propagation Delay

RC values will be taken from WLM for given length-fanout. With this RC info, tool calculates the wire delay and net propogation