WLM -> net Length & net Area LUTs -> why needed?

[ivlsi]

Hi All,

Actually WLM has 4 types of tables:
- capacitance vs fanout
- resistance vs fanout
- area vs fanout
- length vs fanout

As for the capacitance and resistance, it's understood - they are needed for the net delay calculation. But what about net Area and Length - why they are needed?

Thank you!

 

[birdy123]

Since Fanout has dependence on the length of the net - so its very important that you should know the Fanout for different length. Now when in you design the there are 2 different net of same type but the length is different - then with WLM, you can calculate the fanout of the circuit.

Similarly for Area also- Area = Length x Width -- so for different width and different length - the fanout can be different. So as a combined effect - it become Area dependent.

Let me know in case any followup questions.

 

[ivlsi]

Re: WLM -> net Length & net Area LUTs -> why needed?

birdy123, thank you for your response!

Since Fanout has dependence on the length of the net

Don't the cells have the maximum fanout definition in their cell libraries (*.lib file)?

I always thought that WLM is used in the following way:
Let's say we have the following WLM:

wire_load("WLM1")***** {************************************ 
...
fanout_length(1,* 0.002)*********
fanout_length(2,* 0.006);
...
fanout_capacitance( 1, 0.002 );
fanout_capacitance( 2, 0.004 );
...
fanout_resistance( 1, 0.01 );
fanout_resistance( 2, 0.015 );
...******* 
fanout_area(*1, 0.11 );
fanout_area( 2, 0.20 );
}

Let's say we have a NAND gate with two nets connected to its output pin. So, in order to calculate a propagation delay over the nets, the tool refers to the WLM and calculate it in the following way:
1) from 'fanout_length(2, 0.006)' it estimates a length of each such net as 0.006(nm?)
2) from 'fanout_length(2, 0.006)' and 'fanout_resistance( 2, 0.015 )' it estimates resistance of each such net as 0.006*0.015
3) from 'fanout_length(2, 0.006)' and 'fanout_capacitance( 2, 0.004 )' it estimates capacitance of each such net as 0.006*0.004
4) propagation delay over the net is R*C = (0.006*0.015)*(0.006*0.004)

But, you wrote that the fanout depends on the net area and net length that is actually a load seen by the cell... So, as for the cell's lib definitions, are they limited by a maximum load or maximum fanout or both of them?

What is not understood, how does the tool calculate the slop (transition time) over the nets.

BTW, do the vendors prepare different WLMs for different delay types (min/typ/max)?

Thank you!

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2 different net of same type

What types do exist?

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Does fanout depend on the net length only without taking into account the net width?