Fanout_Load value of an input in Synopsys DC

[giggs11]

hi all,

can anyone explain to me how to calculate or determine the fanout_load value for a pin of a std cell library core cell; and the drive strength value of an output?

I know that fanout_load value is important in determining whether the maximum_fanout of an output has been violated. However, how do we determine the most suitable max_fanout value or limit for an output. Can it be calculated...?

As for the drive strength....I knoe drive strength means the total nodal capacitance that can be discharged/charged or driven by an output. But how do we calculate drive strength value...?

Thanks in advance.

 

[niuniu]

every input pin in the library has a load information. add all that connect to one output pin, and you get the fanout_load. set max_fanout_load based on experience, or just use the default value in lib.

drive strengh is a parameter of the output, you do not need to calculate it.

 

[quake]

I think you can discard the fanout_load, since it does not need that much attention.
Inside your circuit the compiler can caculate it and deal with it according to the value from the std_cells, for the ports of the design, just estimate.

 

[giggs11]

hi niuniu,

you mentioned that drive strength is a parameter. What is the unit for the parameter...? Is it the total capacitance that can be driven by the output...?

 

[haosg]

get_attribute fanout_load
drive_of

 

[cnspy]

it is a rate for a standard cell.

 

[mark_yeh]

you can calcute the fanout load by yourself based on the library information. you can specify a big fanout_load value to save area. but it must meet the timing constrain. if you wanna get high speed performance, the fannout_load value should be small.

 

[tukken]

you could find these information in vendor lib