encounter global net connections

xialishui1987

感觉此处关于global net的讲解比较清楚  

www.edaboard.com/thread192787.html
   about global net connections
   hi,
   can anyone tell why do we need to declare Global net connects, and why do we need to add Tie off cells and tap cells?
   Thanks,
   chandra.
   Re: about global net connections
   Hi, you need to associate your various power domains to net names so that the power router connects the right stuff up.  
   Do your wells get tied off automatically in your technology?
   Re: about global net connections

global net connections are used to define the pwr/gns nets to be connected to various pwr/gns pins of std cells/macros/IO Pads etc in your design.

Tie cell :
When any signal pins in your design has 1'b1(to be conncted to pwr net) and 1'b0 (to be connected to gnd) connections then, you need to connect these signal pins to the correspinding pwr/gns nets.
But, since there is lot of noise in the pwr/gns nets due to flactuations which can degrade or damage the gate of the std cell so, the library vendor recommned to NOT connect the required siganl pis to the pwr/gnd nets dircetely and rather connect the required siganl pins to the pwr/ns nets using the Tie cell (i.e Tie high cell for 1'b1 connection and Tie low cells for 1'b0 connections).

Tap cell:
They are required to do the well conncetions of the std cells to the corresponding pwr/gnd net. They are required to be done if your library vendor has not done them for the std cell libraries provided to you.
For Low power design, These connection are required to do the well biasing which is helpful for leakage control.