Power planning

Why power planning is required?
To ensure that all the components in the chip have adequate power and ground connections.
Power network generally includes following things:
Power pad -to supply power to entire chip
power rings -run around the periphery of the die to supply power to standard cell 's core area, and individual hard macros.Rings are put in higher level routing layers so that lower level are used for signal routing.
Power rails-The horizontal wires are often referred as rails and vertical wires are called as straps.They run from end to end,crossing the entire die ,sections of the die.
Rails and straps are placed uniformly spaced array.Rails connect the power pins of the standard cell and extend to power rings,There they connect with vias.Widest trunks are put in higher level routing layers as with power rings.After the straps and trunks are inserted ,they are all well tied together using the vias and via stacks.

The uniformly spaced array of straps and rails may get modified to allow hard macro power rings , wiring keep out area and other restrictions.

Using lower level routing (typically metal1) ,rails are created only in standard cell placement areas that aren't already blocked by hard macros or wiring keep out.













There are two types of power planning and management.
1.core cell power
2.I/O cell power

In core cell power -VDD and VSS power rings are formed around the core and macro, depending on power requirement straps and trunks are created for the macros.

I/O cell power Power rings are formed around the I/O cell, and trunks are created connecting core power ring an power pad.
For Flattened design top to bottom approach is suitable..
For macros bottom to top approach is suitable.




Why power rings are made up of top metal layers?Why top metal layers should have low resistance and not lower metal layers?

R=rho*L/A;
Top metal layers have larger width than lower one.Lower metal layers are used for signal routing and to have adequate supply of power to all the components ,to decrease IR drop due to the resistance in the metal layers ,top metal layers are made wider,hence low resistance and are made up of copper .Lower metal layers are made up of aluminum.