Power Planning Basics

Introduction

Power Planning is one of the most important stage in Physical design. Power network is being synthesized, It is used provide power to macros and standard cells within the given IR-Drop limit. Steady state IR Drop is caused by the resistance of the metal wires comprising the power distribution network. By reducing the voltage difference between local power and ground, steady-state IR Drop reduces both the speed and noise immunity of the local cells and macros.
Power planning management can be divided in two major category  first one is core cell power management and second one I/O cell power management. In  core cell power planning  power rings are formed around the core and macro.In IO cell power planning  power rings are formed for I/O cells and trunks are created between core power ring and power pads. 
In addition  trunks are also created for macros as per the power requirement.
power planning is part of  floor plan stage. In power plan, offset value for rings around the core and vertical and horizontal straps is being define
I/O cell library contains I/O cell and VDD/VSS pad cell libraries. It also contain IP libraries for reusable IP like RAM, ROM and other pre designed, standard, complex blocks.

Input Required In Power Planning

1. Database with valid floorplan 

2. power rings and power straps width

3. Spacing between VDD and VSS Straps

      

Output Of Power Planning

Design with Power Structure

Detailed Power planning Concepts

Detailed Power planning Concept

There are two types of power planning and management. They are core cell power 

management and I/O cell power management. In former one VDD and VSS power 

rings are formed around the core and macro. In addition to this straps and trunks are 

created for macros as per the power requirement. In the later one, power rings are formed 

for I/O cells and trunks are constructed between core power ring and power pads. Top to 

bottom approach is used for the power analysis of flatten design while bottom up approach 

is suitable for macros.

The power information can be obtained from the front end design. The synthesis tool 

reports static power information. Dynamic power can be calculated using Value Change 

Dump (VCD) or Switching Activity Interchange Format (SAIF) file in conjunction 

with RTL description and test bench. Exhaustive test coverage is required for 

efficient calculation of peak power. This methodology is depicted in Figure (1).

For the hierarchical design budgeting has to be carried out in front end. Power is calculated 

from each block of the design. Astro works on flattened netlist. Hence here top to bottom 

approach can be used. JupiterXT can work on hierarchical designs. Hence bottom up 

approach for power analysis can be used with JupiterXT. IR drops are not found in 

floor planning stage. In placement stage rails are get connected with power rings, straps, 

trunks. Now IR drops comes into picture and improper design of power can lead to large 

IR drops and core may not get sufficient power.

Figure (1) Power Planning methodology

Below are the calculations for flattened design of the SAMM. Only static power reported by 

the Synthesis tool (Design Compiler) is used instead of dynamic power.

• The number of the core power pad required for each side of the chip

= total core power / [number of side*core voltage*maximum allowable current for a I/O pad]

= 236.2068mW/ [4 * 1.08 V * 24mA] (Considering design SAMM)

= 2.278

~ 2

Therefore for each side of the chip 2 power pads (2 VDD and 2 VSS) are added.

• Total dynamic core current (mA)

= total dynamic core power / core voltage

= 236.2068mW / 1.08V

= 218.71 mA

• Core PG ring width

= (Total dynamic core current)/ (No. of sides * maximum current density of the metal layer 

used (Jmax) for PG ring)=218.71 mA/(4*49.5 mA/µm)~1.1 µm~2 µm

• Pad to core trunk width (µm)

= total dynamic core current / number of sides * J_max where Jmax is the maximum current 

density of metal layer used

= 218.71 mA / [4 * 49.5 mA/µm]

= 1.104596 µm

Hence pad to trunk width is kept as 2µm.

Using below mentioned equations we can calculate vertical and horizontal strap width and 

required number of straps for each macro.

• Block current:

I_block= P_block / V_ddcore

• Current supply from each side of the block:

I_top=I_bottom= { I_block *[W_block / (W_block +H_block)] }/2

I_left=I_right= { I_block *[H_block / (W_block +H_block)] }/2

• Power strap width based on EM:

W_{strap_vertical} =I_top / J_metal

W_{strap_horizontal} =I_left / J_metal

• Power strap width based on IR:

W_{strap_vertical} >= [ I_top * R_oe * H_block ] / 0.1 * VDD

W_{strap_horizontal} >= [ I_left * R_oe * W_block ] / 0.1 * VDD

• Refresh width:

W_{refresh_vertical} =3 * routing pitch +minimum width of metal (M4)

W_{refresh_horizontal} =3 * routing pitch +minimum width of metal (M3)

• Refresh number

N_{refresh_vertical} = max (W_{strap_vertical} ) / W_{refresh_vertical}

N_{refresh_horizontal} = max (W_{strap_horizontal} ) / W_{refresh_horizontal}

• Refresh spacing

S_{refresh_vertical} = Wblock / N_{refresh_vertical}

S_{refresh_horizontal} = Hblock / N_{refresh_horizontal}

                         

Other Topics Related to Power Plan

1. IR Drop