IR Drop Analysis in Physical Design | IR Analysis in VLSI

In this article, we will discuss what is IR drop in ASIC design, Why IR drop issue occurs, what are the effects of IR drop and how to analyze and prevent the IR drop issue.

What is IR drop issue:

The power supply (VDD and VSS) in a chip is uniformly distributed through the metal rails and stripes which is called Power Delivery Network (PDN) or power grid. Each metal layers used in PDN has finite resistivity. When current flow through the power delivery network, a part of the applied voltage will be dropped in PDN as per the Ohm’s law. The amount of voltage drop will be V = I.R, which is called the IR drop. Figure-1 shows the IR drop in the Power net. Any metal net can be assumed as a combination of small R and C. 

Figure-1: IR drop in metal net

If the resistivity of metal wire is high or the amount of current following through the power net is high, A significant amount of voltage may be dropped in the power delivery network which will cause a lesser amount of voltage available to the standard cells than the actual amount of voltage applied. 

Standard cells or macros sometimes do not get the minimum operating voltage which is required to operate them due to IR drop in power delivery network even the application of sufficient voltage in the power port. Voltage drop in the power delivery network before reaching the standard cells is called IR drop.

This drop may cause the poor performance of the chip due to the increase of delay of standard cells and may cause the functional failure of the chip due to setup/hold timing violation. To avoid this issue, IR analysis must be done and consider its effect in timing analysis in the design cycle. 

Types of IR drop:

There are two types of IR drop in the ASIC design:

1. Static IR drop

2. Dynamic IR drop

Static IR drop is the voltage drop in the power delivery network (PDN) when there are no inputs switching means the circuit is in the static stage. Whereas dynamic IR drop is the voltage drop in the power delivery network when the inputs are continuously switching means the circuit is in a functional state. Dynamic IR drop will depend on the switching rate of instance.

When the inputs are switching continuously, more current would flow in the instances and also in PDN. So there will be more IR drop in the PDN. Therefore dynamic IR drop is more than the static IR drop.

Reasons for IR drop:

IR drop could occur due to various reasons but some main reasons are as bellow.

• Poor design of power delivery network (lesser metal width and more separation in the power stripes)
• inadequate via in power delivery network 
• Inadequate number of decap cells availability
• High cell density and high switching in a particular region
• High impedance of the power delivery network
• Rush current 
• Insufficient number of voltage sources 
• High RC value of the metal layer used to create the power delivery network

Effects of IR drop:

Delay of standard cells depends on the available power supply to the cell and if the power supply decreases the delay of cell increases. The increase in delay of a cell could affect the performance of the design. It is also possible that if the available voltage to a standard cells gets below a particular level, then the cell may stop function completely and could result in functional failure of the design. Or sometimes the IR drop is within the limit and only delay of cells get increased which affects the setup and hold timing of design and sometimes it causes failure of setup and hold timing. 

A sudden drop in the VDD line is also possible if the demand of current gets increased suddenly due to a large number of switching activities in a particular area of design. Such type of drop in VDD level is called voltage droop. Or it may cause sudden raise the level of ground voltage, which is called ground bounce. These are collectively called power noise. figure-2 shows the power noise due to IR drop.

Figure-2: power noise due to the IR drop

In short IR drop could result 

• Change in the delay of cells
• Could violate the setup and hold timing 
• Introduction of power noise in power supply nets

IR analysis and fixes:

Every EDA companies have their own IR analysis tool which performs the IR analysis and based on the analysis the techniques for the IR fixes are applied. Two most popular tools for IR analysis used in industry are:

• RedHawk of Ansys
• Voltus of Cadence Design System

Based on the analysis there are various techniques to fix the IR drop are applied. Some of the fixes which generally performed are:

• Insertion of the sufficient number of de-cap cells which will boost the power delivery network. 
• Reconstruct the power delivery network if it is not built properly. We could increase the width of the metal stripes or could decrease the separations between them
• We could spread the logic cells in a region so that the load can be distributed

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