Hands-On Power Analysis with OpenSTA — A Comprehensive Guide

6 min readSep 15, 2024

In modern VLSI (Very Large Scale Integration) design, power consumption is one of the key metrics of interest for efficient IC (Integrated Circuit) design. This blog post provides a detailed guide on power analysis using OpenSTA, a widely used open-source tool for static timing analysis (STA). We will walk through the basic concepts of power consumption in digital circuits and use OpenSTA to calculate dynamic, switching, and leakage power.

Setting Up the Environment

To get started with power analysis using OpenSTA, the following prerequisites are essential:

OpenSTA: For those unfamiliar with OpenSTA, please refer to A Step-by-Step Guide to Installing and Running OpenSTA in a Docker Environment for Static Timing Analysis Tutorial for installation instructions.

Required Files:

Design file: test.v
OpenSTA script: test.tcl
SDC file: test.sdc
Technology library: NangateOpenCellLibrary_typical.lib

All these files are available at Understanding Delay Calculation and Static Timing Analysis Using OpenSTA: A Comprehensive Tutorial

Understanding Power Models

Before diving into the hands-on exercise, it’s important to understand the power consumption components in digital circuits. Power can generally be divided into three categories:

Dynamic Power (P_dyn): This is the power consumed due to switching activity. The key components include load capacitance (C_L), supply voltage (V_DD), and the clock frequency (f_clk).

The dynamic power is given by:

Where:

CL = Load capacitance
VDD = Supply voltage
α = Switching activity
fclk = Clock frequency

2. Switching Power: Power consumed due to signal transitions.

Switching power is calculated as:

3. Leakage Power (P_leak): Even when no switching occurs, leakage current causes power dissipation. Leakage power is determined using the technology library.

Running Power Analysis with OpenSTA

1. Dynamic Power Computation

Let’s compute the internal power consumed during a signal transition. The given design and SDC files will help you simulate the power using OpenSTA. Start by calculating the energy dissipated in a single transition:

The fall and rise transitions are in a library file; let us assume we are given different energy values based on load capacitance and rise/fall times.

For the fall transition (with C=0.1fF):

Tr=0.1ps, energy is 1 fJ
Tr=100ps, energy is 2 fJ

For the rise transition (with C=0.1fF):

Tr=0.1ps, energy is 2 fJ
Tr=100ps, energy is 4 fJ

The average energy consumed per transition is calculated by taking the average of the fall and rise values for C = 0.1 Ff and Transition (Tr) = 0.1ps :

Clock Period and Frequency:

Assume the clock period is 1000 ps, which is 1×10^−9 seconds.

The number of clock cycles per second (fclk) is calculated as the reciprocal of the clock period:

Switching Activity (α):

The switching activity (α) is the number of transitions per clock cycle. Here, it is given as 0.1, meaning there are transitions happening during 10% of the clock cycles.

Internal Power Calculation:

The internal power is calculated by multiplying the energy per transition with the number of transitions per second:

Pdyn=Energy per transition×Number of transitions per second

2. Switching Power Computation

Now let’s calculate the switching power. The energy dissipated per transition is:

Given the number of transitions per second, the switching power becomes:

3. Leakage Power Computation

Leakage power can be extracted from the library file.

Interpreting the Results

Once the analysis is complete, you will get the total power consumption by summing the dynamic, switching, and leakage power. Here’s how the total power is formulated:

Using OpenSTA’s output, you’ll be able to pinpoint where most of the power dissipation occurs and optimize your design accordingly.

Understanding the Output

The table in the output is divided into several categories, each contributing to the total power consumption:

Group: Represents different sections of the circuit:
Sequential: Refers to storage elements such as flip-flops and latches.
Combinational: Represents the combinational logic gates (AND, OR, etc.) in the circuit.
Clock: Typically shows the power consumed by the clock network, including clock buffers.
Macro: Represents any larger macro components in the design.
Pad: Refers to I/O pad cells that connect the internal logic to the external pins.

Each category is further broken down into Internal Power, Switching Power, Leakage Power, and Total Power.

Key Insights from the Output

Combinational Logic Dominates Power Consumption: In this particular analysis, we can see that the combinational logic is the only contributor to power consumption. It accounts for:

Internal Power: 1.50e-07 Watts, which constitutes 96.7% of the total power.
Switching Power: 5.00e-09 Watts, accounting for 3.2%.
Leakage Power: 1.50e-10 Watts, which is a minimal 0.1%.

The total power consumed by the combinational logic amounts to 1.55e-07 Watts, which is 100% of the overall design power. This indicates that there is no sequential or clock-driven power consumption, and the entire power is being dissipated within the combinational portion of the design.

Switching Power is Minimal: Switching power, which accounts for signal transitions between logic levels, is very small in this design, representing only 3.2% of the total power. This could imply that the design is relatively low on activity or that the clock rate is low, which would reduce the frequency of transitions.
Leakage Power is Insignificant: Leakage power, which is a result of static current flows even when the circuit is not switching, accounts for only 0.1% of the total power. This is a good indicator of a well-optimized design, as high leakage power can significantly reduce the efficiency of a circuit, especially in low-power applications.

Total Power Breakdown

Here is the summary of the power distribution:

Internal Power: 1.50e-07 Watts (96.7%)
Switching Power: 5.00e-09 Watts (3.2%)
Leakage Power: 1.50e-10 Watts (0.1%)
Total Power: 1.55e-07 Watts (100%)

The majority of the power is being consumed internally within the combinational logic, with very minimal leakage and switching activity. This suggests that most of the energy is dissipated due to the intrinsic operation of logic gates, rather than through switching or static leakage currents.

Optimizing Power Consumption

Given that the majority of the power is coming from internal consumption, there are several ways to optimize the design:

Reduce Internal Power:

Minimize capacitance within the logic gates.
Use power gating techniques to turn off idle blocks.
Opt for low-power libraries that reduce the inherent power consumption of gates.

2. Minimize Switching Power:

Reduce signal activity by optimizing logic paths.
Lower the clock frequency if applicable.
Use clock gating techniques to disable clocks in inactive sections of the design.

3. Tackle Leakage Power:

Use libraries that incorporate low-leakage transistors.
Employ sleep transistors to cut off power to idle blocks.

Power analysis using OpenSTA is a critical step in VLSI design. By breaking down the power into its individual components — dynamic, switching, and leakage — you can gain insights into how different factors like load capacitance and switching activity affect overall power consumption. The hands-on tutorial presented in this post should help you get comfortable with the power analysis process and enhance your designs for efficiency.