Guide to Power Measurement A Cadence EDA Tools Help Document Created by Casey Wallace, Spring 2006 Document Contents Introduction General Steps Static Power Dynamic and Average Power Peak Power Energy Measuring Power using Voltage and Current Introduction This document describes how to use the spectre simulator and Cadence tools to measure various power metrics for a circuit design. It is a reference, not a tutorial; therefore, you can apply these steps to any circuit (schematic or extracted layout) that you wish. General Steps These are steps common to all the types of power measurements. Before following any of these steps, find the section corresponding type of power measurement you want to make, and refer to these steps when instructed. 1. In the Analog Design Environment (ADE), set up your simulation stimulus file location, and other relevant simulation parameters that must be assigned. This includes setting up simulation for an extracted cell if necessary, as described in Tutorial C. 2. In the ADE, Select Outputs → Save All. A Save Options window will appear. See Figure 1. Figure 1: Save Options Window, configured to save all pwr waveforms Guide to Power Measurement 13. In the Save Options window, For “Select power signals to output (pwr)”, click on the appropriate checkboxes, depending on what power signals are of interest. Click OK to apply the changes. a. “Total” will save the waveform of the total power consumption of the design. b. “Devices” will save the waveforms of power for each component (e.g. transistor, resistor, etc.) c. “Subckts” will save the waveforms of power for each instance and level in a hierarchical design d. “All” will save all available power waveforms. This option uses the most memory and may slow simulation; however, for small circuits, it is not noticeable. 4. Run the simulation. If you have selected an output to be plotted, the waveform window will appear. If you don’t wish to plot any outputs, you can open the Waveform Window from ADE by selecting Tools → Waveform… 5. In the Waveform Window, select Tools → Results Browser… A Browse Project Hierarchy window will appear (see Figure 2). Click OK to accept the default path (or enter the path of previously-saved results). The Results Browser should now appear. Figure 2: The Browse Project Hierarchy window, left to default 6. At the very left side of the Results Browser, click on the name of the cell view from which you are simulating. Note that multiple cell views may appear depending on past simulations you have conducted on the cell. 7. Click PSF. In the next level of the tree that appears are the “runs” of the simulation. If you have done parametric sweep, there would be a single run corresponding to each value of the swept variable. If you have done a single transient simulation, there is only one run, which is selected for you automatically. 8. Assuming you are doing transient analysis, click tran-tran. See Figure 2. Figure 3: The Results Browser Window after selecting “tran-tran” Guide to Power Measurement 2Static Power Case 1: You are simulating from a transistor-level schematic or you are simulating from a general schematic and want to know the static power consumption of the whole design. 1. Define a stimulus file where all inputs are DC voltage sources. This ensures that no switching activity is happening in the circuit. 2. Follow “General Steps” for power measurement. 3. In the Results Browser, there should be a yellow box named “pwr”. If you want to plot it, right-click on this box. This will display the instantaneous power waveform for your circuit in the Waveform Window. 4. Since there was no switching activity in the circuit, the power waveform should be a straight line. Place your mouse cursor on the line and read the “y” value. This is the static power consumption of your circuit. See Figure 3. Figure 4: Waveform window after plotting the static power (inverter) Case 2: You are simulating from schematic that uses instances of other cells and you want to know the static power consumption of those cells. 1. Define a stimulus file that ensures that no switching activity is happening in the circuit. Depending on the circuit, you may need to disable clock signals or any part of the circuit that generates switching automatically. 2. Follow “General Steps” for power measurement. Make sure that you select either “subckts” or “all” when selecting power signals to save. 3. In the Results Browser, there should now be a branch corresponding to each instance in your top-level schematic. If you do not name the instances manually, then Cadence gives the instances names such as “I10”. If you have left the automatic instance label in the symbol for each instanced cell (i.e. [@instanceName]), then you can easily identify its red instance number right next to the instance symbol in the schematic. Find the name of the instance that you want to measure power for, or that contains the instance you want to measure power for, and left click on its branch name in the Results Browser. This should expand the tree by one level. Guide to Power Measurement 34. If you are at the desired level, right click on the yellow “pwr” box to get the power consumption of that instance and all of its constituent instances. You can descend further into the hierarchy by recursively following step 3. 5. This will display the instantaneous power waveform for your circuit in the Waveform Window. Place your mouse cursor on the line and read the “y” value to determine the static power consumption. Dynamic and Average Power In order to get an accurate measurement for dynamic and average power, first determine the appropriate range of time for which to run a transient simulation. Since the measurement involves an average of the instantaneous power value over the simulation window, choosing a transient simulation length that is too large or small can give very inaccurate measurements. Also, note that dynamic power is a function of the switching frequency. Make sure that you have chosen an appropriate input switching frequency, and that you are measuring dynamic power over an appropriate number of cycles. An arbitrary choice of switching frequency will give an arbitrary average/dynamic power measurement. Case 1: You are simulating from a transistor-level schematic or you are simulating from a
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