14.4 Calculating Power

Use the following steps to calculate the power consumption of your design. The list of screens and steps appear in the order in which you should view them to analyze your power accurately.

1. Summary tab: view global power at the design level and view its impact on junction temperature.
2. Domains tab: define clock domains and specify a clock frequency and a data frequency for each clock domain.
3. Analysis tab (optional): view detailed hierarchical analysis of your power consumption. If your power consumption exceeds your budget, this step helps you understand where there is room for improvement.
4. Frequencies tab (optional): specify individual pin frequencies. This step provides pin-by-pin control of the frequency.
5. Probabilities tab (optional): specify probabilities. You can use the default probabilities or set your own.

1. Set SmartPower in toggle-rate mode. This is the default setting. To confirm that SmartPower is in toggle-rate mode, choose Preferences from the File menu. Select the Use Toggle Rates check box.
2. In the Domains tab, set all the Clock Frequencies to zero except for the clock domain for which you are calculating the power consumption.

SmartPower allows you to estimate the power consumption related to I/Os. You can then analyze power consumption in detail.

1. In the Summary tab, select By Type from the Breakdown drop-down list. The grid displays the total power consumption of all I/Os in the design, as well as all other components. If you use different I/O standards, and different Vcci power rails, you can also display the power consumption for each rail by selecting By Rail from the Breakdown drop-down list. Breakdown By Rail shows the power consumption and current for each Vcci rail in the design.
2. In the Analysis tab, select the highest hierarchy level in the Hierarchy tree in the left pane. Select By Instance from the Breakdown drop-down list.
3. In the Analysis tab, unselect all categories under Instances Contributions except I/Os, and then click Apply. The table then displays a list of all I/Os in the design, along with the power contribution of each individual I/O. The columns display important I/O attributes: external port name, load, standard, drive- strength, slew, and macro.
4. In the Probabilities tab, you can control the output probability of each tri-state and bidirectional I/O. The probability is the percentage of time that the I/O is used as an output. The default value can be changed and a specific value can also be set for each bidirectional or tri-state I/O.

When you run SmartPower, it researches your existing clock domains and partitions your design automatically.

1. Click the Domains tab, and then click the Add Domain button.
2. Select Clock Domain from the drop-down list.
   The Create Clock Domain dialog box appears.

   

3. To create a new clock, select a Potential Clock Pin, specify a clock and data frequency and click Create.
   The new clock domain appears in the Domains window. If you select an existing clock pin from the drop- down list, the lists of clock pins and data pins of this new clock domain are computed automatically based on the netlist topology.

   Note: Select Use Toggle Rates in the SmartPower Preferences to define your data frequency as a percentage of your clock frequency. If your data frequency is 20% of your clock frequency, type 20 in the Data Frequency text box.

   

   If you want to create an empty clock domain and fill the lists of clock-pins and data pins manually, do not select a clock pin. Instead, type a new name for your clock domain.

Beyond the verification of the list of clock domains, you may also wish to verify that the lists of clock pins and data pins computed for each clock domain are correct.

1. Click the Domains tab and select a specific Domain in the list.
2. Display the list of clock pins or data pins of this Domain. A drop-down list in the Domains tab allows you to select clock pins or data pins. SmartPower displays the list of pins corresponding to your selection below the drop-down list. You can add or remove clock pins and data pins as necessary.
3. Remove a pin from a clock domain. Highlight the selected pin and click the Remove button. The pin is removed from the clock domain and is made available in the list of pins that you can add in another clock domain.
4. Highlight the selected pin in the list of pins that are not yet in a domain and click the Add button to add a pin in a clock domain.
   This pin is added to the clock domain. The pin is a clock or data pin, depending on the specification of the drop-down list when you clicked the Add button.

   Note: You cannot add a pin that exists in another domain until you free it from its existing domain. The pin is unavailable until you remove it from that domain.

1. Open the Create Clock Domain dialog box.
2. Click the Domains tab and click the Add Domain button.
3. Select Set of Pins from the drop-down list.
4. Create a Set of Pins. Name your new set of pins, specify a data frequency, and click Create

   

   The new set of pins appears in the Domains window.

1. Click the Domains tab.
2. Select the Clock/Data frequency cell and enter a new value.

   SmartPower defaults to 0 MHz for the clock frequency. Import your clock frequency from SmartTime by using the Initialize Frequencies dialog box in SmartPower or input your target for each clock frequency in the Domains tab. You must also specify a data frequency (5% of your clock frequency is a typical guideline for your data frequency—this corresponds to a toggle rate of 10%).

   Not all the pins/gates/nets of your design are associated with a specific clock. For example, the frequency of a design input port is not always correlated to a clock frequency. By extension, all pins that are upstream of the first level of sequential elements are not associated with any clock. SmartPower creates an InputSet by default that it uses to group all the pins that are controlled by design inputs (instead of sequential elements). You may want to view and verify the InputSet to further evaluate your design.

1. Click the Domains tab and select the domain named InputSet in the list.
2. Verify the list of pins of this domain. All the input ports of your design (except the clocks) belong in the InputSet. All pins between these input ports and the first level of sequential elements belong in the InputSet. You can add or remove pins as necessary.
3. Specify an average input frequency. SmartPower uses the same frequency for all pins of the InputSet. The default InputSet frequency is 1 MHz. Enter a new value to change it.

You might want to split the InputSet into several sets in order to specify different frequencies. For example, you might want to create a ResetSet—a reset tree with a very low frequency.

1. Create a new set of pins: In the Domains tab, click the New button, and select Set of Pins from the drop- down menu.
2. In the Create Set of Pins dialog box, type a name and a frequency for the new set and click Create. The new set of pins appears in the Domains window. You can only create an empty set of pins, but it is possible to add pins to this domain later.
3. Remove a group of pins from the InputSet. Click the Domains tab and select the domain named InputSet in the list.
4. Select the pins that you want to remove and click the Remove button.

The Frequencies tab allows you to specify an average clock and data frequency for each clock domain, and also an average frequency for each set of pins. This gives you an initial estimate of the power consumption of your design. However, if this estimate is not sufficiently accurate, you can refine it with a pin-by-pin annotation of the frequency.

1. Locate the pin in the Frequencies tab. You may need to select different clock domains from the drop-down list on the Frequencies tab. You can use filters to facilitate this search.
2. Select the pin(s) in the grid and click the Set frequencies for selected pins button.
3. Enter a new frequency value and click OK. This specifies a new frequency for the selected pin(s).

1. Locate the pin in the Frequencies tab. You may need to select different clock domains from the drop-down list on the Frequencies tab. You can use filters to facilitate the search.
2. Select the pin(s) in the grid and click the Set frequency for selected pins button.
3. Enter a new frequency value and click OK. This specifies a new frequency for the selected pin(s).

1. Locate the pin in the Frequencies tab. You may need to select different clock domains from the drop-down menu on the Frequencies tab. You can use filters to facilitate the search.
2. Select the pin(s) in the grid and click the Reset frequencies for selected pins button. This removes the specified frequency from the selected pins.

The Probabilities tab allows you to specify the default output probability value for memory blocks, and tristate and bidirectional I/Os. In addition, you can increase the accuracy of the power estimation by annotating the probability of specific pins.

1. Locate the pin(s) in the Probabilities tab. You can use filters to facilitate this search.
2. Select the pin(s) and click the Set probabilities for selected pins button.
3. Enter a new probability value and click OK.

Click the Summary tab to view global power consumption at the design level. The Summary tab shows your design's estimated power consumption and temperature information.

1. Verify your package. You cannot change your package directly in SmartPower, because it may render your place-and-route information (and thus it may severely impact the total power consumption). To choose another package, use Designer > Tools > Device Selection.
2. Click the Summary tab and select a Cooling style in the list. Thermal resistance changes automatically when you update the cooling style.
3. Specify an ambient temperature. Enter an ambient temperature (default value is 25ºC) and click the Set button.
   Note: The junction temperature value changes according to the package, cooling style, and ambient temperature values you choose.

The Analysis tab shows the estimated power consumption of individual blocks, gates, nets, I/Os, and memory, and allows you to make a hierarchical analysis of your power consumption.

1. Use the Analysis tab to expand the design hierarchy. The Analysis tab allows you to expand your design hierarchy and view a complete list of the blocks in your design. In the hierarchy window, click the + next to your design to view the items in the hierarchy. Click the + next to a sub-block to view its sub-elements.

   

2. Click to select a block. By default, SmartPower selects the design-level block, but you can always select another block in the hierarchical tree. The Report window displays the list of sub-elements of the selected block. Initially, this list includes all sub-elements. The grid in the Analysis window displays a breakdown of the power of the selected block by type. This breakdown provides you a good overview of which areas of your design you should optimize for power.
3. To find the components that use a significant amount of power, choose By Instance from the Breakdown drop-down list.
4. Sort and filter the sub-elements to find the component that is using the most power. The Analysis window displays the list of sub-elements of the selected block. By default, this list includes all sub-elements. The grid contains columns for name, type, power, driver, fanout, macro, I/O standard, output load, output drive, slew, port, domain and frequency. You can limit the list of sub-elements to a list of sub-blocks, nets, gates, I/Os, memories, or any combination of these five classes of sub-elements. You can sort the list according to any column by double-clicking the column header.