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likewise stepping, process and power mode, if applicable. All of these parameters will have a considerable impact on the overall power required—especially the process, which has the settings of “maximum” or “typical.” The typical setting will give you the power your application would statistically see, while the maximum will give the worst case. It is important to ensure the power solution can handle the maximum case, but this can become challenging with larger devices that have higher current requirements. You can also define the operating environment here as well, with the ambient temperature, heat sinking and airflow. Defining the maximum ambient temperature at this point will provide a more accurate power estimation, because the power required will increase as the device junction temperature rises. Including heat sinking, airflow or both will therefore lower your power estimation. Also at this stage, don’t overlook the Xilinx ISE® optimization setting. This setting will also have an impact on the power estimation, as different optimization schemes—for example, timing performance against area minimization—will result in varying implementations, each with its own resource usage and fan-out. They too will affect the power estimation. The next stage of the estimation is to go through the tabs at the bottom of the XPE spreadsheet and fill in details on your proposed solution as accurately as possible. To ensure the most accurate estimation at an early stage, it is very important to define at least the resource usage, clock frequencies, toggle rate and enable rate. It is also a good idea to include a level of contingency to address the ever-present requirements creep. This process will result in XPE providing an overall power and junction temperature estimation on the summary tab, like the one in Figure 1. 48
Xcell Journal
Figure 1 – Summary of the XPE device settings and power estimation
Figure 2 – The warning issued when your power supply voltage is outside of acceptable tolerance
Once your estimation is complete, you can, if you wish, export the settings from XPE for use later within the Xilinx Power Analyzer (XPA) by using the “export file” option on the summary page. This ensures the settings in XPA are aligned with those you initially used for the estimate. As the development progresses through RTL production, trail synthesis and place-and-route, it is important to keep the estimation up to date as more-accurate information becomes available. Remember also to keep the hardware team—especially the group
responsible for power engineering— informed of any change in the estimation. The power engineers should be able to provide worst-case maximum voltages for the supply rails, which can further improve the accuracy of your estimation. A higher worst-case maximum supply voltage will increase the power dissipation. XPE is intelligent. It will color the voltage cells on your spreadsheet orange and issue a warning if the worst-case maximum supply voltage is outside of the acceptable tolerance for the device, as shown in Figure 2. Second Quarter 2013