Using XPower

This section describes the settings necessary to obtain accurate power and thermal estimates, and the methods that XPower allows. This section refers specifically to FPGA designs. For CPLD designs, please see Xilinx Application Note XAPP360 at http://www.xilinx.com/support.

VCD Data Entry

The recommended XPower flow uses a VCD file generated from post PAR simulation. To generate a VCD file, you must have a Xilinx supported simulator. See the Synthesis and Verification Design Guide for more information.

XPower supports the following simulators:

ISIM
ModelSim
Cadence Verilog XL
Cadence NC-Verilog
Cadence NC-VHDL
Cadence NC-SIM
Synopsys VCS
Synopsys Scirocco

XPower uses the VCD file to set toggle rates and frequencies of all the signals in the design. Manually set the following:

Voltage (if different from the recommended databook values)
Ambient temperature (default = 25 degrees C)
Output loading (capacitance and current due to resistive elements)

For the first XPower run, voltage and ambient temperature can be applied from the PCF, provided temperature and voltage constraints have been set.

Xilinx recommends creating a settings file (XML). A settings file saves time if the design is reloaded into XPower. All settings (voltage, temperature, frequencies, and output loading) are stored in the settings file. See the “-wx (Write XML File)” section of this chapter for more information.

Other Methods of Data Entry

All asynchronous signals are set using an absolute frequency in MHz. All synchronous signals are set using activity rates.

An activity rate is a percentage between 0 and 100. It refers to how often the output of a registered element changes with respect to the active edges of the clock. For example, a 100MHz clock going to a flip flop with a 100% activity rate has an output frequency of 50MHz.

When using other methods of design entry, you must set the following:

Voltage (if different from the recommended databook values)
Ambient temperature (default = 25 degrees C)
Output loading (capacitance and current due to resistive elements)
Frequency of all input signals
Activity rates for all synchronous signals

If you do not set activity rates, XPower assumes 0% for all synchronous nets. The frequency of input signals is assumed to be 0MHz. The default ambient temperature is 25 degrees C. The default voltage is the recommended operating voltage for the device.

Note: The accuracy of the power and thermal estimates is compromised if you do not set all of the above mentioned signals. At a minimum, you should set high power consuming nets, such as clock nets, clock enables, and other fast or heavily loaded signals and output nets.