Altera Cyclone III LS FPGA - Manual (Page 33)

Chapter 6: Board Test System 6–15
The Power Monitor
© October 2009 Altera Corporation Cyclone III LS FPGA Development Kit User Guide
Power Rail —Selects the power rail to measure. After selecting the desired rail, click
Reset to refresh the screen with new board readings.
f A table with the power rail is available in the Cyclone III LS FPGA
Development Board Reference Manual .
Power Information
This control displays current, maximum, and minimum power readings for the
following units:
 mVolts
 mAmps
 mWatts
Power Graph
This control displays the mWatt power consumption of your board over time. The
green line indicates the current value. The red line indicates the maximum value read
since the last reset. The yellow line indicates the minimum value read since the last
reset.
Graph Settings
These controls allow you to define the look and feel of the power graph.
Scale Select —Specifies the amount to scale the power graph. Select a smaller number
to zoom in to see finer detail. Select a larger number to zoom out to see the entire
range of recorded values.
Update Speed —Specifies how often to refresh the graph.
Reset
This control clears the graph, resets the minimum and maximum values, and restarts
the Power Monitor.
Calculating Power
The Power Monitor calculates power by measuring two different voltages with the
LT2418 A/D and applying the equation P = V × I to determine the power
consumption. The LT2418 measures the voltage after the appropriate sense resistor
(Vsense) and the voltage drop across that sense resistor (Vdif). The current (I) is
calculated by dividing the measured voltage drop across the resistor by the value of
the sense resistor (I = Vdif/R). Through substitution, the equation for calculating
power becomes P = V × I = Vsense × (Vdif/R) = (Vsense) × (Vdif) × (1/.003).
You can verify the power numbers shown in the Power Monitor with a digital
multimeter that is capable of measuring microvolts to ensure you have enough
significant digits for an accurate calculation. Measure the voltage on one side of the
resistor (the side opposite the power source) and then measure the voltage on the
other side. The first measurement is Vsense and the difference between the two
measurements is Vdif. Plug the values into the equation to determine the power
consumption.