Preparation

Before students can perform the lab, the instructor needs to prepare the device for measurements. You will need to physically modify a PDA. This isn't extremely difficult, but should be done with some care. In particular, be sure to take precautions (such as ground straps) to avoid static electricity discharges into the device.

You may modify the PDA of your choice. An older PDA is fine so long as it can run some programs. We used an HP iPaq, which is shown here with an open case.

The case can be opened using standard small screwdrivers. Once the case is open, the printed circuit board will be apparent. The main processor should be easy to locate. The exact processor to be used will, of course, vary with the model of PDA; the Intel XScale is a popular processor in WinCE PDAs. Once you have located the processor, you need to identify which pins on the processor are used for power. To identify these pins you will need to obtain the CPU data sheet from the manufacturer. XScale data sheets can be found at http://www.intel.com/design/pca/applicationsprocessors/index.htm.

You now need to insert a measurement resistor into the PDA. Ideally, you want to locate this resistor at a point between the power supply and CPU such that you measure all the current going through the CPU but no current going to any other part of the system. You will use a small resistor, such as a 1 W resistor, to measure current.You should insert the measurement resistor into the supply path using standard 19th century brain surgery techniques: cut the printed circuit board trace and solder the measurement resistor across the cut. Depending on the type of measurement resistor you will use, you may need to add a small physical support for it to the board.

Your students will need some sort of measurement device. A basic voltmeter can make rough measurements, but can't accurately measure voltage or measure transient variations in power consumption. Some voltmeters are designed to be accurate under transient conditions. You can also use an oscilloscope.

An alternative to a measurement resistor is a sensitive ammeter. The typical ammeter isn't accurate enough to measure the time-varying character of current flow, but some ammeters are designed to measure transient currents. When using an ammeter, install it across the cut in the circuit board trace.

You may also want to install other measurement resistors, such as one for the touch screen system. The most power-hungry component of a PDA is the display, so measuring display power can be instructive.

Your students will need some sort of voltage/current measurement device.

At this point you are ready for your students to use the board. Remind them not to spill coffee on the exposed PDA parts.

Pre-Lab

In this lab you will measure the power consumption of the processor in a PDA. The measurement technique is fairly simple. Your instructor has prepared a PDA that has had its connection between the power supply and the CPU cut, then installed a measurement resistor across the cut. By measuring the voltage across that resistor, you can determine the current into the processor. Knowing the power supply voltage of course lets you determine the power consumption of the processor. If you use a sensitive measuring device, you can measure not just the average power consumption but the transient power consumption.

Here are some questions to answer:

  1. Where is the CPU? memory? touch screen?
  2. Draw a schematic of the power measurement setup, including the CPU, power supply, measurement resistor, and the voltmeter.
  3. What is the maximum current drawn by the CPU? (Find this from the CPU data sheet.)
  4. What voltage drop do you expect to see across the measurement resistor?

Measurement

You will measure the PDA's power consumption in several different modes---that is, with different programs running. Within each mode, you should measure the average power consumption. You should also measure the transient power consumption over some interval.

First, measure the power consumption when no program is running.

Next, measure the power consumptoin when you start Word. How much does power consumption change?

Now measure the power consumption of some activities in Word:

  • Enter some text using handwriting recognition.
  • Enter some text using the virtual keyboard.
  • Write the file.

Switch modes from Word to Outlook. What does the PDA's transient power consumption look like?

Post-Lab

Here are some questions to answer:

  1. What was the mode in which you measured the lowest power consumption? The highest power consumption?
  2. What was the largest transient variation in power that you observed? What system activity caused this event?