UMass Amherst Department of Computer Science
CMPSCI 291E: Electronic Identification Lab
Instructor: Kevin Fu
Handout 4
October 9, 2007
Lab 2
• Deadline: Please return the completed handout to the lab staff before the class on Monday,
October 15th.
1
Introduction
Power constraint is the most important issue in RFID tag development. Passive RFID tags have
limited computation due to the amount of power they receive from the reader. The Wireless
Identification and Sensing Platform (WISP) is a battery-less embedded computing platform for
perpetual sensing and computing developed by Intel Research Seattle. WISP enables battery-less,
wireless sensing and is powered and read by standards-compliant RFID reader. The WISP works by
being in power save mode while it harvests enough power to do computation. When it is powering
on it resets itself, then when it reaches 3.3 Volts it wakes up from power save mode and begins
computation and generates the packet. Finally, it transmits and receives data to and from the
reader. In this lab you will be using an oscilloscope to measure the voltage on the WISP on two
different pins at the same time. You will be observing, analyzing and understanding the waveforms
on the oscilloscope. One waveform will be the power of the WISP, and the other should be activity
on the WISP.
2
Reading
1. WISP Tutorial
WARNING:Please have at least 24 inches between you and the UHF antenna.
3
WISP on Power Up
3.1
Instructions
Locate the ground (GND) and Voltage Out (Vout) pins on the WISP and hook up the probe to
these pins respectively. You will be looking for a waveform that shows you the WISP powering on
from zero volts. In order to achieve this, the WISP must have no power initially. You are looking
for a waveform that shows the voltage stepping up.
1. Setting the oscilloscope
Open the channel one menu by pressing the yellow CH 1 MENU button. This will create
new options next to the soft buttons located on the right of the screen. A soft button can
toggle the option that appears to the left of each button.
Make sure that you have the settings mentioned in the Table 1.
Now open the trigger menu by pressing the grey TRIG MENU button on the right side of
the controls. Verify the settings in Table 2.
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Handout 4: Lab 2
Coupling:
BW Limit:
Volts/Div:
Probe:
Invert:
DC
Off
Coarse
10x, Voltage
Off
Table 1: The parameters that should be set for the Oscilloscope
Type
Source
Slope
Mode
Coupling
Edge
CH1
Rising
Auto
AC
Table 2: The settings should be verified after opening the trigger menu.
Now set the Volts/Div of channel one by turning the knob under the CH 1 MENU button.
The actual value is displayed on the bottom of the screen on the left side. Set this value to
500 mV. Set the Sec/Div to 500ms using the Sec/Div knob.
Use the vertical position knob that is right above the CH 1 MENU button to lower the
position of the starting value so it is on the bottom of the screen instead of the top.
At this point you are ready to capture the WISP’s voltage. Take the first measurements with
the WISP very close to the reader. Wait until the moving line gets to the end (right) of the
screen and start the reader. This will result in a trace across the screen, stop this trace when
it almost comes back to the right edge of the screen by pushing the RUN / STOP button.
The trace that you should get is a rough version of the trace in figure 1. Take the measurements below by zooming in on the waveform with the Volts/Div knob and adjusting the part
of the waveform that is shown on the screen with the horizontal and vertical position knobs.
2. Graph the trace of the voltage.
(a) The voltage on the WISP goes up in a line that looks like a saw blade. The rising
part is the when the WISP is harvesting energy while the flat part is when the CPU is
functioning. Measure the distance on the x-axis of one of a “saw tooth” on the line from
Handout 4: Lab 2
3
where one begins to where the next one starts.
(b) Measure the Y-axis distance of a “saw-tooth”.
(c) Initially the ripples are all the same size but after about one second there is a sharp
ripple that slopes downward. Measure the y-axis and x-axis values at which this ripple
occurs.
(d) After some time, the voltage on the WISP stops rising. Measure the voltage the WISP
reaches and how long it takes.
(e) Increase the distance between the WISP and the antenna and repeat the steps to capture
the trace. Repeat the only the measurement in part 1c above, that is find when the WISP
reaches maximum voltage and how long that takes. Record the increase in distance.
Checkoff: Be sure to get a checkoff before you go to the next step.
4
Handout 4: Lab 2
3.2
Questions
1. Determine how many volts per second the WISP charges by dividing the y-axis value by the
x-axis value from measurement 3 above.
2. What is the WISP doing when the ripple described in measurement 1c occurs?
3. If you rerun the experiment, the results will be consistently a “saw-tooth” line that runs into
a sharp ripple. Why does this happen every time? That is, why doesn’t the voltage fall at a
different point?
4. How much did the WISP loose in maximum voltage when it was moved farther away from
the reader?
5. Explain why the WISP has a “saw-tooth” shape when it charges instead of rising in a straight
line. Hint: Use your measurement from 1a and think about how the WISP is getting power
in the first place.
Handout 4: Lab 2
4
5
WISP Activity
4.1
Instructions
Press the RUN / STOP button one more time to start a new trace. Open the channel 1 menu and
change the coupling to AC. Change the Volts/Div to 100mV and keep the Sec/Div at 500ms. Also
raise the starting value to the middle instead of the bottom of the screen. Start the reader and
wait a few seconds for the waveform to become periodic, then capture a full screen of the waveform
by stopping the trace as it gets to the right side of the screen. You should get a repetition of three
upward spikes and one larger downward spike.
4.2
Measurements
1. Graph one period of the voltage.
2. Measure the y-axis distance of the downward spike. The zero line is shown by a yellow arrow
on the right of the screen. Measure the distance from that arrow down to where the low spike
ends.
3. Measure the y-axis distance of an upward spike in the same way.
4. Measure the distance between downward spikes on the x-axis.
5. Repeat the screen capture with the WISP as far away from the reader as possible (while
the reader can still read the WISP). Measure the distance between downward spikes on the
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Handout 4: Lab 2
x-axis.
4.3
Questions
1. What is happening at every downward spike?
2. Roughly how many times per second is the WISP read by the reader?
3. From what you learned in previous labs, you know that the signal strength is weaker as
you move away from the reader. Can you tell that the WISP is farther from the reader by
comparing measurements 2c and 2d? Can you explain why or why not?
4. Clearly at every downward spike, voltage is being reduced by the y-axis distance of the spike;
likewise, the voltage is increased in the same way by an upward spike. According to your
measurements, do the voltage values of the upward and downward spikes match? If they do
not match can you explain why the voltage still stays constant?
Handout 4: Lab 2
5
7
WISP Holding Charge
5.1
Instructions
Switch the coupling back to DC, change the Volts/Div to 1V, the Sec/Div to 5s and bring the zero
level down to the bottom of the screen.
Enable a trace by toggling the RUN/STOP button and start the reader. Let the WISP charge all
the way and then stop the reader. Capture the falling voltage on screen by toggling the RUN/STOP
button. The voltage on capacitor decays exponentially as you can see in the trace, however we can
approximate the first third of the voltage drop by measuring it as a line.
Press the RUN / STOP button one more time to start a new trace. Open the channel 1 menu
and change the coupling to AC. Change the Volts/Div to 100mV and keep the Sec/Div at 500ms.
Also raise the starting value to the middle instead of the bottom of the screen. Start the reader and
wait a few seconds for the waveform to become periodic, then capture a full screen of the waveform
by stopping the trace as it gets to the right side of the screen. You should get a repetition of three
upward spikes and one larger downward spike.
5.2
Questions
1. How many volts/second are lost in the first third of the decay? How does this compare to
volts/sec gained by WISP from part 1?
6
Maximum Read Range
6.1
Instructions
Disconnect the WISP from the oscilloscope and determine how far away from the reader you can
successfully read the WISP.
6.2
Questions
1. What is the range of the WISP (i.e. how far can the WISP still communicate with the
reader)?
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Handout 4: Lab 2
2. Think about the result in part 3. How is that related to maximum read range?
3. This Alien Reader used employs the EPC Class 1 protocol. Discuss three of the main differences between EPC Class 1 and EPC Gen 2 standards.