Troubleshooting on the move
Engineers need lightweight, and compact, but powerful tools to take with them on-site to validate device functionality, identify failures, and trace the root cause by capturing waveform errors such as timing faults, crosstalk, transients, power quality issues and more.
USB oscilloscopes are small enough to fit in a jacket pocket or a laptop bag, so can be taken anywhere. But they have functionality and performance that match, or exceed, many laboratory instruments, enabling engineers and technicians to tackle these challenges – quickly and easily.
This article will review some common field-failure examples and show how the PicoScope 2200A and 5000 Series oscilloscopes can be used to capture and isolate waveform anomalies to identify source of the problems.
Design verification in the lab . . .
Product design is no easy task, but engineers in the lab do typically have access to all the basic tools they need to get the job done: design automation and simulation tools, plus an array of hardware verification tools including an oscilloscope, signal generator, logic analyser, DVM, precision power supply and the rest. On top of that there is generally a design team – if the FPGA engineer is struggling with a memory interface problem he or she can tap the shoulder of the hardware design manager and they can consult other team members to examine and resolve the issue together.
. . . and in the field.
If design problems show up once the product is deployed, the dynamics are quite different. First, the customer is likely not best pleased and will expect prompt action to resolve the issue. If the product is being used in a manufacturing process the economic consequences of field failure can be very significant, so time really is of the essence. Worse still, if the product is being used in a safety-critical or medical application, people’s lives can be put at risk. The pressure to find the root cause of a design problem and rectify it is enormous.
Pity, then, the unlucky design engineer who is plucked from the warmth of the design lab and sent long distance to find and solve an obscure design flaw that didn’t show up during the development process. She or he will be working in an unfamiliar environment, overseen by the dissatisfied, and potentially hostile, customer with no immediate team on hand to help. Furthermore, transporting the much-needed lab equipment on-site is impractical due to shipping constraints – and the in-house design team are using it for the next project in any case!
What’s needed is a small, lightweight, hardware verification tool with functionality that matches or surpasses the verification tools that were available in the laboratory. Further, it needs to enable sharing of results and data so that other members of the design team, back in the lab, can see what is going on in the field, do their own analysis, and behave as an integrated team to find the cause of the problem and resolve it.
PicoScope
PicoScope USB oscilloscopes are small, light, alternatives to traditional benchtop instruments. A PicoScope fits in a laptop bag, yet offers 2 or 4 channels, up to 500 MHz bandwidth, a built-in signal source and 16 digital channels on MSO models. High-end features such as advanced triggering, serial bus decoding, mask limit testing and waveform mathematics are included as standard. Buffer memory up to 2 gigasamples enables in-depth analysis of complex systems that can be performed in real time by the PicoScope user, or off-line and remotely by other engineers using their own license-free copy of the PicoScope software.
In this article we’ll look at examples of the PicoScope 2200A two-channel pocket oscilloscope and the PicoScope 5000 four-channel flexible resolution model.
Basic Signal Integrity Measurements
The first task for an engineer who needs to find a design flaw is to perform basic signal integrity and timing checks. Does the clock distribution look okay? Are the logic edge rates in spec? Is there evidence of noise or crosstalk that could be interfering with the circuit behaviour? Is the design stable over time and with changes of external parameters such as temperature, supply voltage and in the presence of EMI?
Figure 1 is a screenshot from a PicoScope 2200A showing capture of a simple I2C clock and data waveform (For full resolution click here). The display has been split into four view panels which enable the user to view different portions of a captured signal by zooming in on the waveform features of interest.
In the top left view we can see the overall captured waveform. The blue waveform is the SDA signal and the red one is the clock. The bottom-left view is zoomed-in on the trigger event, in this case a pulse width > 200µs. Top right is zoomed-in to show clock activity on the sixth packet in the captured sequence. Bottom right is showing timing relationships of clock and data of the second packet using PicoScope rulers.
Looking For Glitches
Having checked the device for normal operation, the next step might be to search for glitches or other waveform errors that could be the cause of intermittent circuit failures.
Figure 2 (for full resulotion click here) shows how the PicoScope 2200A can be set to trigger on common digital error conditions such as pulse width, drop-out or logic state.
Another way to search for intermittent errors is to capture a known good waveform from the device under test and create a mask, with user-defined tolerances, that can be used to test waveforms over extended periods of time. In the event of a violation of the mask, PicoScope can trigger an “Alarm” to save the failing waveform, ring a bell or take some other user-defined action. This is ideal for unattended soak-testing of a device.
Figure 3 (for full resolution, click here) shows the PicoScope mask limit testing, which works in both the time and frequency domains. If spikes from a switched mode power supply are causing interference on the data bus, it would show up clearly in the frequency domain view and trigger an alarm.
Checking Program Execution
A logical step in the debug process is to check the data sequence to make sure expected values are being written to and read from each device in the chain. For that, the engineer might want to trigger the oscilloscope on a start pulse or, perhaps, on an error condition, and see the data flow leading from or up to those events.
Figure 4 (for full resolution click here) shows decoding of an I2C bus. Notice the glitches on both the clock and data waveforms that appear to be the result of crosstalk on the board.
This example is showing just a few packets of decoded data with a short buffer memory. PicoScope models such as the 5000 Series have huge buffer memory up to 256 megasamples, which enable hundreds or thousands of data packets to be decoded to trace detailed program activity. Any data errors that are identified can be immediately related to the corresponding waveform for analysis and rectification.
Power Analysis
If the device is mains-powered, or if it is situated close to mains-powered equipment, the failure may be due to power supply quality problems or radiated emissions. Non-linear loads and modern power-conversion equipment produce complex voltage and current waveforms with significant harmonic content. Proper measurement and analysis of these harmonics is essential to resolve power quality problems. On the supply side, power quality issues such as sags and dips, swells and spikes, flicker, interruptions and long-term voltage and frequency variations also need to be evaluated to ensure the device continues to work correctly under all conditions of supply.
Figure 5 (for full resolution click here) shows the supply voltage (blue channel), and the current drawn (green channel) from a laptop computer (the one I’m using to write this article!). The laptop SMPS is non-linear, so has significant harmonic content, shown in the frequency domain view at the top. The power waveform is the black maths channel, calculated as ABS(ChanA x ChanB) and calibrated in kW. In this case the overall current draw is low and the power transmission system can easily supply the demand. But industrial installations switching high currents can create a lot of problems for transmission systems, which can cause problems for other devices connected, or close, to the same supply.
Stress & Margin Testing
Evaluating a design using inputs and signals from the actual sensors that are deployed in the final design is a sensible, and necessary, part of the design validation process. But, by the very nature of real-world signals, you may have to wait a long time for the sensor to deliver a “bad” signal. (And remember, with the customer breathing down our necks, time isn’t on our side!)
A useful strategy then, is to simulate incoming waveforms with a signal generator, and vary parameters such as frequency, amplitude, rise- and fall-time, duty-cycle etc., and see how the device responds.
Of course, many input signals aren’t simple sine, square or pulse shapes. For complex waveforms the PicoScope’s built-in Arbitrary Waveform Generator is incredibly powerful in being able to deliver real-world waveforms that can be stressed with known, measured imperfections such as jitter, noise, phase, and frequency changes to create worst-case operating conditions and check that the device processes the full range of inputs correctly.
The beauty of this approach with PicoScope is that waveforms captured by a scope channel can be easily moved to the AWG memory, edited as necessary, and output.
Figure 6 (full resolution here) shows a serial data stream that has been captured by the PicoScope, edited to add a glitch, and output from the AWG – a process accomplished in just a few seconds.
Rarely is a product designed by one engineer. More likely there is a team of engineers who have developed the hardware, written the FPGA code and firmware, drivers, packaging and so on. When trouble strikes it is important that all the team members can have access to test data to view and resolve their element of the design. Sending screen shots and CSV data files is a good start, but PicoScope software can be run, without license fees, on any number of PCs. So the field engineer can collect raw waveform data and send it back to the design team, who can then perform their own detailed analysis to identify design flaws, suggest additional tests, and contribute new AWG files. So cooperation helps to find the cause of problems quickly and agree on steps to resolve the issue.
About the author
Trevor Smith is Business Development Manager at Pico Technology. His email address is Trevor.smith@picotech.com
If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :
