Protocol Analysis in a Scope
Digital designers reach for an oscilloscope as the tool of first choice for debug. As products under development can have a myriad of issues to debug, oscilloscopes with versatility give faster insight to shorten development cycles and produce higher quality products. Additionally, a recent scope-based innovation provides three instruments in one: oscilloscopes with built-in logic and protocol analysis. What are the advantages and trade-offs of these tools vs. traditional scopes plus a separate protocol analyzer?
Oscilloscope technology advances enable new classes of fast, easy-to-share products with deep memory. For example, a scope with 100 Mpoints of memory per channel can capture 10 ms of time while sampling every 100 ps. These high-speed sample rates enable capture of high-speed serial buses such as USB 2.0 or PCIe. Or, turn down the sample rate and they make instruments to capture long streams of slower serial buses like I2C, SPI, RS-232, CAN, or FlexRay. Ten MS/s with 10 Mpoints of memory yield 1 second of real-time capture on an I2C bus.
Traditional scopes include ADCs that produce 8-bit analog waveforms. Increased hardware FPGA technology fuels protocol analysis in a scope innovation. For instance, between the front-end scope input circuitry and the scope's trigger circuitry, some vendors incorporate a chip that provides real-time clock recovery for buses with embedded clocks and evaluation of serial packet structures.
Serial packet content is fed from this circuitry to the scope trigger to enable real-time serial packet triggering. Users can specify a number of protocol-specific trigger conditions. The oscilloscope circuitry compares these conditions to the incoming serial stream and, in real time, can determine when to trigger, either repetitively or via a single-shot measurement.
Scope vendors with access to state-of-the-art technology can customize these protocol analysis ICs to support a variety of serial standards for triggering. This extends scope protocol analysis capability from lower speed buses like RS-232 through multigigabit serial links like PCIe
Figure 1. Infiniium 9000 Series Oscilloscope With Application Options Providing Protocol Analysis Capability
In addition, scope technology, implemented in either hardware or software, can provide real-time decode of serial packets and display this content time-correlated with the scope's analog waveforms. On most oscilloscopes, this requires some post-processing work and will have a negative impact on the scope's capability to process waveforms at the highest rate.
Scopes that include hardware-based serial decode capability speed the time required for processing and update rate, minimizing dead time between scope acquisitions. If you are making single-shot measurements, processing via either software or hardware is adequate. If you are running the scope repetitively and looking for anomalies in real time, hardware-based decode will provide the best results.
Quality protocol analysis in a scope requires one additional attribute: a big display. Larger displays allow serial signals to be viewed in more meaningful ways and can even mimic protocol analysis displayed on a PC monitor. For example, with a 15″ integrated display, Agilent's 9000 Series Oscilloscopes have room to show serial transaction symbols on waveforms as well as protocol analysis in a compact protocol decode listing window on the lower portion of the display (Figure 2). Serial data can be saved as graphic files or exported from the oscilloscope in a number of methods including .csv file export.
Figure 2. Agilent's 9000 Series With Protocol Options Providing Protocol-Level Triggering and Packet Decode
Who Needs Scope-Based Protocol Analysis?
Protocol analysis in a scope is designed to extend the debugging capability, not to replace traditional protocol analyzers. Scope-based protocol analysis targets engineers who regularly use scopes for debug.
If an oscilloscope is your primary debugging tool, scope-based protocol analysis is likely a great extension for you. It provides a method for extending the scope to look into another technology domain as well as several advantages over using a traditional protocol analyzer.
A single scope can be configured for protocol analysis of a variety of buses all in a single instrument. The addition of protocol analysis extends potential equipment utilization for the scope.
Easy Access to Serial Signaling
Attach nonintrusive scope probes for mid-bus probing. The scope doesn't need a standard port or special IO for connectivity. For differential buses that incorporate noise immunity like USB and CAN, differential probes work nicely. Protocol analyzers may require a standard IO port such as a PCIe or USB connector.
Don't Break the Bus
Oscilloscopes passively monitor serial signals while protocol analyzers typically provide retransmit and retiming. The passive monitoring can uncover physical-layer problems that traditional protocol analysis can mask.
Correlation Between Physical-Layer and Protocol-Layer Measurements
Scopes with protocol analysis provide time-correlated views of waveforms, symbols, character, link, and transaction-layer packet data down to the bit level. This allows users to isolate communications faults to logic or analog sources.
For example, this capability makes it easy to trigger on an issue that manifests itself at the protocol layer and then quickly zoom to the physical-layer measurement to see if it's caused by a signal-integrity issue. Traditional protocol analyzers operate exclusively at the protocol layer and can't provide physical-layer information.
Easy-to-Specify Triggering Conditions
As each protocol requires unique triggering hardware investments, users will find variance among protocol triggering conditions for various oscilloscopes. All are fairly simple to configure and set up. The difference typically lies in how deep into the serial packets triggering conditions can be specified.
When to Stick With a Traditional Protocol Analyzer
If you don't use an oscilloscope regularly, a traditional protocol analyzer is likely a better fit. It has several advantages over protocol analysis in a scope and is more likely to be used by engineers who aren't dealing with hardware design.
For established slower-speed buses like I2C, RS-232, CAN, LIN, and SPI, traditional protocol analyzers are extremely inexpensive. Typically, $500 will suffice.
Traditional protocol analyzers usually are small and use an external PC to display the protocol viewer and configure the trigger.
Long Capture Time
Protocol analyzers characteristically will capture much longer windows of time. Scopes capture and store on each acquisition cycle and fill memory rapidly. While this provides great physical-layer parametric detail, some applications don't require this. Traditional protocol analyzers store just 1s and 0s to preserve memory. In addition, they have state-based acquisition systems that store on bus-aligned transactions, further extending memory usage.
High-Speed Serial Protocol Analysis
For some buses like PCIe, the question of which tool to use for protocol analysis remains unanswered. Traditional protocol analyzers can be hard to find and more expensive than a scope. PCIe protocol analyzers also typically are tailored for computer applications, often support multiple lanes, and include exerciser/stimulus capabilities.
As high-speed serial buses such as PCIe go into embedded markets, design teams often find oscilloscopes to be a great fit for debugging needs. The higher bus speeds increase the probability of a physical-layer signal integrity issue that will need resolving.
For a new high-speed scope, adding PCIe protocol analysis costs less than $2,000 incrementally. The associated scope-based protocol viewer is extremely similar to what can be found at a 10x price on a traditional protocol analyzer. Scope-based protocol analysis also provides views while the link is undergoing training and coming in and out of idle states.
PCIe is just one example of many high-speed serial buses that use 8B/10B encoding for symbols to achieve DC balance and bounded disparity and provide enough state changes to allow reasonable clock recovery. Many design teams, especially those using FPGA technology, incorporate their own proprietary 8B/10B lightweight protocol or use a standard for which off-the-shelf tools don't exist. Scopes that provide generic 8B/10B protocol decode capability allow users to trigger and decode at the bit or HEX level.
Oscilloscopes continue to use increasingly sophisticated protocol analysis capabilities. Protocol triggering and decode application options are available on newer scopes and can be added to most recently purchased scopes. As multigigabit serial links are deployed into embedded designs, the choice of a traditional protocol analyzer vs. outfitting a more general-purpose oscilloscope with protocol analysis becomes blurred.
For guidance on what to choose, ask yourself the following questions: What is the primary tool I want to use for debugging? Do I have the potential need for debugging across both physical and protocol layers? How can I evaluate protocol analysis in a scope for the specific serial bus of interest?
Scope vendor websites and datasheets contain helpful information on features, pricing, and availability.
About the Author
Joel Woodward is the senior product manager for oscilloscopes in the Digital Test Division, Electronic Measurement Group at Agilent Technologies. He joined the company, then Hewlett-Packard, in 1989 as a technical support engineer and has held several management positions in the past 20 years. Mr. Woodward received a degree in electronic engineering from Brigham Young University and a master's of business administration from Regis University and completed coursework from Harvard Business School. Agilent Technologies, 5301 Stevens Creek Blvd.,Santa Clara, CA 95051, e-mail: email@example.com