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Pros and cons of different high speed digital tests

Posted: 10 Oct 2011 ?? ?Print Version ?Bookmark and Share

Keywords:dynamic protocol reconfiguration? Protocol analyzers? Oscilloscopes? Logic analyzers?

There are many different good tools on the market for debugging digital designs. Each has advantages and disadvantages that solve specific niche problems. No tool can solve all problems at all speeds and for all protocols. This paper provides a discussion of the relative merits and demerits of high speed digital test approaches and dynamic protocol reconfiguration.

Protocol analyzers
Protocol analyzers are ubiquitous and come in two main categories: software-based acquisition and hardware based acquisition.

A good example of a software based protocol analyzer is WireShark. As it is free, it's a real bargain. It supports decodes for over 980 protocols. One big limitation to these software based systems is that they are only as good as the acquisition system used to capture the data. Many acquisition systems drop frames, have few ports, or only interface with one protocol at a time (and possibly only one protocol ever). They work by putting the ports in a 'promiscuous mode' that attempts to capture all frames. Because of hardware limitations, frames are often dropped. Partial frames, or corrupted frames are typically filtered out. Users may end up wasting time troubleshooting a system for a dropped frame that wasn't really dropped. Dropped frames might also conceal a problem.

This type of analyzer typically acts as an endpoint or it connects via a monitor port or external TAP. Monitor ports often filter, and drop traffic. Without a TAP, these systems don't act as a 'man-in-the-middle' to watch a dynamic conversation. Man-in-the-middle systems are able to watch the link as it comes up and down and troubleshoot the auto-negotiation process.

Hardware based protocol analyzers can be sourced from companies such as Network Scout Systems, and Network Instruments. If you spend enough money you can often buy hardware that runs wirespeed without dropping packets. However, this isn't available for all technologies.

Disadvantages of hardware based protocol analyzers:
? They typically only support one protocol.
? They have poor physical layer capability (because they were developed for IT guy to use in troubleshooting a network).
? The hardware can be expensive and not very scalable (because it was designed for low-port-count IT purposes).
? They probably don't have real-�time triggering and viewing capability (not needed for IT work).
? They may not be able to cross trigger external devices in real time for advanced diagnostics.
? This approach is inadequate for information assurance and cyber security applications where the ability to see the physical layer bits can be important.

Today's modern high speed real-�time oscilloscopes are often used to evaluate communications links such as Ethernet or USB. This market is dominated by Tektronix (Danaher), Agilent, and LeCroy. Two of the key uses of a fast oscilloscope are the ability to display eye diagrams and measure jitter. Oscilloscopes can be used to measure and solve many problems (especially at the physical layer).

Limitations to real-time oscilloscopes include:
? Low channel count.
? Poor protocol awareness (and therefore poor protocol triggering).
? High performance oscilloscopes will also be very high priced. Depending on bandwidth, speed, memory and port count modern oscilloscopes can cost $100,000 to $1,000,000.
? Oscilloscopes are usually standalone instruments. Integrating the oscilloscope into a system requires the use of LabView (or equivalent) or an engineer to write software code.

Logic analyzers
The logic analyzer market is dominated by Agilent and Tektronix. Logic analyzers can be very helpful especially if you need to monitor many channels. One of the strengths of a logic analyzer is that it can be used to look at the kernel, IO, and other parts of the board simultaneously. These instruments are versatile and can look at almost any kind of digital signal within the bandwidth and voltage ratings of the instrument.

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