Don’t be illogical when choosing logic-analysis tools

Logic analysis is evolving to address changes in electronic-product design and the makeup of design teams. Logic-analyzer manufacturers are collaborating closely with makers of other types of development tools. ADVERTISEMENT Vendors are introducing new types of products that complement (and perhaps compete with) general-purpose logic analyzers. Bus analyzers ease bus debugging by triggering on protocol violations and presenting complex data in an easy-to- digest format. Products that resemble logic analyzers are displacing traditional in-circuit emulators in both hardware and software debugging of µP-based systems.

Bus monitors versus bus analyzers: there is a difference The Web becomes a logic-analysis tool For more information

Like so much of electronics, logic analysis is changing profoundly. Microprocessors with complex architectures and clock speeds that already top 0.5 GHz present incredible challenges. Ever-faster buses that depend on intricate communication protocols now pervade electronic systems. Circuitry that once resided in multiple ICs whose pins afforded ample opportunities for probing now resides deep within ASICs, where many layers of logic separate it from probe points.

Product-development teams, which once focused on hardware, now usually include more software engineers than hardware designers. To hardware designers, general-purpose logic analyzers are important allies. Software engineers, on the other hand, either avoid using the instruments or maintain a strained, arm's-length relationship with them.

Logic analysis must adapt to this new landscape or risk irrelevance. Instrument makers have responded by introducing new species of logic-analysis tools ( Table 1 and Table 2 ) and collaborating more closely with suppliers of other types of tools, especially software-based tools. The new products include accessories for general-purpose instruments as well as specialized stand-alone units. Though not true logic analyzers, these units perform related functions. Many of the new tools come from companies that don't produce general-purpose logic analyzers (see sidebar " For more information... ") To ensure that you make logical decisions about logic analysis, you need a clear understanding of these new tools—what they're good for and what they're not.

The end of the ICE age?

For nearly two decades, the logic analyzer has been a vital tool for debugging µP-based system hardware. For debugging µP code, however, software engineers have relied more heavily on another hardware-based tool—the classical in-circuit emulator (ICE). Now, however, advanced processors are confounding these ICEs. Although traditional ICEs are holding their own with 8- and 16-bit processors of older architecture, in systems based on newer µPs, the ice man cometh for the classical ICE.

Hardware-based debugging tools for newer processors bear a faint resemblance to traditional ICEs and a stronger resemblance to logic analyzers. Gone are most of the classical-ICE features that provided real-time control of processor operation and some of the features that allowed real-time observation of processor activity. In their place, you now find run-control via µP serial ports—background-debugging mode (BDM) on Motorola ( www.motorola.com) devices and IEEE 1149.1 (JTAG) on other manufacturers' chips.

In the realm of debugging tools for newer buses, at least a dozen manufacturers offer hardware-based products aimed at filling software engineers' needs. Many of the new buses are serial and even more of them depend on complex communication protocols. The new bus-analysis tools aim to display bus traffic in a way that makes apparent the significance of the bus activity. They also strive to detect and trigger on protocol violations in real time. For most modern buses, this task requires fast, complex hardware.

The multitalented instrument

A logic analyzer provides two views of digital-circuit activity. In the timing-analysis or asynchronous mode, the analyzer acts like an oscilloscope, though usually with many more channels than you find in scopes. In the timing mode, the logic analyzer's internal clock provides the timebase. However, unlike a scope display, the waveform presentation shows only two levels (1 and 0) in any clock period.

In the state-analysis, or synchronous, mode, the target system's clock acts as the instrument clock. In the state view, time usually runs from the top of the screen to the bottom, and the analyzer usually presents the data as meaningful mnemonics, such as processor op codes and high-level-language instructions.

Recently, logic analyzers have expanded their capabilities to include perform-ance analysis, which often involves statistical analysis of measurements. Once the exclusive domain of the traditional ICE, performance analysis helps software engineers determine how efficiently the target-system software is running. For example, performance analysis can indicate the amount of time (typical and worst case) that a target system spends executing subroutines.

Married to the bus

A few bus-analyzer manufacturers proclaim that their instruments make general-purpose logic analyzers obsolete and that only a timid engineer would purchase a logic analyzer for bus analysis. According to these manufacturers, such engineers hesitate to invest in an instrument that is married to only one bus, because next year's project might use a different bus.

Bus-analyzer manufacturers insist that you can buy two bus analyzers for less than the cost of one general-purpose logic analyzer and its associated preprocessors. Thus, if next year's project doesuse a different bus, you can buy a new bus analyzer and still spend less than the cost of a general-purpose logic analyzer. These manufacturers also assert that the bus analyzer is more compact and easier to use.

Still, most bus-analyzer vendors don't see their products competing with general-purpose logic analyzers. Instead, they see the products as complementary. The logic analyzer, together with a good oscilloscope, excels at hardware debugging. The bus analyzer's forté is debugging bus-related software problems—of course, only on the bus for which the instrument was designed. Many systems have multiple buses, and not all development teams have a bus analyzer for each of those buses. If no bus analyzer is available for one or more of the system's buses, a general-purpose logic analyzer can come to the rescue. The logic analyzer is also the tool of choice for problems unrelated to buses.

Really, it's state analysis

Bus analysis with a logic analyzer is really a special case of state analysis. Although a logic analyzer may not always be the most efficient bus analyzer, the substitution is often quite effective. Moreover, employing a general-purpose logic analyzer for bus analysis may help you get to the root of hardware problems that confound many dedicated bus analyzers. Logic analyzers excel at finding such problems because they perform timing analysis, a function that few dedicated bus analyzers are capable of (see sidebar " Bus monitors versus bus analyzers: There is a difference .")

At least one company supports using a general-purpose logic analyzer for bus analysis. FuturePlus Systems has built one of its core businesses by providing hardware and software that adapt HP logic analyzers to bus analysis.

Perhaps because you can often use a logic analyzer for bus analysis, some in the industry have begun interchanging the terms "bus analyzer" and "logic analyzer." Such reckless terminology is unfortunate because you usually can't make the opposite substitution—that is, you generally can't substitute a bus analyzer for a logic analyzer.

More memory please

As systems become more complex, it is not surprising that users are demanding logic analyzers with greater memory depth. Engineers commonly use a logic analyzer to obtain a record (trace) of the target-system activity preceding a malfunction to find and fix the problem. The chain of events that culminates in a malfunction often begins many clock cycles before the first aberrant behavior is noticeable. The deeper the buffer, the better the chance it will capture the offending event.

Although all logic-analyzer manufacturers are committed to producing instruments with memory depth as great as the technology permits, Hewlett-Packard derides the practice of gathering ever-increasing amounts of data. In fact, HP engineers have named the technique "swallow (lots of data) and wallow (in all of the data)." HP already supplies logic analyzers with 4-Mbit/channel trace memories, but the company's application engineers argue that you are usually better off refining the criteria that trigger a trace and capturing less data. Of course, this practice presupposes that you can determine exactly how to refine the trigger criteria.

One bus-analyzer manufacturer talks about developing an analyzer with a memory depth of 1 Gbit. The analyzer would capture 128-bit-wide traces, so it would fill its memory in 8 million clock periods. The bus's clock speed is 100 MHz, so the trace buffer would fill in just 80 msec. Without an automated searching method, you might spend days scrutinizing that 80-msec record for the event that precipitated the failure. On the other hand, for automated searching to work, you must be able to define what you are searching for. And if you can define what you are searching for, why not trigger on it instead of searching after you've captured the data?

One reason is that the malfunction may occur so infrequently that, even if you know what to search for, it may be days before the condition arises. In such situations, the deep trace may be extremely valuable, notwithstanding the difficulty in extracting the secrets it contains.

In bed with ASICs

Just as high-speed buses that use complex communication protocols are a fact of life in modern electronic systems, so are complex ICs—even so-called system-on-chip ASICs. Consequently, the idea of embedding logic analyzers or portions of logic analyzers within ASICs seems destined to garner increasing interest.

More and more, complex ASICs hide the points that you need to connect to a logic analyzer to verify the ASIC design. In response, companies, such as Boulder Creek and Intellitech, are developing technologies that make the logic analyzer part of the IC. A serial interface provides access to the instrumentation functions. However, if the vendor produces the IC in significant quantities, the area that the instrumentation functions occupy can raise the IC cost to an unacceptable level. Therefore, to save area after verifying the design, ASIC designers must often remove the instrumentation functions from the chip layout.

An analyzer for every bench

Conventional logic analyzers come in a range of prices and capabilities. Market leader Hewlett-Packard's pricing begins at $4995 for the 54645D mixed-signal oscilloscope ( Picture). This instrument has 16 logic-timing-analysis channels that acquire 200M samples/sec (or 400M samples/sec if you use only eight logic inputs). The 54645D also has two 100-MHz-bandwidth analog channels that acquire a maximum of 200M samples/sec to a depth of 1M sample. Just $5 more buys the 34-channel 1664A, which does 500-MHz timing and 50-MHz state analysis. This unit has a monochrome CRT display. All other HP logic analyzers now sport color flat-panel displays with resolutions of 640X480 pixels or more.

The top of HP's benchtop-analyzer line is the $20,000 1660ES, with 136 logic channels that perform 500-MHz timing and 100-MHz state analysis. This instrument also includes two 500-MHz- bandwidth, 2G-sample/sec scope channels. The $16,300 1660EP substitutes a 32-channel digital-pattern generator for the scope channels.

HP's modular systems, the $25,740, 204-channel 16600A; the $10,040 16700A ( Picture); and the $11,500 16702A are now Web-enabled (see sidebar " The Web becomes a logic-analysis tool "). Your choice of plug-in modules determines the 16700A's and 16702A's logic-analysis characteristics. HP supplies modules that perform state analysis to 1 GHz and timing analysis to 4 GHz. The 16600A accommodates one emulation module; the 16700A and 16702A each accommodate two emulation modules. These modules communicate with a variety of µPs via the chips' serial ports (IEEE 1149.1 or BDM, for example). Thus, these systems incorporate both logic analysis and a nontraditional form of in-circuit emulation.

Tektronix's TLA 700 family ( Picture) includes both portable ($9500) and system-level ($14,750) logic-analysis mainframes. The units provide 500-psec timing resolution (equal to 2 GHz), 200-MHz state-analysis speed, and 512-kbit/channel maximum memory depth. They can simultaneously perform state and timing analysis through the same probes. Portable mainframes accommodate 272 channels. System-level mainframes accommodate 680 channels. The required 34- to 136-channel plug-in modules cost $8400 to $27,300 each.

Don't break the bank

Many vendors offer lower cost logic analyzers. Link Instruments' PC-based instruments come in enclosures external to the host PC. The company offers units with speeds to 500 MHz and with as many as 160 channels. Prices range from $1350 to $7000. The company also makes PC-based digital scope units that are good companions to the logic analyzers.

Boulder Creek's Pod-A-Lyzer PC-based logic analyzers are unusual for several reasons. The most noticeable reason is their size: The units are no larger than the pods that interface between conventional logic analyzers and the target system. The devices also need no dedicated interface card within the PC; they connect to standard serial (RS-232C) or Universal Serial Bus ports. An 18-channel unit with memory of 64 kbits/channel does timing analysis at 100 MHz and state analysis at 66 MHz and costs $1295.

NCI offers two PC-based logic-analyzer lines. The 400-MHz Mobilogic line comes in a 3X6X15-in. external enclosure. Prices range from $3995 for 48 channels with memory of 64 kbits/channel to $8990 for 96 channels with memory of 256 kbits/channel. The company also makes logic analyzers on PC plug-in cards. These units offer as many as 96 channels (two boards), timing-analysis speeds as high as 400 MHz, and state-analysis speeds of 50 MHz. Memory depths reach 1 Mbit/channel. Prices range from $2425 to $3995.

Bus-analyzer prices are at least as varied as the buses themselves—ranging from $295 for a controller-area-network bus analyzer that you must use with a low-cost logic analyzer to $30,000 for a Fibre Channel analyzer. Options can even further raise the price of the Fibre Channel analyzer.

Scopes

Emulation

Dan Strassberg, Senior Technical Editor

You can reach Senior Technical Editor Dan Strassberg at 1-617-558-4205, fax 1-617-928-4205, e-mail ednstrassberg@cahners.com.