FROM EDN EUROPE: The two-horse race of logic analysis

The list of manufacturers of test equipment for use in the development of electronic products is not a very long one; nevertheless, there cannot be many categories of test equipment in which the total number of suppliers has shrunk to just two. Logic analysers is one of them, though; today, only Agilent and Tektronix sell them (www.agilent.com, www.tektronix.com). There are a number of specialist products at the margins of the market with logic analysis functions, as well as some relatively low-speed, PC-add-on devices that provide a measure of logic analysis—but classifying logic analysers as mainstream, benchtop (or rack-mount) instruments capable of capturing and interpreting data from today's leading-edge designs, there are just two sources.

What, in today's design environment, is the logic analyser for? Tektronix' Dave Ireland, European Market Segment Manager for Design and Manufacturing, says that the predominant uses in embedded system design continue to be where the instruments have been traditionally employed—intercepting signal streams from component to component, between functional blocks, within the confines of a PCB, and between PCBs. At present, parallel buses still dominate, although the shift to serial communications is making an impact. In today's designs, Ireland says, serial buses are most likely to appear as external interfaces to another system or network. Serial interfaces, he emphasises, are becoming the major debug and test challenge.

At Agilent, Tim Colls, Logic Analysis Marketing Manager at the Colorado Springs Design Validation Division, notes that a big percentage of the market still lies with designers of PCs and other processor-based IT equipment, with other sectors emerging. However, whereas in the past, the analyser would find a large part of its use in decoding processor instruction stream, today, it is more likely to be employed in debugging high-speed I/O and interfaces to devices such as DDR memory. "There was a time when a long list of supported processor families [with comprehensive disassembly] was the key to success in logic analysers. Now, that focus has shifted to memory and I/O."

All too often, over the years, the logic analyser has been the instrument or last resort. As Tim Colls puts it, "When you had to reach for the logic analyser, you knew you were having a bad day." This became a self-fulfilling prophesy—unfamiliarity with the user interface made the instruments difficult to operate, so engineers would use them infrequently, reinforcing the lack of familiarity. The current version of both companies' products are Windows-based, and both cite this familiar user interface as a major factor in improving the ease-of-use of the whole class of instrument.

Tektronix has just launched an upgrade to its logic analysis line, with new mainframes (Figure 1) and a new version of its analysis software. TLA 7012 is a portable mainframe and TLA7016, a benchtop version. Both use existing TLA series acquisition modules, and the basic measurement capabilities of the series are unchanged with, for example, a peak acquisition rate of 8 GHz. The 7012 supports two modules allowing a maximum of 272 channels of acquisition, features an integrated 2-GHz Pentium M processor running Windows XP, and has its own 15-in. TFT LCD display. Touch screen operation allows direct selection of waveforms. The 7016 is the module-rack version with six slots. It can run under the control of an external PC, or one of its slots can host a 3 GHz P-4-based controller. The instruments are among the first to employ a Gigabit Ethernet LAN connection to speed the transfer of very large sets of acquired data. Using two graphics controllers, it is possible to view a long trace split over two physical screens placed side-by-side to allow greater expansion of complex waveforms.

In the TLA software version 5.0, Tektronix has added a number of "scope-like" automated measurements that the user can apply directly to selected waveforms. As well as measurements that are literally equivalent versions of those found on scopes (for example, period and frequency), there are also logic-analysis-specific routines such as counting the instances of a data pattern, or cycle-count within a burst. Measurements are set up by drag-and-drop; dropping a trigger or waveform icon on to a waveform initiates the required sequence with no further action. The software will run with all previous TLA series instruments.

Both Agilent and Tektronix offer the facility to synchronise signal acquisition and display on both logic analyser and oscilloscope. This feature evolved over many generations of instruments, with assorted combinations of analyser and scope facilities being tried over time. The original motivation remains valid; the single-bit representation of the logic waveform transition on the logic analyser display can show a glitch or other failure, but it gives very little information about the signal integrity effects that might be causing it. The detail available on the scope waveform can reveal exactly that, but only if it is precisely time-correlated with the logic analyser display. Tek calls its facility iView, implemented as a direct connection between instruments; on the Agilent products, an external piece of hardware, the Time Correlation Fixture, links the two instruments. In either case, triggering of each instrument by the other is supported, together with display of fully-detailed scope waveforms alongside logic traces on a single screen. In the case of a high-speed serial interface, a full-resolution waveform acquisition capability is used to present an eye diagram of the bit stream.

Now, both companies are moving to facilitate linking the logic analyser to other instruments; this time in the RF domain. The motivation behind this evolution is the prevalence of digital modulation in communications and of software radio techniques. Logic waveforms in the baseband section of a software-defined radio have a direct impact on the output RF spectrum in a transmitter. An application note from Tektronix describes how to use the TLA 700 logic analyser together with a TDS7000B scope and a real-time signal analyser, RSA3308A, in the analysis of the behaviour of a frequency-hopping software-defined radio system (Figure 3). If the RF spectrum contains any out-of-limits spurious emissions or transients, a spectrum mask in the RSA can trap that condition, triggering the scope to capture (for example) waveforms from the radio's digital frequency synthesiser, and at the same time triggering the logic analyser to determine what control signals led to the failing signal condition. Tek's Ireland notes that the integration is not yet as tight as that available between logic analyser and scope alone, but that it is under constant development.

Despite the head-to-head competitive atmosphere engendered by a two-player market, a de-facto standard does exist between the two companies' products in one important practical area: that of probe connections. Logic analyser probing was traditionally carried out by miniature "grabber" probes on a per-signal-line basis. As leaded components grew rarer and bus widths greater, this became less practical. At the same time, with edge speeds rising, it became more critical to provide a well-matched connection to the signal line. Designers had—and have—the option of inserting a pinned-out connection into their PCB layout at points which are likely to be of interest for logic probing. The popular Mictor connector remains widely-used for this purpose. Both Agilent and Tektronix now offer a probing connector with a common footprint (Figure 2); signal lines are brought out to a pad pattern on the PCB, with associated mechanical attachment points. No connector is necessary—so-called "connectorless" probes pick up the PCB surface pads and transmit the signals to the analyser. The technologies used by the two companies to make contact with the PCB differs, but in each case, a compression contact system links the 34 pads (which can carry 17 differential signals) to the logic analyser's ribbon cable probe lead. The result is that designers can lay down a single pattern knowing that whichever brand of instrument is to hand will be able to use it. Agilent's Colls says that his feature has been widely adopted and that since its introduction, most designers using it have accepted the small area penalty and left the probe patterns in place on production boards. Agilent calls its connector "SoftTouch", and Tektronix "D-Max".

An expanding area of use for the logic analyser cited by Agilent's Colls is in FPGA design. In contrast to ASIC designers, who (of necessity) spend as much time as possible in simulation of their logic before committing to a mask set and first-pass silicon, designers working with FPGAs move into hardware much more quickly. They continue, says Colls, to favour finding and fixing logic problems in the real rather than the simulated environment—despite increasing device complexity. The logic analyser becomes the tool of choice for examining logic behaviour and has to adapt to the fact that the logic is expressed internally within a single complex package.

Last year, Agilent, in a joint announcement with Xilinx, introduced the B4655A FPGA dynamic probe. This is an application for the 1680, 1690 and 16900 family logic analysers. The FPGA designer uses a Xilinx design software option called ChipScope, which is available with the Virtex FPGAs (including the Virtex 4 family), to insert extra structures into the FPGA configuration code. The extra circuitry provides visibility of internal circuit nodes, multiplexing up to 64 signals out through a single physical pin on the programmable device, with no impact on the timing of the main circuitry. The Dynamic Probe software running within the logic analyser presents those signals as though they were individually-probed physical lines, and also associates the signal names gleaned from the Xilinx device programming tool set with the appropriate signal on the screen.

Agilent says that this feature has been well-received by developers working with Xilinx parts who consider it as a significant productivity boost, and has increased the use of its analysers. The feature is only available with Xilinx FPGAs—Colls notes that the programme is the result of a close co-development effort that has not been repeated with other FPGA vendors.

Users of Altera or other FPGAs can turn to third-party software, for example that from Temento (www.temento.com), to secure logic-analysis-style insight into the inner working of their FPGA configurations. Once again, the principle is to insert extra functionality into the chips' configuration code streams, allowing access to internal nodes of the programmable array.

It still is not clear why a market that continues to represent considerable value should have shrunk to just two players. In the phase of logic analyser development, when support of a myriad of processor architectures was de rigueur, the support costs involved may have eliminated some of the smaller players from the scene. Now that the focus has shifted to high speed and, in particular, high-speed serial interfaces, the barriers to entry (or re-entry) for any further participants may be just too high. Pat Byrne, President of Agilent's Electronics and Solutions Group, says, "We [the two remaining companies in the market] just out-innovated everyone else. Logic analysis has become a natural duopoly."

With the focus being on high-speed serial, does the boundary line between what constitutes a logic analyser—as opposed to a protocol analyser—begin to disappear? For Agilent, Tim Colls thinks it doesn't. The software engineer who has to verify the detail of one serial bus at a time will prefer the protocol analyser. The logic analyser, although it will have to embody ever-growing understanding of the serial protocols, remains the tool of the hardware engineer, seeking to correlate activity across multiple buses and sub-system cores.