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Logic analyser selection

Discussion in 'Embedded' started by Nitesh, Apr 28, 2004.

  1. Nitesh

    Nitesh Guest

    Hello All,

    We are going to purchase logic analyser. As it is huge investment I would
    like to have some guidance from you experienced people. We are looking for
    logic analyser that can support a 32bit processor running at 300Mhz.

    1) What are the most critical parameters we should consider while purchasing
    logic analyser (considering our processor)?
    2) Which is a best brand and make you can suggest to go for (considering
    ease of handling, time to learn the handling as we are new to the logic
    analyser)?
    3) How much multiple state aquisition frequency of the logic analyser should
    be compared to processor clock frequency?
    4) Our processor has about 350 pins. How much logic analyser channels could
    be sufficient?
    5) What things we need to change if our target processor is changed?

    your suggesion will help us quite a lot,

    thanks in advance,
    Nitesh.
     
    Nitesh, Apr 28, 2004
    #1
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  2. As one of the old hands here I have to say that I have never ever
    needed a Logic Analyser to resolve any embedded system project
    problems. So the first question you have to ask yourself is why do
    you fdeel you need one. As you say, it is a large amount of money.
    I will not say to you don't buy one but I think you need to be very
    clear in your justification for purchasing one.

    Purchase, of course, is not the only option. You could always hire
    one when you desparately need one. However, I have found that the
    only time in my experience when the hire of a logic analyser was
    considered the problem was solved before the manager could raise the
    order paperwork.

    --
    ********************************************************************
    Paul E. Bennett ....................<email://peb@a...>
    Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
    Mob: +44 (0)7811-639972 .........NOW AVAILABLE:- HIDECS COURSE......
    Tel: +44 (0)1235-811095 .... see http://www.feabhas.com for details.
    Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
    ********************************************************************
     
    Paul E. Bennett, Apr 28, 2004
    #2
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  3. Nitesh

    Al Gosselin Guest

    Nitesh,

    I've embedded :) some questions and answers below
    The actual clock rate of the processor is of relatively little
    importance. For example, Intel processors are at ~3 GHz, but the
    front-side bus is ~800 MHz and the memory runs at ~300 MHz. What are
    the clock and/or data rates of the various buses that you need to look at?
    Without knowing which processor you want to use, this question is
    largely unanswerable. If you want Inverse Assembly, then the
    manufacturer has to support your processor. If the processor has other
    built-in buses that you want it to decode, then they should also be
    supported.
    There are two major brands, Agilent (was HP) and Tektronix (Note that
    I work for one of them, but I'm trying to be agnostic here). The
    relative merits of one vs. the other are endlessly debated. Get a
    demo, ask a LOT of questions, borrow one for a week or two (each
    company has loaner equipment). Pay a LOT of attention to the
    Applications Engineers. The prices are basically the same.
    Largely irrelevant question. They key, again, is bus rates. LA's
    basically have two modes, Timing and State. In Timing mode, the
    analyzer is like a multichannel digital scope. It captures lots of
    data as fast as it can. Up to a point, faster is better, because you
    might be using it to figure out why a memory access isn't working, so
    you need to look at setup and hold times, for instance. In State mode,
    the bus sets the acquisition speed. You can capture all bus accesses,
    or only specific ones, such as writes to a specific memory address.
    It depends. What buses do you want to look at? How wide are they? Many
    of the 350 pins will be power, ground and other less relevant buses.
    Again, it depends. It depends on what your current processor is and
    what your next one is. If you were going, for instance, from a
    Motorola 8260 to a Motorola 8560, you might only need a new processor
    support package for a few thousand dollars. If you were going from ARM
    to Pentium, then it might be a LOT more.
    Take your time, read a lot, go to both the Agilent and Tek web sites
    and download every application note that you can find. Most of the
    operation, user and processor support manuals are available online. If
    you can't get what you want, then call the company and complain.
     
    Al Gosselin, Apr 28, 2004
    #3
  4. Nitesh

    CBarn24050 Guest

    One major problem youve missed is how are you going to connect it to your
    system? You may well have to add circuitry and/or extra connectors to your pcb.
     
    CBarn24050, Apr 29, 2004
    #4

  5. Most important: design your board IN ADVANCE for probing. Probing a 32-bit,
    300-MHz CPU (probably with 100-MHz bus) is impossible using pin clips. Use
    Mictor connectors for all important signals, have your bus clock connected
    to the Mictors' clock pins, and include the connectors and their probe load
    in your signal-integrity analysis (of course you do SI, don't you?). Any
    serious LA will have the connector and probe data available; if it doesn't,
    don't consider it.

    If you want to do disassembly or bus analysis, make sure your your LA has
    this (usually add-on) packages; connect your Mictors EXACTLY as specified,
    or it will not work.

    Leave a lot of big/thru-hole ground pads available all around your board
    (both sides). You will need them (solder a short wire to them and connect
    to the LA Gnd pins with Alligator clips).

    Connect one or more Mictor connectors to other stuff you may want to probe;
    you probably won't have to use the CPU bus analysis all the time, but if
    you have tricky or custom I/O devices you will probably have to debug them
    as well.

    Other hints: an LA is only half your lab; you also need a good digital
    oscilloscope (is that how you write it? must be at least 2-channel,
    preferrably 4, with reasonable memory depth). You will probably use it
    more than the LA.

    Performance: the LA should have state clock-rate at least 25% faster than
    your bus clock; the 'scope should have a sampling-rate x5 of your clock
    and matching channel bandwidth.

    You may be able to find used instruments at eBay or similar sites - much
    cheaper and good instruments don't degrade much (if at all) over a year
    or two, unless they've been really abused.
     
    Assaf Sarfati, Apr 29, 2004
    #5
  6. Nitesh

    Jan Homuth Guest

    Depending on the CPU/MCU you are using, -- have you considered using an
    in-circuit emulator instead ?

    Using a logic analyser to debug the application is a stony and time
    consuming way.
    Specially at 300MHz you are starting a real adventure (as explained in the
    other posts)

    /jan
     
    Jan Homuth, Apr 29, 2004
    #6
  7. Maybe go for a processor which has internal trace capability like some ARM9s
    instead?
     
    Ulf Samuelsson, Apr 29, 2004
    #7
  8. Nitesh

    Jim Stewart Guest


    Others have given you good techical advice,
    here's a little political advice from an old geek.

    Nothing pisses off management like seeing a
    logic analyzer they spent $40,000 on sitting
    in the corner gathering a layer of dust.
    If you *need* to buy a new LA for what you're
    doing, make sure you use it.

    If you're not sure, buy a used one on ebay.
    HP 16500A's on ebay are a huge bargain.
    For $500-$1,000, you can have a 100/500mhz
    logic analyzer, digital scope and pattern
    generator all in one box, with floppy disk
    configuration save and load, color touchscreen,
    and laserprinter screen print. The 16500
    series has modular function cards so you can
    configure a machine to the specific task at hand.

    The only drawback is noise and big footprint.
     
    Jim Stewart, Apr 29, 2004
    #8
  9. Nitesh

    rickman Guest

    When we design a new board, we do a prototype version with the main
    layout on N layers, then we add two layers and enlarge the outline for
    routing to connectors just for a logic analyzer. Then if the board is
    ok, we just strip off the extra layers and reduce the outline and the
    target board layout is done. Of course the routes for debug will affect
    issues with signal integrity, so any high speed signals are not really
    done until you have speed tested the final board. But that really
    should be handled in the design phase. SI issues are very hard to see
    with a scope or analyzer.

    When purchasing a logic analyzer the main parameters are speed, channel
    count and channel depth. If you are debugging code, you will want a lot
    of depth. But for other design issues, depth is not likely to be a
    problem. Speed is important, but it can be very hard to connect a logic
    analyzer to busses of 100MHz or above without distorting the waveforms.
    I normally only use a logic analyzer to debug logic issues with a slower
    clock. Anything that works at low speed and fails at high speed will
    more likely need a scope rather than a logic analyzer.

    --

    Rick "rickman" Collins


    Ignore the reply address. To email me use the above address with the XY
    removed.

    Arius - A Signal Processing Solutions Company
    Specializing in DSP and FPGA design URL http://www.arius.com
    4 King Ave 301-682-7772 Voice
    Frederick, MD 21701-3110 301-682-7666 FAX
     
    rickman, Apr 29, 2004
    #9
  10. Nitesh

    rickman Guest

    I am not sure what a Mictor is, but I think you are talking about the
    little clips that can attach a single wire. This is a royal PITA for
    any type of bus. I recommend that you find a pinout (I can give you one
    for the older HP style connector) for the probe connector and connect up
    16 or 32 pins at once. When debugging a complex board, the small effort
    to add these connectors will easily pay for itself in the time you save
    in debug. If you have room on the board and the cost is not a problem,
    you can even leave them in the final design for service work.

    --

    Rick "rickman" Collins


    Ignore the reply address. To email me use the above address with the XY
    removed.

    Arius - A Signal Processing Solutions Company
    Specializing in DSP and FPGA design URL http://www.arius.com
    4 King Ave 301-682-7772 Voice
    Frederick, MD 21701-3110 301-682-7666 FAX
     
    rickman, Apr 29, 2004
    #10
  11. Nitesh

    Al Gosselin Guest

    <SNIP>

    Oh Yeah! Probing.

    The AMP Mictor connector came out in 1996 or so. It was twice as dense
    as the previous solution (pins on 0.1" centers). It is supported by
    both Agilent and Tek (There is some inconsistency in pin numbering,
    but they're basically the same layout).

    Agilent added a Samtec solution in 2000. It was denser, supported
    higher speeds, was full-surface mount, and was much easier to route.

    Tek came out with an elastomeric connectorless solution in '02 and
    Agilent came out with SoftTouch last year. The newest solutions can
    support signals above 1 GHz, with loading of 0.7pF or less.

    Again, spend a lot of time on the web, look at everything and ask a
    lot of questions.

    Alan
     
    Al Gosselin, Apr 29, 2004
    #11
  12. Nitesh

    Eric Smith Guest

    I have never *needed* a logic analzyer, in the sense of not being able
    to solve a problem without it. But on multiple occasions using one has
    saved me a great deal of time. Sometimes problems can be easily seen on
    a logic analyzer that are much harder to spot with just an oscilliscope.
     
    Eric Smith, Apr 29, 2004
    #12
  13. I have never *needed* a logic analzyer, in the sense of not being able
    to solve a problem without it. But on multiple occasions using one has
    saved me a great deal of time. Sometimes problems can be easily seen on
    a logic analyzer that are much harder to spot with just an oscilliscope.[/QUOTE]

    I have never used a logic analyser to debug a micro, but sometimes usueful
    to work out the sequences on digital video processing circuits, or why
    a peripheral (or the software sequence to it) is not working.

    Not yet needed more than 24 channels, sometimes 32 would be nice, but
    can be a pain seeing too many channels if they are not 'bussed' together.
     
    Paul Carpenter, Apr 29, 2004
    #13
  14. Find an analyzer with a probe for your processor; state analysis is
    great, but deciphering long instruction words is no fun. Be prepared to
    spend for a new probe when you change processors.

    Design lands into your boards for the new elastomeric probes. There's
    nothing worse than an unreliable connection to your board.

    My rule of thumb for number of channels is:
    Size of Address Bus used + Size of data bus + size of any other bus +
    all strobes used/required by each of those (read, write, chip selects,
    bus grant/ack, etc), plus 8. The buses can be cascaded together so you
    don't have to look at 40 separate traces.

    In my experience, I have found speed to be much less important than
    memory depth. Murphy's Law demands that the glitch or race condition
    you are trying to find always occurs hundreds or thousands of clock
    cycles from where you trigger, just past the end of the captured data.
    If you can see it, you can always measure it more precisely with a good
    high-speed scope than you will with the LA.

    Tek and Agilent both make fine products. It doesn't matter which you
    get, except that once you have mastered one, you won't want to have to
    learn how to operate another.

    If you are tracing a single nasty problem, you might try renting one for
    a few weeks. You'll have a much better appreciation for what you want
    it to do afterwards.

    --Gene
     
    Gene S. Berkowitz, Apr 30, 2004
    #14
  15. NO, these are not singe-pin connectors. They are high-density connectors
    for high-speed signals: two rows with a GND plane in the middle. The
    signal pins are SMT, the GND connector are thru-hole (and the guy who
    edited the PCB was really annoyed about it).

    We've used the ones by AMP, but there are other manufacturers. Both
    Textronix and Agilent LA probing guides give exact information (and,
    surprisingly, both LAs' probles are pin compatible, even though their
    pin numbering scheme is entirely different). Each carries 32 signals
    plus clock and IIRC, clock gating signals.
     
    Assaf Sarfati, Apr 30, 2004
    #15
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