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Thread: USB FPGA Logic Analyzers: Worth Investigating?

  1. #1
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    Default USB FPGA Logic Analyzers: Worth Investigating?

    Let me start by saying I have never seen a logic analyzer, stand-alone or PC-based, in operation. It would be a stretch to say I need one. But lately I have seen, on eBay for example, a new class of FPGA-based analyzers that are small, lightweight and use an USB bus to communicate with a PC, which runs the analytical software and displays the results.

    I see them on eBay for less than $100. I know there is public domain software that [runs] these devices. I just don't know what the limitations are. I am leaving aside the issue of how useful they would be but I'm sure someone has an opinion about that as well. If so, feel free to chime in.

    Thanks,

    -CH-

  2. #2

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    depends what you need it for. The USB ones are streaming, they dont do real 'analysis' like hardware does. USB ones lack bandwidth. If you want to capture i2c, spi etc they are fine (well some SPI can be too highspeed for a lot of them), can do protocol decoding most of the time.

    the real hardware non-usb ones have proper state analysis and timing, you dont get this in the usb stuff. Mine has 136 parallel high speed channels.

    they are really two very different kinds of devices for different things.

    Also, most of the USB ones steal the Saleae software, you should check Sigrok to see if the one you want to buy is supported by them, that way your not stealing Saleae stuff (The exception is the DSLogic+ which uses a modified version of Sigkrok which is open source).

    oh, be aware a lot of the USB ones dont clamp voltages so poking real RS232 can be hazardous! they are often 0-5v things and not -23V to +23V!

    If you want to poke the bus on a Z80, C64 or something. You need a real hardware LA.
    If you want to poke TTL RS232, i2c, a usb one is usually fine.

  3. #3
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    I use a DSLogic all the time. It's USB, has 16 input channels, and will accurately do 200 MHz with 64 MBit of capture RAM. Will also do streaming at lower aggregate data rates. It's USB and has none of the limitations called out above.

    Inside, it's the same hardware architecture as the rest of the FPGA USB analyzers: Spartan 6, 64 Mbit of DRAM, and a FX2LP. Could use open sump firmware with it. But DSLogic is a bit better. The only main difference is most of the lower priced options don't have a RAM chip. They only have internal EBR. So your capture depth at high speed is limited.

    -Alan
    "Good engineers keep thick authoritative books on their shelf. Not for their own reference, but to throw at people who ask stupid questions; hoping a small fragment of knowledge will osmotically transfer with each cranial impact." - Me

  4. #4

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    A logic analyze requires a connecting a number of things to be useful. Often by the time you have thing connected, you'd have been better off with a scope. In my many times fixing old computers, I've never used more than a two channel scope with delayed sweep. One time I used a logic analyze was to find a logic design error. Even then, I found the logic analyzer wanting in more ways to qualify the particular even I wanted to see happening.
    Still, there are many times an analyzer would be handy. A computer can do a lot of tings really fast. You may want to capture something that happens after 2298 clock cycles into the boot code. Even there, I find it more useful to have a EPROM programmer, A few machine pin sockets, to create adapters, some EPROMs and some basic knowledge of the machine code used on that machine. Usually test code can be a short as 10 or so instructions. One can use it in ways that are not practical, using the normal boot code.
    I'll admit that it takes year of knowledge to be useful but no more than to be useful with a logic analyzer. Many analyzers have special pod to handle particular processors. With these one can decode instructions executing on the hardware. These can be quite useful debugging a new design. You won't find much of these features on a cheap analyzer.
    If you've reached the point in understand the advantages and uses of an analyzer, you my find that for old computers you may find other ways to deal with them. The cheap ones are not to useful at capturing glitches, either. For a design, this can be a frustrating problem.
    Dwight

  5. #5

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    A logic analyze requires a connecting a number of things to be useful. Often by the time you have thing connected, you'd have been better off with a scope. In my many times fixing old computers, I've never used more than a two channel scope with delayed sweep. One time I used a logic analyze was to find a logic design error. Even then, I found the logic analyzer wanting in more ways to qualify the particular even I wanted to see happening.
    Still, there are many times an analyzer would be handy. A computer can do a lot of tings really fast. You may want to capture something that happens after 2298 clock cycles into the boot code. Even there, I find it more useful to have a EPROM programmer, A few machine pin sockets, to create adapters, some EPROMs and some basic knowledge of the machine code used on that machine. Usually test code can be a short as 10 or so instructions. One can use it in ways that are not practical, using the normal boot code.
    I'll admit that it takes year of knowledge to be useful but no more than to be useful with a logic analyzer. Many analyzers have special pod to handle particular processors. With these one can decode instructions executing on the hardware. These can be quite useful debugging a new design. You won't find much of these features on a cheap analyzer.
    If you've reached the point in understand the advantages and uses of an analyzer, you my find that for old computers you may find other ways to deal with them. The cheap ones are not to useful at capturing glitches, either. For a design, this can be a frustrating problem.
    Dwight

  6. #6
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    Well, thanks as always to those who responded.

    When I bought my first scope it was used, analog and slow; it had one channel and the first time I tried calibrating the probes I thought there was something wrong because all I could see were the horizontal traces, like a series of dashes across the graticule. I didn't know enough to turn up the brightness so I could see the vertical trace.

    When I thought about a replacement I briefly considered another used machine, maybe something in the Tektronix line with faster speed and dual traces. The more I investigated the more hesitant I became to buy a used scope, no matter whose name was on it. I chose a modest dual-trace DSO instead.

    I think that will have to suffice; I realize I'm not there yet and maybe never will be. I'm not sure I will live long enough to acquire a new discrete, hardware based logic-analyzer, plus its attendant probes, learn how to use it effectively and then learn when to use it and when not to.

    Thanks for saving me the trouble.

    -CH-

  7. #7
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    Used logic analyzers are usually pretty good bargains, provided that you can find one with all of the probes and pods "Screen lights up with display" isn't good enough. Usually, one surplused from an equipment rental outfit will be in good condition.

    Some logic analyzers include an oscilloscope, as the basic LA has most of the circuitry anyway.

    While setup is an onerous task when you're dealing with lots of signals, sometimes you just want to capture a single signal. You could use a modern DSO, but an LA will do the job as well.

    Some older LAs have "personality modules" that can disassemble code for a processor on the fly. I've never found that to be particularly useful, but YMMV.

    My .02, at any rate.

  8. #8
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    Quote Originally Posted by Chuck(G) View Post
    Used logic analyzers are usually pretty good bargains, provided that you can find one with all of the probes and pods "Screen lights up with display" isn't good enough. Usually, one surplused from an equipment rental outfit will be in good condition.

    Some logic analyzers include an oscilloscope, as the basic LA has most of the circuitry anyway.

    While setup is an onerous task when you're dealing with lots of signals, sometimes you just want to capture a single signal. You could use a modern DSO, but an LA will do the job as well.

    Some older LAs have "personality modules" that can disassemble code for a processor on the fly. I've never found that to be particularly useful, but YMMV.

    My .02, at any rate.
    $2.50, at 2-cents-a-word, but wisdom is priceless . Thanks; I'll keep my eye open. A particular make or model most favored?

    -CH-

  9. #9

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    There are different approaches to USB logic analyzers.

    The LogicPort (http://www.pctestinstruments.com/) for example tries to do an entire capture to its internal buffer, and then transmits it to the PC at USB 2 speeds. This allows it to do 34 channels at 500 MHz, but only for 2K samples. That buffer can be fine, but it often requires careful trigger configuration to get the data you want to see. I think their software is very nice.

    The Saleae (https://www.saleae.com/) approaches things differently. They are more of a we'll stream forever type of solution. With USB 2 that was around 8 channels at 24 MHz or so. With USB 3 many times faster. They have 8 and 16 channel models, but the less channels you enable, the faster you can sample. No worries about setting up a trigger, you can just capture and let it run for a long time and then dig out the data you want. Their software is excellent.

    I've tried a few other USB LA's and most of them were disappointing in software especially, but sometimes in quality as well.

    For oscilloscopes the story is much the same, there are a lot of ones out there that disappoint in both software and quality, but there is one USB oscilloscope company that does it right : Picoscope. British company and their products / software are excellent.

  10. #10
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    Quote Originally Posted by clh333 View Post
    $2.50, at 2-cents-a-word, but wisdom is priceless . Thanks; I'll keep my eye open. A particular make or model most favored?

    -CH-
    HP/Agilent/Keysight pioneered the field. Still some of the best. Buy the best one you can find.

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