Help Picking the Right Logic Analyzer - Page 1
Forum section: electronics / beginners

author: thakidd
author_rank: Regular Contributor
timestamp: October 26, 2010, 01:06:25 pm
content: Hello all. Well I have been having a lot of fun with my Rigol DS1052E but ran into a problem the other night debugging a serial issue between an AT&T Motorola GoPhone and a Max3232. I did manage to solve the problem eventually by pulling some kung fu with the Rigol's run/stop button, but am wondering if I could have saved a crap load of time if I had a logic analyzer on my desk.To clarify, most of my work is done between MCUs of the AVR variety and attached 'peripherals'. I am wondering what kind of logic analyzer I should be looking at. I am on a limited budget but would prefer to buy something better now than have to re-buy later. I am also about 3 to 6 months away from jumping out of the Atmel 8-16bit stuff and possibly playing with the Atmel ARMs.I have looked at all of the USB logic analyzers and, after some research, found seriously issues with their bandwidth. I have also checked out theOpen Bench Logic Snifferwhich looks like a better option than USB type stuff (@ +70MHz) and it appears that GadgetFactory has come out with theButterfly Onewhich does 100MHz and possibly 200MHz.So what do you recommend? Is there something nice I might build myself (i.e. +100MHz; better than the Seeed OLS)?Thx for your suggestions in advance.

author: slburris
author_rank: Frequent Contributor
timestamp: October 26, 2010, 02:18:21 pm
content: So the first questions you need to ask are:1) How many signals do you want to monitor simultaneously?2) What's the highest clock rate you need support for?3) Do you want stuff that does protocol decoding for you, i.e. SPI, I2C, and the like?4) Standalone or use a PC?5) What voltage levels do you need to look at? TTL, 3.3V CMOS, 2.5V CMOS, something else?Scott

author: thakidd
author_rank: Regular Contributor
timestamp: October 26, 2010, 03:04:57 pm
content: 1) 32 signals2) Not sure..currently playing with AVR Megas but am considering moving to AVR ARM3) SPI, I2C, etc would be quite nice as I do use them a lot4) PC preferable...may not be necessary though5) All of those...I believe I would never analyze something over 5V or so, but I could be mistaken.TBH, I am not sure what I need. At this point in development, I would say I need something simple to diagnose 8-16bit MCU peripherals...but moving to 32bit is a whole new world to me that I am just starting to explore. I can tell you that currently a logic analyzer capable of max 200-100MHz on 16-32 channels should work fine. Not sure though if that will suffice for the future. I guess I am looking for a device with a bit of growing room when it comes to USB, etc...at a reasonable cost of course.

author: slburris
author_rank: Frequent Contributor
timestamp: October 26, 2010, 04:38:35 pm
content: The big problem with the open source logic analyzers I've seen is that they are littlemore than FPGA's with memory attached. That's fine if what you are working onfits nicely with I/O voltages of the FPGA. If it doesn't, what do you do? I'm notsure how reliable just exposing the FPGA pins with no input conditioning is going to be.Given your clock requirements, none of the USB analyzers that stream data to yourPC's memory are going to cut it. You need stuff with onboard memory for capturethat can later be sent to your PC.Have you looked at the Intronix LogicPort? 34 channels, sampling to 500Mhz,decoders for CAN, I2C, SPI and RS232. The logic threshold can be adjusted from-6 to +6 volts and I think it can tolerate +- 40 volts without breaking.http://www.pctestinstruments.com/index.htmThe downside is that capturing is into an internal block RAM on an FPGA (for speed)so you only get 2K of memory. However, only transitions are stored, so that cangive you a lot of effective compression, depending on what you are looking at.There are also some cheaper clones out there, but I've heard the software isn'tnearly as good as the Intronix software:http://www.thefamousbrandsoutlet.com/test-measurement-instruments/pc-usb-logic-analyzer/pc-based-usb-logic-analyzer-la5034.htmlhttp://circuitspecialists.com/prod.itml/icOid/8251Alternatively you could take one of the open source FPGA boards, add some sortof input conditioning to let you adjust the logic thresholds and roll your own.Scott

author: TheDirty
author_rank: Frequent Contributor
timestamp: October 26, 2010, 07:26:36 pm
content: Do you really need that much bandwidth for a logic analyzer? I have a USB one and I've never been limited by its bandwidth. 99% of its use for me has been interface and protocol decoding. I suppose if I ever had a problem with the signals going too fast I can always artificially slow them down in code in order to debug them.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: October 26, 2010, 08:25:41 pm
content: I have a Saleae Logic and quite like it. It might be a bit limited with advanced triggers, and only be 8 channels, but the unlimited sample depth is valuable. Modern microprocessors and interfaces tend to be serial rather than parallel, so you don't need 16 channels to deal with a 16 bit processor.

author: SuperMiguel
author_rank: Regular Contributor
timestamp: October 26, 2010, 10:06:07 pm
content: Whats the difference between a scope and a logic analizer?

author: dimlow
author_rank: Frequent Contributor
timestamp: October 26, 2010, 11:44:52 pm
content: @thakid i had a very similar problem to you, so i got out my trusty and crap 32 channel 500 Mhz logic analyser. Only to realise later that all i really needed was three channels at around 800Kh sampling rate, i mainly work with PICs and most on the time, do not need a LA. But in the times that i do, the requirement for a high end LA is never met. One day soon a may knock up my own LA with a pic and some simple software. Heck the Pickit 2 has a built in La, maybe that could have done the job for me ? problem is i was using that to program the chip.Look very carefully before you spend lots on a LA, my best advice is to not take too much notice of the hardware side, but make sure you look into the software and triggering,that's the important bit in my view, crappy software makes for a useless device.

author: McPete
author_rank: Regular Contributor
timestamp: November 02, 2010, 10:06:57 am
content: Quote from: SuperMiguel on October 26, 2010, 10:06:07 pmGenerally, a logic analyser only provides digital indication. It'll show you the data on your lines and pulse widths, but you won't be able to see if your square wave is distorted or non-compliant with a certain protocol due to ripple/debouncing problems, for example. For the most part, they're pure digital indication. Like a scope, they're available as both stand-alone and computer-connected units.They do tend to have more channels than your average scope-the least I've seen is the Saleae with 8 channels, but they can range up to hundreds of channels for bench mounted units.Like a scope though, attenuation via the probes and inputs will change the characteristics of your circuit. You may encounter the infuriating situation where a circuit doesn't work when you're not measuring it, but as soon as you've connected up your test equipment, it starts workingI've probably missed a lot there, but that's my impression.P.