Electronics > Beginners
Which logic analyzer?
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roncromberge:
Hello,

I don't know if these logic analyzers and YouTube movies already mentioned in this thread? But the are worth looking at.

https://youtu.be/dobU-b0_L1I

https://youtu.be/xZ5wKYnCNcs



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roncromberge:
Just another video.

https://youtu.be/4FOkJLp_PUw


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rstofer:

--- Quote from: tggzzz on November 10, 2018, 09:24:00 am ---I am told the Rigol will only decode what is on the screen. I suspect that makes it of little use with long data comms and/or for triggering on a specific message.

--- End quote ---

That is true!  The thing is, I just want to see a single frame.  I am interested in the state of the clock with CS* goes low, the subsequent data transitions relative to the clock and the fact that all the data has shifted before CS* goes high.  I don't need to decode "War and Peace", just a single short frame to verify timing.  Everything else can be done with printf().

After I get the LA trace, I can compare the actual timing to the datasheet.  To do this accurately, I probably need to sample at somewhere around 10x the signal rate.  That's why I built the 200 MHz Sump analyzer.  One of my projects is clocking SPI at around 12.5 MHz.  I was in a hurry, I guess.
tggzzz:

--- Quote from: rstofer on November 10, 2018, 04:37:15 pm ---
--- Quote from: tggzzz on November 10, 2018, 09:24:00 am ---I am told the Rigol will only decode what is on the screen. I suspect that makes it of little use with long data comms and/or for triggering on a specific message.

--- End quote ---

That is true!  The thing is, I just want to see a single frame.  I am interested in the state of the clock with CS* goes low, the subsequent data transitions relative to the clock and the fact that all the data has shifted before CS* goes high.  I don't need to decode "War and Peace", just a single short frame to verify timing.  Everything else can be done with printf().

--- End quote ---

Just so. That's signal integrity, which is best done with a scope. If that's all you want then there's little need for an LA.

LAs excel with complex arm/trigger/capture options, e.g. if X occurs more than 52 clock1s after Y then wait 12ms and capture all 64 bits on the databus on the rising edge of clock2 when Z is inactive. The extent to which that can be done with a scope would be very scope-specific, or with printfs it would be application dependent.
rstofer:
I built the Sump.org LA because it offered layers of triggering across all 32 channels.  If this, then if that, then if the other thing, occurred in that order, start sampling.  Very powerful!

The FPGA board I was using at the time had 50 pin headers and it was possible to bring out a full 32 signals plus the state clock.  Since everything in the logic was referenced to the state clock, this was about as good as it could get.  The logic was running at 50 MHz and the LA ran at 200 MHz (16 channels) or 100 MHz (32 channels) (IIRC).

The Java client application runs on Linux (easier) or Windows (a PITA to set up).  All in, the tool was quite helpful.

But the thing is, I was looking at logic that might be running a million cycles in as the OS was trying to boot and getting hung.  I had to gin up logic to create trigger events to see what was happening.  It was all very educational.

What did I learn?  One-hot encoding doesn't work well with LAs because the state word is too wide (over 100 bits for my project) so I had to create additional signals within the FSAs.  Integer encoding is easier to debug but requires additional decoding logic in the project itself.  I stayed with one-hot and created the triggers.  A scope is definitely not the right tool for this kind of thing.  Maybe an MSO but those get pricey and they aren't a really good substitute either.

I don't see either tool, LA or scope, as being acceptable replacements for the other.  They do different things in different ways and both have a place on the bench.  But the order in which they are needed may favor buying one before the other.  Price is also a factor.  If a LA will do the job, it will likely be a lot cheaper than a scope with decoding.
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