View Full Version : Would a logic analyzer suffice over an oscilloscope

04-18-2010, 11:19 AM
Hopefully, this post isn't too off-topic for this forum, it looks as though similar topics have been covered here, but not comparing the two in the detail that has helped me come to a decision.

I come from a Physics degree background, and would like to program PIC micro's as a hobby. So I know the basics of how to use an oscilloscope as this was covered in my degree.
I don't get much time to do the PIC stuff, which is why I say would like to.

Past projects have so far been simple LCD and LED circuits, and a simple 1-wire reader.

However, I would like to move to doing a wireless system, including 1-wire, and GLCD. I am not decided as to whether to use Zigbee/MiWI, or an analogue RF chip I have found with UART output. (I do have the PICDEM RF projects for looking at the MiWi stuitability, but the stability and range appear to be too low for what I need I think).

On my current project, I have been having an issue with what I believe is timing when using a 48MHz PIC chip, and trying to drive a 1-wire bus, so I thought I would buy an oscilloscope to try and watch the wire. (The issue is that only 1 of the chips I have [out of 5] works, and then only when attached to the end of a 5m wire. Current setup is that I have the PIC controlling the GLCD, and I want to start getting the PIC working as the 1-Wire busmaster).

So a couple of months ago I attempted to buy a 2 channel digital oscilloscope off ebay. The specs seemed to be OK, for what I thought I needed. However, the item was obviously faulty when I tried to use it. Two months later I have managed to get a refund for the item, and the hassle with the supplier has put me off of that brand altogether. I am not going to name them, as this may be a one-off situation.

Whilst looking for an alternative oscilloscope, I have come across logic analisers, specifically the item at http://www.saleae.com/logic/ has caught my eye, as being a cheap and cheerful device which might really help out with working out why the protocols are not working.
It also fits in the buget I am thinking of, up to 250-300, but lower being a lot more preferable, as this is a hobby I do not get a lot of time for (comes out around 110 currently).

However, my concern is, what if the problem that I am experiencing it capacitance related, or under/over-voltage. This device may not be enough to to determine what the problem is.
Another down-side, is that currently the software will not run on my laptop (linux), but will run on my desktop (XP, and I have quickly tested it in the simulation mode).

Could someone with experience in both types of devices, provide a bit of assistance with the decision? Currently my 'test' tools go as far as an in-circuit debugger (ICD2), and a volt/ampmeter combined. I do a little PCB etching via ferrous chloride, but most of my stuff at the moment is at the breadboard stage.
Thanks in advance,

04-18-2010, 12:20 PM
If it came down to choice between an oscilloscope and a logic analyser I would go for the oscilloscope every time. You will get much more use out of it than out of a logic analyser. When I worked in electronics an oscilloscope is something you would use every day whereas a logic analyser is something you would pull out on the rare occasion you were having timing or protocol problems. But having said that the unit from Saleae does look intriguing and at the price you can't really go wrong. In fact I've had a play with their software and ordered one last week, it should arrive some time next week with a bit of luck (if the planes are flying).

But back to your problem, my advice is get an oscilloscope now, wait for Saleae to bring out their Linux software and then think about whether you can justify a logic analyser.

04-19-2010, 08:41 PM
Thank you for your answer. You have confirmed what I really thought, my heart says Logic Analyser, my head really says Oscilloscope and save up.
Thank you again.
Do please let me know what you really think of the device once you receive it. To me, the only off-putting thing is that it might just be a small aluminium box that doesn't look like 100 worth of kit!

04-19-2010, 11:41 PM
www.saelig.com (http://www.saelig.com)

they have really good and inexpensive electronics equipment - scopes (PC based and desktop) and logic analyzers as well.

i would suggest buying from them, i own this scope and am very pleased with it.


almost forgot!

you should consider buying an MSO scope (mixed signal) - generally they come with at least 2 traditional scope probe channels and then a 16-bit digital logic analyzer bus. you can trigger between digital and analog as well, which may come in handy.

they are very expensive - but if you can hold out long enough and save it will be worth it.


04-20-2010, 07:34 AM
To me, the only off-putting thing is that it might just be a small aluminium box that doesn't look like 100 worth of kit!

You're absolutely right about the box. It is just a few ICs inside and not worth such high price.

However, the real worth of any logic analyser(LA) is 95% the HOST software running on your PC. It retrieves the useful data at the correct moment and presents it in a form that tells you what actually is happening. It saves you precious time in trouble shooting.

LA or scope cannot replace each other but part of their usage overlaps, meaning you can solve some of your problems with either.

For very tricky problems only a LA or scope can give you a definite answer. I agree with AlexR that 99% of time one would use a scope as 99% of the problems are of the type that can be solved using a scope.

You still have to choose between a conventional scope of a few ten MHz BW or a digital scope and there are big differences in how they operate too.

People buying LA because that was the exact moment in time a LA can really save his butt and sure enough the LA will almost be forgotten and be stashed on a shelf afterward. Few months old LA can often fetch 90% of its selling price on eBay so selling them after use can be an answer too.

04-20-2010, 09:54 AM
Since you are going to program PICs. Get a PICKIT 2, as it can be used as 4 channel logical analyzer. I do not know if this future has been included in the PICKIT 3 update. Microchip has said they will do so.

05-08-2010, 09:26 AM
Thanks for all of the replies, I have been away for a while.
I am not sure at the moment whether the pickit is the right way to go. The V2 seems to do the logic part, but apart from this, it doesn't appear to do anything more than the ICD2.
I might be looking at the ICD3 or pickit 3 at a later date, as the ICD2 isn't guaranteed any more to be compatible with the new chips coming out.
The pickit 3 doesn't mention the ability to be a logic analyser. Also, as programming/debugging would probably go alongside tweaking logic, I am not sure that combining these two features is really much help.
I think at the moment I am going to go for a cheap analogue oscilloscope and a separate analyser. I can't justify the cost for a worthwhile digital oscilloscope.