Author Topic: How many channels do I need to decode any serial interface with oscilloscope?  (Read 4922 times)

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Offline Galenbo

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... SPI for example uses 4 lines,...

I am currently working on an SPI (=serial) problem, and it's more parallel than you think.

I try to connect 3 different modules (ETH, GLCD, MCP) on the same SPI "port" of a PIC processor, the way it's theoretically described for ages in every book. Mosi, Miso, Clock. Finished.

in practice:
-every SPI module needs his own CS_EN pin.
-to rule out problems and easy time-based debug, I chose to give every module temporarily it's own RST pin.
-for some reason, the GLCD has an AO pin: data or command. You can drive the backlight led with pwm.
-the ETH and MCP module can give feedback on an interrupt pin.
-to control start/stop sampling of the analyser, I control an output bit in my program on suspicious places

This gives:
3-clk, miso, mosi
3-cs0 cs1 cs2
3-rst0 rst1 rst2
2-AO, led_pwm
2-int0 int1
1-bit for start/stop analyser

"only" 14 channels, to debug a "serial" protocol, witch I consider easy and basic. (and shitty)
I have a 8-channel analyser witch is enough for this, but less is really not workable.

You may also consider Saleae Logic-products in addition to the oscilloscope. The Saleae Logic-products are very handy and can really save your day. There might be some other similar products in the market, but I have used the Saleae Logic-devices in my professional life with great success.
+1, due to their "sold out" at the time I bought the cheapest $10 replica, that device seems to replace everything in the -y2010 -500$ market.

After this experiences, I would never choose a scope with integrated analyser, except maybe a $10K scope with a $4K analyser, for a $50BN project, witch is far away from my abilities and needs at the moment.

The only need to get it integrated I see at the moment is the ability to see synchronisation between analog and digital, but in my accuracy and frequency range this can easily be done by triggering one device with the other, or record 1 channel with both devices the same time.
« Last Edit: July 23, 2016, 01:14:05 am by Galenbo »
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Offline Galenbo

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... with built in 16 channel Logic analiser. Saleeaeie is a toy, with other USB logic analisers you lose time instead of working.
Please enlight me on this.
 
-What I do not mean: You are wrong, I do not agree, you know nothing,...
-What I mean: Can you give me some personal examples and situations where you clearly saw the differences and where the better soft/hard where really worth the money? Where did the others fail or made you loose a day?
If you try and take a cat apart to see how it works, the first thing you have on your hands is a nonworking cat.
 

Offline NANDBlog

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... with built in 16 channel Logic analiser. Saleeaeie is a toy, with other USB logic analisers you lose time instead of working.
Please enlight me on this.
 
-What I do not mean: You are wrong, I do not agree, you know nothing,...
-What I mean: Can you give me some personal examples and situations where you clearly saw the differences and where the better soft/hard where really worth the money? Where did the others fail or made you loose a day?
I was working a lot with LAs when I was working with ADCs, switched front end. So if you dont see the analog signal, your conclusion was wrong. Also, you cannot quickly check that signal integrity is correct. The reason that the LA is connected to a system is 90% because something went wrong. Now, with an DSO, you dont usually have enought channels or capabilities to trigger correctly on a bit error, or CAN error frame or anything like, with an USB LA you cannot see what is causing the issue. I had this with digital isolators. The USB LA told me that everything is fine, when in reality, it was  signals from the glitchland. When you need a USB LA or a MSO... There might be a very fine line. I think it is better to spend the company's money on something that I'm sure will do the job, than on something that "might be enough".
And take a look at the list prices (farnell) for scopes, like the MSOX3024T (personal opinion to be minimum for electronics related work at the workplace). It is the same price as the DSOX3024T. But check other price. DPO2024B and MSO2024B has 600 EUR difference. It is hardly justifiable to buy just a DSO for work these days.
 

Offline suicidaleggroll

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FYI - the newer Saleae models also do analog sampling.  It only runs at 50 MSps so it can't do high speed lines, but it's more than enough to check for signal integrity on I2C, UART, CAN, and other <5 MHz digital signals.
 

Online JPortici

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SPI is shitty?
not really.
it's dead simple.
it can be goddamn fast.
itrather easy to make automated transfert with dma or internal buffers and the right peripheral.
there are not all simultaneously verified for many other of the popular transfer methods. the downside is that you need a separate chip select line for each peripheral, i'll give you that
(hint to reduce number of pins: demux with inverted outputs and enable)

+1 for a MSO, get a proper tool. I would use a LA only if i had to check differential lines or a parallel bus or i have to keep track of more than 2 analog channels, otherwise i always use the analog channels and the decoder, i need to see what the hell is really going on, not ones and zeros.
and even in that case the keysight 3000 would be the starting point, an instrument one can trust with only an hint of skepticism.

another one i would pick is the picoscope because the advanced triggering comes out of the box.. that's the one i use at work, at the time it was the only scope that offered SENT decoding without asking for your future son and grandson as a collateral

.. by the way, how good is the GWI GDS2000A Series logic analyzer?
« Last Edit: July 23, 2016, 03:55:09 am by JPortici »
 

Offline Kilrah

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I was working a lot with LAs when I was working with ADCs, switched front end. So if you dont see the analog signal, your conclusion was wrong. Also, you cannot quickly check that signal integrity is correct.

So you had a particular application where an USB LA was not enough, fair enough, but that's no reason to say it's just "a toy".

I've had a Saleae device for 5 years (and just bought a new one after it was stolen), and it's always done a perfectly fine job of letting me debug/analyze things at protocol-level which is what I typically need. When I have to debug logic signals I'll typically do a 5-min check with the scope to see if signal integrity is OK just in case, once confirmed I can forget about it and go on spend my couple of days of protocol coding/fixing for which a scope would just be unnecessary and excessively bulky.
 

Offline Galenbo

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I was working a lot with LAs when I was working with ADCs, switched front end. So if you dont see the analog signal, your conclusion was wrong. Also, you cannot quickly check that signal integrity is correct...
You are clearly on a whole other level than me.
But for checking signal integrity, what is the difference between me connecting my probe+LA and you connecting the probe+LA cable?

And take a look at the list prices (farnell) for scopes, like the MSOX3024T (personal opinion to be minimum for electronics related work at the workplace). It is the same price as the DSOX3024T. But check other price. DPO2024B and MSO2024B has 600 EUR difference. It is hardly justifiable to buy just a DSO for work these days.
You are right about the $4K scopes, it makes hardly a difference. The "leather option" is cheaper for a Mercedes than for a Lada.
But for my 4ch $1K scope (2009), the price for a model including a LA was double.
On top of that, I was somewhat scared the internal LA would be limited, not be upgradeable, difficult to control,... compared to a PC-based device.

A disadvantage I have, is that my scope doesn't have a Pass/Fail output, to connect to my LA. The newer 1054Z has this on the back.

But the topic starter seems to have his reason for not wanting a PC-based LA.
Maybe the Topic starter can say what the budget for 1 MSO scope is?
« Last Edit: July 23, 2016, 11:19:37 am by Galenbo »
If you try and take a cat apart to see how it works, the first thing you have on your hands is a nonworking cat.
 


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