Logic analyzers

Often when developing or debugging radio-electronic devices consisting of several components (microcontroller, ADC, I2C memory, etc.) I want to look at the communication of these components among themselves. To finally understand why everything is written “correctly,” but nothing works. In general, you can use a digital oscilloscope for this, but 4-channel oscilloscopes are quite expensive, and this is not very convenient. Much nicer such a problem is solved using a logic analyzer. A logic analyzer is essentially similar to an oscilloscope, but if the main task of an oscilloscope is displaying a change in an analog signal with which it does a great job, then the goal of a logic analyzer is to record a digital (but not always only digital) signal with its subsequent analysis in convenient programs.

Below I will describe the experience with logic analyzers Logic Pro 8 from Saleae at the cost of about $ 480 and DSLogic Pro from DreamSourceLab (most likely a copy) at the price of $ 99 I got for $ 73.
To whom it became interesting welcome under kat.

Specifications

Logic analyzers differ from each other mainly in the number and type of channels, the presence of a buffer, and the frequency of sampling.
A small comparison table with the characteristics:

	Logic Pro 8	DSLogic Pro	Description
The number of digital channels	eight	sixteen	The more the better, but for the year of use more than 4 while it was not really needed
The number of analog channels	eight	0	For an analog, an oscilloscope is preferable, but sometimes it is convenient to see what is going on not only in the digital on the channel, but also in the analog.
Interface	USB 2.0 / USB 3.0	USB 2.0	USB 3.0 is preferable if you have it of course. Although the moment is controversial, I will describe below why.
Sampling frequency of digital channels	500 MS / s - USB 3.0 100 MS / s - USB 2.0	400 MS / s - for 4 channels 200 MS / s - for 8 channels 100 MS / s - for 16 channels	The more the better, but in the case there is one nuance, but more on that below.
Analog Channel Sampling Frequency	50 MS / s - USB 3.0 10 MS / s - USB 2.0	-	The bigger, the better.
Voltage range digit / analog:	1.2V - 5.5V / -10V - + 10V	0V - 5V / -	Here Logic Pro 8 has a choice of three threshold voltages of 1.2V / 1.8V / 3.3V. DSLogic Pro has a choice from 0V to 5V in 0.1V increments. It also has an overvoltage protection of -30V - + 30V.
Internal channel resistance	1.8 MOm	250 kOm	The more the better, the less turns out to be on the circuit under study.
Buffer	-	256Mbits	The more the better, this parameter determines how many samples can capture the analyzer without transferring them to the computer. The thing is extremely useful especially for UBS 2.0
Sampling depth	unlimited in theory	16MS per channel	The more the better, Logic Pro 8 uses PC memory. The depth of sampling can be very large, but DSLogic Pro also suffices.

In general, in many respects, Logic Pro 8 has an advantage, especially in terms of the analog part. However, the lack of a buffer has a negative effect, firstly, the speed drops twice when using 4 channels, and 4 times when using 6 or more channels. But this is not all, since the speed is tightly tied to the USB bus, incidents happen if this bus is loaded, sometimes it is enough to plug the UART cable into one USB hub so that the program curses and says that the USB speed is not enough and the capture speed will be reduced. Not that it happens often and strongly hinders, but it’s worth remembering about it.
Of the features of DSLogic Pro, we would like to note the presence of an external line of the clock, and signals for external trigger input, and trigger output.

Appearance and equipment

Logic Pro 8 Kit:

• analyzer itself
• two connectors with multi-colored wires, 4 channels per connector
• 16 clips for connection with the legs of the microcircuit, wires, etc.
• USB 3.0 Cable USB-A to MictoUSB 3.0

DSLogic Pro Kit:

• analyzer itself
• 16-channel wide connector plus additional signals
• 16 multi-colored clips for connecting with the feet of microcircuits, wires, etc.
• USB 2.0 Cable USB-A to MictoUSB

Comparison

If we compare the equipment, it is approximately the same. However, in my opinion, the connector with wires for Logic Pro 8 is much more convenient and pleasant to handle, the wires are softer and more flexible. Clips at Logic Pro 8 is several times more convenient. First, they have two pins for connecting to the analyzer wire, which is convenient, and secondly, the pins for connection are on the side, which makes it easier to hook the clips to the wire already connected to them. DSLogic Pro has a pin on the top, which makes it difficult to connect at times, although the clips are no different in terms of flexibility, but the overall quality is still lame.

Software

Whatever a good logic analyzer is, without good software there will be no convenience in working with it. Good software should quickly display captured data, allow you to watch their characteristics such as frequency, duty cycle. And it is very good that he would be able to decipher well-known protocols. It is inconceivably convenient to indicate where the I2C legs are and see exactly what was happening on the bus from the point of view of the I2C protocol, and not sit and scratch the head peering and trying to figure out whether it is all right and what address was written here. Each analyzer is equipped with its own program:

Saleae Logic

Saleae analyzers are equipped with Saleae Logic

Multiplatform program, all Windows versions are supported starting from XP (a year ago there were problems with Win 7 and higher, the new version of Win 7 works without complaints, did not test above), Mac OS X 10.7 Lion + installed and worked without any complaints , Ubuntu 12.04.2+ didn't work for me (I tested it a year ago on the old version of the program now it can work), but I didn't really try.

It supports a large list of protocols for the analysis of I2C / SPI / UART / CAN, work without complaints. It is possible to capture both by manual start and by trigger on one of the channels. Of the drawbacks on long samples, it starts to slow down quite strongly.

Of the features of the program, it is worth noting the possibility of management via a telnet-like protocol, the presence of an SDK for writing its own protocol decoders. Protocols are written in C ++, which complicates the process of writing them.

DSView

DSLogic Pro comes with the DSView program:



Multiplatform Windows support program also starting from XP, Mac OS X 10.11.4, Linux (the program is in the source). The program is superior in capabilities to Saleae Logic, there are more advanced triggers with support for several channels at once, the list of protocols is quite extensive. But here the developers have gone further and the protocol can also be used to add the ability to decrypt work with a specific implementation of this protocol. For example, you can choose the I2C protocol and the implementation of the 24CXX and then immediately read what exactly happened on the bus from the point of view of this chip. Both protocols and their implementations are written in Python, which in theory should simplify writing and debugging. Brakes in the work was not observed.

It is also worth noting that the project is partially OpenSource and the source code of the software and the HDL parts are uploaded to GitHub . And on the Wiki is a schematic. Therefore, it may soon be a bunch of clones of this device (maybe I also have a clone).

Simple testing

For a simple test of the analyzers, the DE0-Nano-SoC dev was used on which the meanders from 200Mhz were generated; each next one was obtained by dividing by 2 previous (200/100/50) I will try to capture them with the available analyzers.

The DSLogic Pro managed to capture 200Mhz / 100Mhz / 50Mhz below, and so it is clear that everything is going fine. To say that 200 and 100 were captured ideally it is impossible sometimes there is a slight loss of frequency and duty cycle, but on the whole it is normal, by 50 everything is already perfect.


Seleae normally could not remove 200Mhz and 100Mhz, but 50 Mhz is already going perfectly.

And yet, I would not be serious about such measurements, 200Mhz is probably a brute force measurement, but it was worth a try.

For the experiment we will try to capture the data transmission over I2C (the transmissions are different, therefore the data do not converge).

Everything is perfectly readable, it is clear where what event occurred, what addresses and data were transferred. No fuss.

Same for CAN:

The result is the same as in I2C everything is clear for the minimum amount of time.

findings

Both logic analyzers do an excellent job with their tasks. In spite of the difference in price, I would not have been able to lead someone, they have both advantages and disadvantages. On the side of Saleae, the appearance, convenient wires, high-quality clips, USB3.0 and analog channels, is it worth giving $ 400 a difficult question, if there is a possibility then why not. On the DSView side there are more channels, an input for an external clock and triggers, a buffer inside the analyzer, OpenSource software, and a price. Both of them can greatly facilitate and speed up the work of setting up their own or others' schemes, and the study of third-party protocols.

If you have questions - ask.

Ps. We have been using Seleae for about a year, there were only problems with early versions of software for Win7 and higher, DSView was bought for personal use about a week ago and there are no complaints yet, but the usage time is small ...