What is a Logic Analyser

Updated 14 October 2016

Saleae logic pro 8If you’ve ever come across the term ‘logic analyser’ and not been sure exactly what is was, you’re in the right place. Logic analysers are one of the less common pieces of test and measuring equipment, but they’re extremely useful in many different ways.

Simply put, a logic analyser does exactly what its name implies. It’s designed to read the high speed 1’s and 0’s which make up every digital signal, and tell you what they mean. Logic analysers generally fall into two different categories: self-contained units which have a display and miniature computer on board and process all of the data within themselves, or those that connect via USB to a host computer. The self-contained units are generally much larger and more expensive than the USB logic analysers, however both can be equally useful. Logic analysers have a variety of functions and features depending on the model, however the two features which almost every analyser should have is the ability to capture and visualise the data, and to interpret various communication protocols such as SPI, I2C, USB etc. and tell you what data type is being transferred and decode each byte.

Oscilloscope vs. Logic Analyser

Now you might be thinking ‘this sounds pretty similar to an oscilloscope, what’s the go?’, and it’s a fair question. Well there’s several key differences, if you’re interested in learning more, check out our tutorial on DSOs MSOs and Logic Analysers, however to put it simply, an oscilloscope is designed to view input signals, display them in real time and take measurements such as time and voltage. It’s designed for viewing waveforms. A logic analyser however is designed to analyse digital communication. It captures and tracks the logic level shifts between high and low, and can graph these for viewing later and tell you what information is being transmitted around the communication protocol. However these article isn’t a comparison between the two, it’s about what a logic analyser is and why it’s useful.

Logic Analyser Features

Whilst a typical oscilloscope only has a couple of input channels, most logic analysers will have anywhere between half a dozen, to hundreds! Think about a low level digital communication protocol like SPI. SPI is a 4-wire protocol and if you want to check to see that your SPI data is being transmitted correctly, you need to view and analyse the four wires; clock, data in, data out, and slave select. This requires 4 channels which caps out most oscilloscopes, however it’s perfect for logic analysers. They can track all for channels at once, and if you specify the communication type, even tell you what data is being transmitted based on the micro-timing of individual bits.

Something to bear in mind is that there are lots of different ways that logic analysers capture data, and different technologies can cost upwards of $10,000. However for most applications, a high quality USB logic analyser such as the Saleae range is going to be the best bet as you’ll get great sample quality, and because it uses your computer’s RAM to process the data, for most modern computers, you’ll have upwards of 4GB of RAM. That’s a ridiculous amount of data! This means that the logic analyser itself can be quite compact, check out the beautiful shots below of how well laid out the Saleae internals are:

Saleae internals 1Saleae internals 2

To understand this a bit more, let’s take a look at what some high speed digital communication looks like.

Analysing a Signal

To get more of a handle on how logic analysers work, and why they are so useful, let’s look at a real world high speed digital signal. We’ll take a look at the communication of a standard I2C bus. For this example we’ve used an Arduino Mega 2560 and a Particle Photon. The Photon is just sending the string “hello” every 0.5 seconds over the I2C bus, and the Mega is receiving it. We can use the Saleae to tap into the I2C bus and take a look at what’s going on:

Saleae sample capture I2C

This is a screen shot which is viewing the two wires used in I2C communication, we’ve set the communication protocol to I2C so the Saleae knows what it’s looking at, and we’ve zoomed in on the data being transferred. What’s amazing is the automatic annotation that the Saleae Logic has given to the bits, revealing the characters we transferred. How awesome is that!?

Logic analysers like the Saleae range are capable of much more than the simple example we used, but hopefully that gave you an insight into our powerful the logic analyser can be in any workshop.

To check out the full Saleae range, head over to Logic Analysers Australia and take a look at our tutorial comparing the various models and which one is right for you.

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