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Review: At Last . . . An Affordable Logic Analyser

If you ever yearned for a logic analyser you will know how expensive they can be, with professional units costing $5,000 or more. The good news is that a new crop of more affordable devices is becoming available and a good example of these is the Logic, from US company Saleae. It costs just US$149 and even after Australian dollar conversion that price is very attractive.

Review by Geoff Graham

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Fig.1: this screen shot shows a typical capture without analysing the data streams. You can type your own labels to identify the signals into the boxes on the left. The buttons down the left let you select the trigger conditions. Logic can monitor eight signals. Each input is supplied with gripping type probes to hold onto IC pins. The colours used for the wires matches the colours used in the software to identify the inputs.

What do you get for your money? Logic has eight channels that can record millions of samples at up to 24MHz. You also get a software package that includes decoders for I2C, SPI, serial (eg, RS232) and other data streams. It communicates with your computer via USB and it uses your computer to do the analysis and display.

Logic analysers

If you are new to digital circuits then you may not know just what a logic analyser is and why you might need one. Simply put, a logic analyser is like a multi-channel oscilloscope that displays logic levels (ie, high voltage or low voltage) rather than tracing the actual voltage.

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Fig.2: this is the same logic trace shown above with two of the analysers turned on. The upper four traces represent an SPI interchange while traces 6 and 7 are a decoded I2C stream. The software shows the value of the data byte in a coloured bubble.

So why not just use an oscilloscope? Because a logic analyser is optimised for investigating digital circuits, it has multiple channels, making it easy to measure timing differences and it can decode the data streams that are transmitted and received.

Let’s say that you are developing a circuit which uses a microcontroller and a chip that communicates via SPI (Serial Peripheral Interface). Many chips use SPI to communicate, including memories, displays, accelerometers and sensors.

You dutifully read the data sheets, build a prototype, write the code and… it does not work.

Hmm... what next?

SPI uses four data lines to communicate. These are chip enable, clock, data in and data out. So you pull out your oscilloscope and check the various lines and observe “something” on each line. The first problem is that the signal is fleeting. There might be a burst of data when the program initialises but then nothing. With most oscilloscopes you need a repetitive signal to keep the trace on the screen long enough for you to see what is going on.

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