Logic Analysers

A Logic analyser allows examination of large numbers of high-speed digital signals to detect problems and verify correct operation.  Logic analysers are able to display the entire contents of memory and I/O buses. While a DSO can display a 2-wire i2c or RS232 signal without any trouble, a Logic Analyser can display an entire 32-bit wide memory bus, for example.

SPECIFICATIONS
Bluewater Systems has two Logic Analysers:

• A Tektronix TLA5204 which is a 136-channel 2.5 GSa device. This is used for most work.
• An Agilent 1660A which is a 136-channel 200 MHz device. This is used for lower speed work and software benchmarking/debugging (or when the Tek is busy!)

The number of channels refers to the number of different digital signals which can be recorded at once. A 136 channel analyser gives extra flexibility for seeing very wide buses. For example, an ARM CPU may have up to 32 address lines, 32 data lines and another 30 or so control lines. This leaves about 30-40 lines free for looking at particular peripherals at the same time.

The number of samples per second is the number of times per second that the scope takes a digital sample. For example, 2.5 GSa/s means that the analyser takes 2.5 billion samples per second. Often we would run the analyser slower than this to catch data over a longer period of time.  The scope also has internal storage - the Tektronix can hold 4 million records. This means that you can capture an event and then zoom in many many times to see the event in detail.

TRIGGERING

As with DSOs, logic analysers capture data starting from a trigger event. For example, a very obscure problem may be that writing a certain value to flash is not operating correctly. You can therefore program the analyser to trigger when it sees a write pulse, a flash chip select and a certain data value on the processor bus. When all three conditions are met, the logic analyser starts recording so you can see what happens.

Logic analysers support extremely complex trigger conditions to obtain the required data even for the rarest events. Because of the enormous power of these pieces of equipment, problems which otherwise might take days or weeks to analyse and fix can often be found and corrected in the time it takes to connect up the cables.

SUMMARY
While logic analysers are not the easiest devices to master they can be extremely useful in certain circumstances. They are therefore somewhat like intercontinental airline pilots - they sit around most of the time, but when they earn their money, the pay-off is enormous.