Pico Makes Capable Logic Analyzer
A common enough microcontroller project is to create some form of logic analyzer. In theory, it should be pretty easy: grab some digital inputs, store them, and display them. But, of course, the devil is in the details. First, you want to grab data fast, but you also need to examine the trigger in real time — hard to do in software. You may also need input conditioning circuitry unless you are satisfied with the microcontroller’s input characteristics. Finally, you need a way to dump the data for analysis. Gusmanb has tackled all of these problems with a simple analyzer built around the Raspberry Pi Pico.
On the front and back ends, there is an optional board that does fast level conversion. If you don’t mind measuring 3.3 V inputs, you can forego the board. On the output side, there is custom software for displaying the results. What’s really interesting, though, is what is in between.
The Pico grabs 24 bits of data at 100 MHz and provides edge and pattern triggers. This is impressive because you need to look at the data as you store it and that eats up a few instruction cycles if you try to do it in software, dropping your maximum clock rate. So how does this project manage it?
It uses the Pico’s PIO units are auxiliary dedicated processors that aren’t very powerful, but they are very fast and deterministic. Two PIO instructions are enough to handle the work for simple cases. However, there are two PIOs and each has four separate state machines. It still takes some work, but it is easier than trying to run a CPU at a few gigahertz to get the same effect. The fast trigger mode, in particular, abuses the PIO to get maximum speed and can even work up to 200 MHz with some limitations.
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