Logic analysis with Bus Pirate

While preparing for my SD tutorial, I realized it would be a long, uphill battle to debug SPI communications with a two-channel oscilloscope, especially when I had no prior experience on implementing SPI with AVR. To break the learning curve to manageable steps, I decided to get a Bus Pirate, a serial protocol tool I had heard a lot of good things about. What made the decision even easier was the fact that my local electronics supplier had just started selling the SparkFun version of Bus Pirate.

In this compact post, I’ll discuss my first impressions of the Bus Pirate, and try out the logic analyzer functionality.

Bus Pirate General Impressions

The first thought that crossed my mind when I saw the Bus Pirate in real life was how small the device was. As you can see, it’s hardly larger than an SD card. All the components are of surface mount type, LEDs included. USB cable was not included but fortunately I had several in my cable drawer. The thing does not need any other setup than plugging it in and installing the FTDI virtual COM port drivers. After that, it showed up as COM19 and I could connect to it with Putty. See the Bus Pirate 101 tutorial for installation details.

The v3 hardware is based on a PIC chip running at 3.3 volts and it took me a while to get comfortable with the power supply & pullup resistor logic – basically you can either use the Bus Pirate pins “normally” so an output pin is driven by the PIC, or in “open drain” mode, where a CD4066B analog switch connects the lines via pullup resistors to whatever is wired to the Vpu pin. This way, the Bus Pirate can interface with other than 3.3V devices (the PIC only pulls the line down or “lets it go” and the pullup takes the line high).
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