The Bus Pirate prototype “Ultra” v1b has a 10 pin 1.25mm connector for a display daughterboard. We wanted a more dynamic way to keep track of the pinout and other handy information like pin states and voltage levels.
There are various reference stickers and labeled probe cables for the Bus Pirate v3 and v4, but a display frees us from strict pinout conventions and makes setup a lot easier for new hackers. More on the display below.
We wanted to use ePaper!
At first we really wanted to use ePaper. A static, easily readable display is reminiscent of the stickers it replaces, and makes reference to its primary purpose as a label. The slow update speeds also dampen expectations of flashy graphics and interaction.
The three color ePaper shown above was a top candidate, but it requires 180 seconds of rest between updates to stay in spec. Slow updates are acceptable, but we need to be able to change the display more frequently than once every three minutes.
While the two color version of this display doesn’t have this limitation, we ultimately decided that the Dots Per Inch is just too low to look really good. ePaper is also quite expensive compared to IPS LCDs with much higher pixel density.
Our attention turned to inexpensive LCDs. These are not the dodgy panels with limited color and ever-changing driver chips you may have used in the past. About $3 buys a 240×320 pixel 2 inch IPS (view from every angle) display supporting 250K colors, with a simple SPI interface and well known driver chip. We bought samples from two manufacturers, a QT020HLCG00 and a HT020SQV003NS.
With a bunch of colors and the ability to constantly update, we can show a lot of useful info. Each pin has a proper name corresponding to the active mode, along with input/output direction indication. Color caps on each pin match the probe cable wires, the terminal interface and the logic analyzer client.
Starting with v1b we can measure voltage on all 8 IO pins, so we’ll show that on the display. We’re particularly excited about this feature because we always end up jugging multimeter probes to verify levels during a rough hack. Now we can just look at the display!
Both LCDs are approximately the same dimensions, but the solderable flex connector is a slightly different length. The LCD daughterboard has extra long pads that fit both displays. The files for the display board are in the git repo.
The image on this LCD is just a static mock-up exported from Photoshop. Next we’ll need to create a font and write actual values to the display. This is will be tricky because LCD fonts typically look jagged and ugly, and we don’t really have enough power to do anti aliasing on the fly. Follow our latest progress in the forum.