Nuts and Volts - September 2013

to solder a couple 1x3 male headers into the module to make it breadboard friendly. When you're done, the module will look like the one in Figure 2.

Down the line, I intend to roll my own. Figure 3 is the schematic for my JM_Pixel module (Adafruit does not provide a schematic for the NeoPixel. There's nothing magic about this circuit; it's taken right from the WS2812 datasheet. The RC components between VDD and VCC keep PWM noise on the VDD line from bothering the control electronics (which are connected to VCC). Easy peasy. Serial data is fed into DIN. When the device has received 24 bits, the rest of the stream will be directed to DOUT for other modules.

The signal from the DOUT pin is re-shaped and amplified so that it doesn't degrade as data propagates from LED to LED. According to the datasheet, we can have up to five meters of wire between WS2812 modules. I'm not sure I'd push it that far, but I like that they don't have to be right on top of each other.

One note on my version (see Figure 4): The pins are organized differently than the NeoPixel; my printed circuit board (PCB) layout matches the polarity of female-to-female servo extender cables. I've included Dip Trace files and Gerbers for the JM_Pixel. If you order from Bay Area Circuits (see File Menu in Dip Trace PCB) using the 10-day production schedule, you can get 100 boards for about $36 with shipping. Add $0.50 for the LED (from Adafruit) and another dime for the resistor and capacitor, and you can build your own pixels for about a buck each — if you're willing to do the soldering.

Cracking the WS2812 Code

As a programmer, I do my best to wear a "Captain Obvioso" costume so that everyone can understand what I'm doing — and most of the time it works. To that end, I like to specify RGB colors in my projects as a long value in the form $RR_GG_BB. Here's the rub: The WS2812 wants the 24- bit packet to arrive as $GG_RR_BB (the RR and GG bytes are swapped).

It's easy to swap a couple bytes in code, but on asking for suggestions in the Propeller forum, a really interesting discussion ensued and in the end, the most clever solution — in my opinion — was by TonyP12 (his handle) who is a frequent and very helpful contributor in the Parallax forums. He came up with a trick that eliminates nine lines of code and three ADVENTURES IN PROPELLER PROGRAMMING