Datasheet

But we wanted to make sure that the LEDs would not get dangerously hot even by

accident. For that reason, the Pixie firmware uses the PIC’s on-chip temperature indicator

to estimate the board’s temperature and would shut-down the LED when it gets too hot

(above about 70 degrees celsius). It will automatically resume operation when it cools

down. Getting this temperature indicator to work with reasonable precision was a challenge.

First, the PIC’s supply voltage needed to be extra-clean (as described above) and second,

to account for variability between different instances of the PIC, each and every unit goes

through an automated calibration process during manufacturing and the temperature

calibration data gets written to the PIC’s flash memory.

Loss of Communication

Have you ever noticed how NeoPixels retain their color if their controller goes away? While

this can be considered a convenient feature in some cases, it is an absolute no-go in a 3W

LED case. Losing communications with the controller for any reason during a high-

brightness pulse, that was otherwise intended to be very short, could potentially result in

LEDs being left on for extended periods, consuming a lot of power and dissipating a lot of

heat (limited by the over-temperature feature described above). Even more, what if we have

a firmware bug (not that we ever have bugs, but just for the sake of the discussion ;D)

causing the PIC to hang while its LED is on? That would be unacceptable.

Watchdog to the rescue! Remember we told you how awesome the PIC12 is? Another

feature that is useful for us is the watchdog. It will reset the PIC if it doesn’t hear from our

firmware that everything is fine for about 2 seconds. In turn, our firmware will only pet the

watchdog every time it gets a valid color and successfully latches it. So unless we hear

from our controller at least every 2 seconds (and in practice, better leave a little margin),

the Pixie resets, causing the LED to turn off until told otherwise.

So unlike NeoPixels, if you want your Pixies to stay on for extended periods, even with no

color change, you need to constantly remind them that you’re alive by sending them their

favorite string.

Conclusion

Who would have guessed that designing a circuit having only about 20 simple parts could

get so complicated? Certainly not us! We have done our best to provide a high quality,

useful product and learned a lot along the way. We’re hoping you’ll like the result and

enjoyed reading about some of the reasoning behind it.