Datasheet

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Advanced Coding

Help! My Arduino servo code stops working when combined with NeoPixels!

Unfortunately the NeoPixel and Servo libraries don’t play nice together; one is dependent on

periodically disabling interrupts, the other absolutely requires interrupts. There are a couple of

options here:

Use a dedicated servo control shield (http://adafru.it/1411) or breakout

board (http://adafru.it/815), offloading that task from the processor so interrupts are a non-

issue.

Use a hardware-PWM-based servo library (http://adafru.it/eda) rather than the stock

Arduino Servo library. This can provide rock-steady servo timing without interrupts, but can

only control a very limited number of servos (2-3), and only on very specific pins.

How fast can I refresh a string of (N) pixels?

NeoPixels receive data from a fixed-frequency 800 KHz datastream (except for “V1” Flora pixels,

which use 400 KHz). One bit therefore requires 1/800,000 sec — 1.25 microseconds. One pixel

requires 24 bits (8 bits each for red, green blue) — 30 microseconds. After the last pixel’s worth

of data is issued, the stream must stop for at least 50 microseconds for the new colors to “latch.”

For a strip of 100 pixels, that’s (100 * 30) + 50, or 3,050 microseconds. 1,000,000 / 3,050 = 328

updates per second, approximately.

However…

That’s only the time needed to push the bits down the wire. The actual refresh rate will be

something less than this, and can’t be estimated as a single number for all cases. It takes time to

process each “frame” of animation. How much time depends on the complexity of the math and

the efficiency of the code (for example, floating-point calculations can be relatively slow). The

technique above gives a maximum theoretical rate, but that’s just a starting point. Reality in

some cases could fall an order of magnitude (or more) below this.

For exploratory benchmarking, you can always write code as if a large number of pixels were

present, and time the result. The extra output bits will simply be ignored by the strip (or you can

even test with no NeoPixels connected at all).

That won’t do. Now what?

Because NeoPixels use a fixed-frequency clock, options are limited. You can’t switch out for a

faster microcontroller and expect substantially different results.

One option is to use a different LED type, such as our LPD8806 strips or WS2801 pixels. These

can be driven at higher data rates, though they do have some other tradeoffs with respect to

NeoPixels (cost, color resolution and/or pixel density).