Recently, I encountered an interesting problem: How do you measure the length of a WS2812 programmable RGB-LED string electronically? That is, just using signals that are already there. This can be useful if you want a LED controller to adapt its pattern according to the string that is connected to it, or simply for diagnostic reasons.
WS2812 strings are usually controlled by using one serial output signal only. The LEDs are daisy chained and you simply push out data for all LEDs without any feedback. If there are fewer LEDs on the string, the data for the last LEDs is ignored.
An intuitive approach to counting the number of LEDs is to feed the output of the last LED of the string back into the microcontroller. According to the WS2812 protocol, the output of LED number n is low until data for n+1 LEDs has been pushed to the string. To count the number of LEDs, data is fed to the string until a rising transition is detected on the output. A relatively easy way to implement this is by using a pin change interrupt on the AVR, or a similar feature on other microcontrollers.
While this works nicely, it has the disadvantage of requiring another wire. Can we do it without introducing any additional connections? Continue reading “Measuring the length of a WS2812 string”