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Charlieplexing

Charlieplexing (also known as tristate multiplexing, reduced pin-count LED multiplexing, complementary LED drive and crossplexing) is a technique for accessing a large number of LEDs, switches, micro-capacitors or other I/O entities, using very few tri-state logic wires from a microcontroller, these entities being wired as discrete components,[1][2] x/y arrays,[3][4] or woven in a diagonally intersecting pattern to form diagonal arrays.[5] The method uses the tri-state logic capabilities of microcontrollers in order to gain efficiency over traditional multiplexing, each I/O pin being capable, when required, of rapidly changing between the three states, logical 1, logical 0, and high impedance. This enables these I/O entities (LEDs, switches etc.) to be connected between any two microcontroller I/Os - e.g.

Also in 2001, Don Lancaster illustrated the method as part of his musings about the " N-connectedness" problem, referring to Microchip Technology, who had already discussed it as "complementary LED drive technique" in a 1998 application note[12] and would later include it in a tips & tricks booklet. The method, however, was known and utilized by various parties much earlier in the 1980s, and has been described in detail as early as in 1979 in a patent by Christopher W. Malinowski, Heinz Rinderle and Martin Siegle of the Department of Research and Development, AEG-Telefunken, Heilbronn, Germany for what they called a "three-state signaling system". […] Charlie Allen originally championed this technique internally at Maxim, and so the shorthand name "Charlieplexing" came into use to distinguish reduced pin count multiplexing from the traditional method.

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