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". 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. ![]() Daniel at the time had created simple circuits with PIC 12C508 chips driving 12 LEDs off 5 pins with a mini command set to set various lighting displays in motion. While Microchip did not mention the origin of the idea, they might have picked it up in the PICLIST, a mailing list on Microchip PIC microcontrollers, where, also in 1998, Graham Daniel proposed it to the community as a method to drive rows and columns of bidirectional LEDs. Īlso 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 and would later include it in a tips & tricks booklet. Allen, an applications engineer of MAX232 fame, who had proposed this method internally. The name "Charlieplexing", however, first occurred in a 2003 application note. The Charlieplexing technique was introduced by Maxim Integrated in 2001 as a reduced pin-count LED multiplexing scheme in their MAX6951 LED display driver. Multiplexing can generally be seen by a strobing effect and skewing if the eye's focal point is moved past the display rapidly. Other issues include duty cycle, current requirements and the forward voltages of the LEDs, but these issues also affect standard x/y multiplexing.Īs with any multiplexing, there is a requirement to cycle through the in-use LEDs rapidly so that the persistence of the human eye perceives the display to be lit as a whole. This shows a six I/O Charlieplexed array that is scalable to any size without difficulty. However, there is no problem if a diagonal array is used (see diagram). The method uses the tri-state logic capabilities of microcontrollers in order to gain efficiency over traditional multiplexing.Īlthough it is more efficient in its use of I/O, there are issues that cause it to be more complicated to design, and render it difficult for larger displays when trying to fit Charlieplexing into a standard x/y array - as described later in this article. When functioning as an LED display or mechanical keypad, the use of reversed pairs of LEDs or standard diodes at the intersections doubles the 8 input matrix size from 28 to 56. The diagram shows that eight inputs creates 16 intersections when used in an x/y array, but 28 intersections when used in a Charlieplexed array. LEDs for output or switches / micro-capacitors for input).Ĭharlieplexing can be used in displays and resistive or projected capacitance keypads and touchscreens, using a simple array (shown). ![]() A Charlieplexed digital clock which controls 90 LEDs with 10 pins of a PIC16C54 microcontroller.Ĭharlieplexing (also known as tristate multiplexing, reduced pin-count LED multiplexing, complementary LED drive and crossplexing) is a technique for driving a multiplexed display, keypad or touchscreen, in which relatively few I/O wires from a microcontroller are woven in a diagonally-intersecting pattern to access a large number of I/O entities (e.g. Six diagonal Charlieplexed I/O lines formed into a very simple and scalable LED matrix. This diagram shows how eight inputs to a lattice touchscreen or keypad creates 28 unique intersections, as opposed to 16 intersections created using a standard x/y multiplexed touchscreen. ![]() Technique for driving a multiplexed display
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