![]() 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. to drive an array of LEDs.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 pins on a microcontroller are used e.g.
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