Charlieplexing - Input Data Multiplexing

Input Data Multiplexing

Charlieplexing can also be used to multiplex digital input signals into a microcontroller. The same diode circuits are used, except a switch is placed in series with each diode. To read whether a switch is open or closed, the microcontroller configures one pin as an input with an internal pull-up resistor. The other pin is configured as an output and set to logic-low. If the input pin reads low then the switch is closed, and if the input pin reads high then the switch is open.

One potential application for this is to read a standard (4×3) 12-key numeric keypad using only 4 I/O lines. The traditional row-column scan method requires 4 + 3 = 7 I/O lines. Thus charlieplexing saves 3 I/O lines; however it adds the expense of 12 diodes, (since the diodes are only free when LEDs are used). This reference shows a variation on the circuit that only needs 4 diodes, however that method qualifies as lossy compression, because when certain combinations of buttons are pressed simultaneously, those signals interfere with the microcontroller's ability to read certain other buttons. The microcontroller can always detect when the data is corrupt, but there is no guarantee it can sense the original key presses, unless only one button is pressed at a time. (However, it is probably possible to arrange the circuit so that if at most any two adjacent buttons are pressed, then no data loss will occur.) Basically though, the input is only loss-less on the 4 diode circuit if only one button is pressed at a time, or if certain problematic multiple key presses are avoided. In the 12 diode circuit, this is not an issue, and there is always a one-to-one correspondence between button presses and input data. However, there are so many diodes that are required to use the method (especially for larger arrays) that there is generally no cost savings over the traditional row-column scan method, unless for some reason the cost of a diode is only a fraction of the cost of an I/O pin, where that fraction is one over the number of I/O lines.