The most common method for interfacing
multiple switches—multiplexing—
allows for the connection of
(N/2)2 switches with N microcontroller
IO lines. The method described
below, which has its roots in an LED interface
technique commonly known as “Charlieplexing,”
makes it possible to interface
N*(N – 1) switches with N IO lines.
In conventional multiplexing of an N-by-
N matrix with 2N IO lines, half of the lines
are configured for input and half for output
(at logic 0), permanently using a pin either
for just the input or for just the output. The
proposed method uses both states of an IO
pin, increasing the number of switches that
can be interfaced to it.
In the proposed method, every IO pin is
connected to every other pin with two switch-and-diode series circuits.
Figure 1 shows a two-pin example. In this configuration, switch
S1 can be read by configuring Pin2 to input mode and Pin1 to output
mode (with logic 0). Switch S2 can be read by configuring Pin1 to
input mode and Pin2 to output mode. This technique allows designers
to interface six switches with only three pins, and a whole numeric
keypad of 12 switches with four pins.
This configuration easily scales to any number of IO pins, but
between two IO pins, the circuit essentially remains the same. For a
circuit with N IO lines, each IO pin has N – 1 diodes leading in, and
N – 1 diodes leading out. At any time, one pin is switched to logic 0,
while the other N – 1 pins are configured in input mode for reading
N – 1 switches attached to this pin. Generally, the N – 1 diodes leading
into this pin can be replaced by a single diode, with
all of the N – 1 switches connected to its anode.
Figure 2 shows a circuit for interfacing 12 switches using only four
microcontroller IO pins. To read switches S1-S3, Pin1 is set to logic
0, and Pin2, Pin3, and Pin4 are set to input mode. Similarly, to read
switches S4-S6, S7-S9, and S10-S12, Pin2, Pin3, and Pin4 are, respectively,
set to logic 0, while the rest are set to input mode.
The circuit in Figure 3 demonstrates how a full numeric keypad
interface can be connected to only four pins of a microcontroller (an
Atmel ATMEGA8 in this case). The switches are pushbutton types,
and the diodes are generic rectifying diodes. The demonstration program
(go to www.electronicdesign.com), written in C using WINAVR,
successively sets one of the IO pins to 0, and the rest to input, and
it sets the state of the switches, which was relayed serially in mode
9600-8-N-1.
References:
1. Anurag Chugh, Dhananjay V. Gadre, “Eight-Pin Microcontroller
Handles Two-Digit Display With Multiple LEDs,” www.electronicdesign.com, ED Online 15512
2. “Charlieplexing” at http://en.wikipedia.org/wiki/Charlieplexing.
Retrieved Sep. 3, 2007.
