- Published: Tuesday, 21 May 2013
- Written by Jon Chandler
Aruna Rubasinghe - February 28, 2013
This is a repost from EDN Magazine's Design Ideas
There are several ways to read multiple switch inputs using a reduced number of microcontroller-unit (MCU) pins. For example, you can use an analog MCU pin to read multiple switches by assigning a unique voltage to each switch through a resistor network, or you can use a one-wire device, such as the Maxim DS2408 8-channel addressable switch.
The first method has several disadvantages: The MCU has to have an ADC function, debounce wait times reduce the polling rate, and an error results if the switch is opened during the ADC sampling time. The second method also has the drawback of comparatively low speed; it uses 1-wire communication, which requires continuous polling; and each poll generates an 8-bit data sequence relevant to switch positions.
This Design Idea describes a method for reading multiple pushbuttons or open/closed switches using only two digital I/O pins and a timer interrupt of the MCU (Figure 1). Optionally, a third I/O pin can be assigned to periodically reset the CD4017 (a cascadable decoded 1-of-10 Johnson counter) for reliable operation should an EMI or ESD event occur that could falsely clock the counter, or you can use the circuit shown in Figure 2 and retain the two-pin feature. The diodes isolate the counter outputs in the event that two or more switches are closed at the same time. You can increase the number of switches connected by cascading multiple CD4017 ICs using a carry-out signal (pin 12) and a clock signal (pin 14).Discuss this article in the forums (0 replies).