Embedded Systems Design with the Atmel AVR Microcontroller Part II

7.3. INPUT DEVICES 187
make connection between the two switch contacts. The pushbutton must be depressed again
to release the connection.
Hexadecimal rotary switches: Small profile rotary switches are available for microcontroller
applications. These switches commonly have sixteen rotary switch positions. As the switch is
rotated to each position, a unique four bit binary code is provided at the switch contacts.
A common switch interface is shown in Figure 7.3(b).This interface allows a logic one or zero
to be properly introduced to a microcontroller input port pin.The basic interface consists of the switch
in series with a current limiting resistor. The node between the switch and the resistor is provided
to the microcontroller input pin. In the configuration shown, the resistor pulls the microcontroller
input up to the supply voltage VDD. When the switch is closed, the node is grounded and a logic
zero is provided to the microcontroller input pin. To reverse the logic of the switch configuration
the position of the resistor and the switch is simply reversed.
7.3.2 PULLUP RESISTORS IN SWITCH INTERFACE CIRCUITRY
Many microcontrollers are equipped with pullup resistors at the input pins. The pullup resistors are
asserted with the appropriate register setting. The pullup resistor replaces the external resistor in the
switch configuration as shown in Figure 7.3b) right.
7.3.3 SWITCH DEBOUNCING
Mechanical switches do not make a clean transition from one position (on) to another (off ). When
a switch is moved from one position to another, it makes and breaks contact multiple times. This
activity may go on for tens of milliseconds. A microcontroller is relatively fast as compared to the
action of the switch. Therefore, the microcontroller is able to recognize each switch bounce as a
separate and erroneous transition.
To correct the switch bounce phenomena, additional external hardware components may
be used or software techniques may be employed. A hardware debounce circuit is illustrated in
Figure 7.3(c). The node between the switch and the limiting resistor of the basic switch circuit is
fed to a low pass filter (LPF) formed by the 470K ohm resistor and the capacitor. The LPF prevents
abrupt changes (bounces) in the input signal from the microcontroller. The LPF is followed by a
74HC14 Schmitt Trigger, which is simply an inverter equipped with hysteresis. This further limits
the switch bouncing.
Switches may also be debounced using software techniques.This is accomplished by inserting
a 30 to 50 ms lockout delay in the function responding to port pin changes. The delay prevents the
microcontroller from responding to the multiple switch transitions related to bouncing.
You must carefully analyze a given design to determine if hardware or software switch debouncing
techniques will be used. It is important to remember that all switches exhibit bounce
phenomena and, therefore, must be debounced.