Page 1146 - The Mechatronics Handbook

For example, an interrupt may occur when a digital logic level changes at a device pin, indicating a sensor
condition, or it may occur when the user presses a button on a keypad, indicating that an action is desired
and is to be performed immediately.
The implementation of a real-time software system may either be custom designed or may make use
of a commercial real-time operating system (RTOS). Since the design of an interrupt-driven real-time
system has many potential pitfalls, the usage of a mature RTOS can greatly speed development time.
Embedded Modules
The functionality of commercially available embedded computing modules has been steadily increasing.
It is common to find powerful microcontrollers, an Ethernet interface, and basic Internet protocol support
all combined in a very small form factor for less than $50 in single quantities. This level of integration
can greatly speed development time for network-enabled control or remote sensing applications.

Hardware Interfacing

Mechanical Switches
Switches are easily interfaced to digital logic with a resistor as shown in Fig. 43.3. The mechanical nature
of the switch may lead to bounce or oscillation of the digital signal for a brief period during the switch
opening/closing action. This bounce may be eliminated in the software or with a small amount of
additional hardware.
Analog Inputs
Analog inputs that indicate one of the two conditions can be interfaced to a digital logic input with a
simple comparator (Fig. 43.4). A threshold voltage is set with a resistor divider. The comparator generates
a digital signal, which indicates whether the analog input voltage is above or below the threshold voltage.
This approach can be used for sensors such as optical interrupters (for part counting, motor movement
detection, etc.), temperature limit sensors, and many others.
When the analog voltage itself is of interest (as in, for example, temperature measurements), an analog-
to-digital converter (ADC) can be used to provide either a serial or a parallel representation of the voltage
with a precision ranging anywhere from 8 bits to 16 bits and above. A serial ADC may require as few as
two digital I/O pins on a microcontroller for transferring data, while a parallel ADC requires at least as

+5V

FIGURE 43.3 A mechanical switch is easily interfaced to digital input
to a digital input on a microcontroller using a single
resistor.

+5V

analog input + to digital input
-

FIGURE 43.4 A digital input driven by a comparator
detects whether an analog voltage signal is above or below
a threshold voltage (set with a resistor divider network).

©2002 CRC Press LLC